Lime-aluminium-phosphate interactions in selected acid soils from Fiji

Transcription

Copyright is owned by the Author of the thesis. Permission is given for
a copy to be downloaded by an individual for the purpose of research and
private study only. The thesis may not be reproduced elsewhere without
the permission of the Author.
L IME-ALUMINIUM-PHOSPHATE INTERA CTIONS
IN SELECTED ACID SOILS FROM F IJI
A thesis presented in par tial fulfilment of
the requirements for the degree of
Doctor of Philosophy in Soil Science
at Massey University
Ravendra Naidu
1985
ii
ABSTRACT
Poor crop production in Fij i has long been associated with Al-toxicity
and/or P deficiency problems .
Although a t t empts have been made to
allevia te t hese problems, the lack of suitable soil-tes ting procedures and
a limi ted understanding of lime -Al-P in terac tions are restricting the
bet ter u tilization of these soils .
Following a preliminary investigation, 4 contrasting Fij ian soils
(Batiri, Koronivia, Nadroloulou, and Seqaqa) were chosen for a lime-Al-P
The soils, which had pH and M KCl-ex tractable Al
-1
values ranging from 3 . 9 to 4 . 9 and 35.6 to 0.3 mmol kg , respec tively,
in terac t ion st udy.
were used t o investigate the effec t of liming on surface charge, P-sorption
characteristics, the amoun ts of P extrac ted by a number of soil- testing
procedures, and plant upt ake of P .
A st udy was c ondu cted to compare M KCl-extrac t ion procedures for
exchangeable Al and analytical techniques used in the determination of Al .
For each soil, different extrac tion procedures and analy tical techniques
measured signifi can tly (P < 0 . 01)
differen t amoun ts of ex tractable Al.
It was recommended that extrac t able Al in Fijian soils could be best
determined by the oxine reagent following a 2 x 1-h shaking wi th M KCl .
The ion reten t ion me thod, which is commonly used to measure charge,
was examined crit ically wi th a view to s tandardising it for the range of
soils used in the presen t study.
The method involves an initial washing
of soils wit h an elec trolyte of high concentration to remove exchangeable
ions, equilibration of the washed soils wi th an ele c trolyte of the desired
concentration
and subsequen t extraction of the equilibra ted soils.
The
concentrations of prewash elec trolyte (0 . 5M CaCl 2, O .lM CaCl2, and
O. O lM CaCl2) used to remove exchangeable ions prior to equilibration with
O. OlM CaCl�nd the soil: solution ra tio were found to have a marked effect
on the m agni t ude of t he surface negative charge of unlimed soils.
However, these differenc es were largely related to t he amount of Al removed
during t he prewash and the equilibra tion procedures .
Thus when the Al
released in the extrac ting solu tion (0.5M KN0 3 ) was included in the
calcula tion of charge, the differences in the measured nega tive c harge
obtained either because of varying concen tra tions of prewash elec trolyte or
for the effe c t of soil: solution ratio were reduced.
iii
Surface charge, det ermined in O . O lM CaCl2, was always found to be
higher than that de termined in 0 . 0 3M NaCl and this difference was more
pronounced in limed soils at high pH values .
Subsequent studies revealed
that this anomaly was largely due to the inabili ty of Na to exchange
wi th Ca at high pH values .
The resul ts of t hese st udies, together wi th
those involving the prewash electrolytes and the soil: solution ra tio,
sugges ted t hat a suitable method of measuring surface charge of limed
soils would use O . OlM CaC1 2 as the equilibra tion elec troly te and include
in the calculation of charge the amount of Al released in the extracting
solution .
Incubation of soils with added lime caused a large increase in
surface negative charge .
However, the magni tude of increase in the
negative c harge varied considerably between soils .
For example, the
nega tive charge in the Seqaqa soil increased from 8 to over 38 cmol(p)kg- 1 ,
-l
compared t o only a small increase of 2 to 10 cmol(p)kg
in the Batiri
soil over t he same pH range.
In contrast to liming, P additions resulted in only a small increase
in negative charge .
Interestingly, all soils possessed positive charge
up to 1 cmol(p)kg- 1 , even at pH values as high as 7 .
Subsequent studies
+
+
showed that this may have been due to substi tution of Ti4 and,hr Mn4 in the
iron oxide lat tice .
Ex traction of lime- and P-treated soils wi th Olsen and Mehlich
reagents s howed that liming had a marked effect on the amount of P removed .
Whereas Olsen P increased on either side of pH values 5 . 5 - 6.0 , Mehlich P
consisten t ly decreased with increasing soil pH .
For example, in the
high P- sorbing Seqaqa soil, Mehlich P decreased from 0 . 2 mmol kg-l at pH
4 . 5 to
<
0 . 0 1 mmol kg- 1 in soils wi th pH higher t han 7 . 0 .
The decrease
in Mehlich P was shown to be due to t he neutralizing effect of lime on the
extrac t ant.
An isotopic-exchange study revealed an increase in exchangeable
P up to a pH approximating 7, above which there was a sharp decrease,
possibly indicating the formation of insoluble Ca-P compounds .
Al t ho ugh liming had only a small effec t on the sorption of added P,
t his was sufficient to have a significant effec t on equilibrium solution P
concentration .
Generally, liming caused an increase in equilibrium
solution P concentra t ion up to pH values of 5 . 0 - 6 . 0, above which there
was a marked decrease .
The initial increase in equilibrium solution P
concentration appeared to result from an interaction between added P,
iv
surface negative charge and electrostatic potential in the plane of
sorption.
Subsequent sorption studies using Nadroloulou soil incubated
with either KOH or Ca ( OH ) 2 showed that the decrease in solution P at high
pH values was probably due to the formation of insoluble Ca-P compounds .
The effects of lime and P addition on the growth of the tropical
legume Leucaena leucocephala were studied in a controlled-climate laboratory .
With all 4 soils, there was an initial increase in the dry matter yield of
the plant tops with liming which was followed by a marked decrease .
This trend was most pronounced in the Seqaqa soil where dry matter yield
of tops increased by -2000%
at the pH at which maximum growth occurred .
Similar but smaller increases were noted in the other soils.
The c oncentration of Al in plant tops increased on either side of
the pH of maximum growth, but Al uptake by the whole plant ( tops + roots )
declined steadily with increasing pH .
Poor growth at low pH values was
attributed t o Al-induced P deficiency within the plant and at high pH
values largely to a soil P deficiency and to a smaller extent to the
increased concentration of Al in the plant tops .
P deficiency at high
pH values was attributed to the formation of insoluble Ca-P compounds
and this was supported by the data obtained from isotopic-exchange and
P-sorption studies.
A furt her plant growth study was conducted on the limed soils,
previously used for the growth of Leucaena ,leucocephala. Ryegrass ( Lolium
perenne L ) plants were initially grown in sand and then transferred onto
the soils.
Plant growth was again retarded at low and high pH values
but comparison with control plants grown in a similar manner but not
transferred onto the soils demonstrated that the poor growth at both high
and low pH was due in part to a toxicity effect rather than simple P
deficiency.
It is likely that Al was responsible .
Comparision of the data obtained by resin extraction and plant P
uptake gave a close 1 : 1 relationship .
In contrast, Olsen-, Colwell-,
Bray (I)-, Bray ( II ) -, and Mehlich-extractable P were only weakly correlated
with P uptake.
The difficulty in relating plant P uptake data to
extractable P levels was attributed to the problems associated with
extracting
P
from limed soils.
V
AC��O��EDGEMENTS
I would like to express
my
appreciation to a number of people
without whose help and guidance this thesis would not have been possible.
In particular, I would like to thank:
Professor J. Keith Syers, who with the cooperation of MAFF, Fiji
and the Fiji Public Service Commission initiated this study.
Also for his
supervision, unending enthusiasm, and encouragement during the study.
His
rigorous standards of scientific thoroughness and presentation , have been
highly rewarding .
Mr. Russell Tillman, to whom I
am
grateful for his patience,
constructive ideas and continual guidance during the planning and
execution of this study.
Dr. J.H. Kirkman for many helpful discussions with several aspects
of this study.
Mr.
Lance Currie and Dr. I. Warrington of the Department of Soil Science
and D.S.I.R., respectively, for guidance in the execution of the glasshouse
trials.
br. R. Lee, Soil Bureau, Lower Butt, for his advice in the studies
related to e xchangeable Al.
Past and present Post-doctoral Fellows and students in the Department
of Soil Science.
Especially, Dr. D. Curtin, Dr. N.S. Bolan, Dr. R. Harrison,
and Miss P. Sorn-srivichai for helpful discussions with certain aspects of
this study.
Mr. D.M. Leslie, Soil Bureau, Lower Butt and Professor R.J. Morrison ,
University of the South Pacific, Fiji, for their advice in the collection
of soil samples.
Dr. D. Horne, Dr. B. Ponter, and Miss M. Wallace for proof reading
the thesis.
Fiji Government for financial support in the form of a scholarship
and to the University of the South Pacific for granting me study leave.
vi
The Faculty of Agricultural and Horticultural Science, Massey
University , for a Johannes August Anderson award.
Carolyn Hedley for excellent illustration of figures.
Dianne Syers, for producing an excellent t ypescript.
Finally to my son Roneal for providing a lively atmosphere at home
and to my wife Shamila who has helped me in many aspects of my work.
Her patience, help, and encouragement have been a constant inspiration.
vii
TABLE OF CONTENTS
Page
ABSTRACT .
.
.
ii
•
ACKNOWLEDGEMENTS
V
TABLE OF CONTENTS
vii
LIST OF FIGURES
xiii
LIST OF TABLES
-- xix
CHAPTER 1
GENERAL INTRODUCTION
1
1. 1
1
Introduction
CHAPTER 2
REVIEW OF LITERATURE .
3
2. 1
Introduction
3
2.2
Mechanism of Phosphate Retention
3
2.3
Effect of Liming on Aluminium and Phosphate
Chemistry of Acid Soils
2.3.1
.
•
. .
.
.
.
•
Sol ution chemistr y of p ure aluminium
s ystems and its imp lication to soils
2. 3.2
2.5
8
Lime-aluminium-phosphate interactions
in acid soils .
2.4
6
Origin of exchangeable aluminium and its
interaction with added phosphate
2.3.3
5
.
.
.
.
.
.
•
10
Effects of Excess Soil Aluminium and
Low Phosphate on Plant Growth . . . .
16
2.4.1
Uptake of aluminium by p lants
16
2.4.2
Toxicity symptoms
16
Conclusions
.
•
•
.
.
.
.
17
viii
Page
CHAPTER 3
GENERAL MATERIAL S AND METHODS
•
18
.
3. 1
Soils
3.2
Analytical Procedures
18
3.3
Selective Chemical Dissolution Techniques
21
3.3.1
•
.
.
•
•
.
•
18
•
Determination of oxalate-extractable
iron and aluminium
3.3.2
•
.
.
.
•
•
•
.
21
•
Determination of citrate-dithionite
bicarbonate-extractable iron and aluminium
3.4
3.5
24
Preliminary Lime Incubation Study
3.4.1
21
24
Pre paration of soil sam p les
Extraction of Aluminium from Soils using M KCl:
24
A Comparison of Methods
3 . 5 .1
Introduction
3. 5 . 2
Methods
24
•
27
•
3. 5.2.1
Aluminium-release curves
3. 5.2.2
Determination of M KCl-extractable
aluminium:
Extraction procedures
27
(a) Short-term shaking
(Pratt and Blair, 1961)
27
(b) Long-term shaking
27
(c) Long-term standing (Yuan, 1959)
27
(d) Leaching (Black, 1965)
29
(e) Sequential extraction
29
Analytical procedures
29
3. 5 . 2 . 3. 1
Calorimetric methods
29
(a)
(b)
29
29
3 . 5 .2.3
3 . 5 .2.3. 3
3 . 5.2.4
3.5.3
3. 5 .4
Oxine
Aluminon
•
27
•
•
•
•
•
•
Atomic absorption
spectrophotometry
Recovery tests
30
30
•
Results and discussion
30
3. 5.3.1
3 . 5.3. 2
Comparison of extraction procedures
33
3 . 5.3.3
Analytical techniques
36
3. 5.3.4
Recovery tests
Conclusions
•
•
•
•
•
•
•
•
•
•
.
•
•
•
30
38
41
ix
Page
CHAPTER 4
CHARGE CHARACTERISTICS OF ACID SOILS
AS INFLUENCED BY L IMING
42
•
4. 1
Introduction
4.2
Materials and Methods
44
4. 2. 1
Pre paration of soil samp les
44
4.2.2
Charg e measurement procedures
45
.
•
42
.
4.2.2.1
4.2.2.2
4.2.2.3
Effect of concentration of prewash
electrolytes on charge subsequently
determined in O.OlM CaClz solutions
45
Effect of soil: solution ratio
on negative and positive charge
determined in O.OlM CaClz
46
Effect of index cations on the
determination of negative and
.
positive charge . .
46
.
4.2.3
.
Effect of soil pH and added phosphate
on ne gative and positive charge determined
46
in 0.0 1M CaClz
4.3
Results and Discussion
4.3. 1
47
•
Charge measurement procedures
47
Effect of concentration of
prewash electrolytes
47
4.3. 1.2
Effect of soil: solution ratio
49
4.3. 1.3
Effect of index cations
52
4.3. 1 . 1
•
4.3.2
4.4
•
Effect of soil p H on char ge
characteristics. of soils
4.3.3
•
•
.
Effect of added phosphate on charge
Conclusions
.
•
.
55
59
61
X
Page
CHAPTER 5
EFFECT OF L IM ING ON PHOSPHATE EXTRACTED
BY TWO SOIL-TESTING PROCEDURES .
63
5. 1
Introduction . . . . . . .
63
5.2
Materials and Methods . . . .
64
5.2. 1
Pre paration of soil sam ples .
64
5.2.2
Chemical analyses
64
5.3
5.3.1
65
Effect of lime and phosphate additions
on isoto pically-exchang eable phosphate
5.3 . 2
Effect of lime and p hosphate additions
on Olsen-extractab le p hosphate
5.3.3
5.3.4
5.4
65
68
Effect of lime and phosphate additions
on Mehlich-extractable phosphate
71
Com parison of soil-testin g procedures
73
Conclusions . . . . .
75
CHAPTER 6
EFFECT OF L IMING ON PHOSPHATE SORPTION BY ACID SOILS
78
6. 1
Introduction
78
6.2
6.3
. . . .
.
79
6.2.1
Prep aration and preliminary analysis of soils
79
6.2.2
Incubation of soils with KOH
79
6.2.3
Phosp hate sorption studies
79
82
6.3 . 1
Effect of liming on p H, extractable aluminium ,
and Olsen-extractable phosphate
•
.
•
6.3.2
Effect of pH on phosphate sorption
6.3.3
Ef fect of background electrolyte on phosphate
82
83
sorp tion b y Nadroloulou soil incubated with
calcium or potassium h y droxide
6.4
Conclusions . .
89
94
xi
Page
CHAPTER 7
LI�-ALUMINIUM-PHOSPHATE INTERACTIONS AND
THE GROWTH OF LEUCAENA LEUCOCEPHALA .
98
7. 1
Introduction . . . . .
98
7.2
99
7. 2. 1
Soils
99
7. 2.2
Plant g rowth stud y
101
7.2.3
Chemical analyses
101
7.2.3. 1
Soils .
101
7.2.3. 2
Plants .
1 02
Results and Discussion . .
102
7.3
7.3. 1
Effec t of liming on EH, extractable
aluminium, and extractable Ehosphate
7. 3.2
. . . . . . . . . . . . .
.
. .
.
.
.
.
. . . . . . .
.
.
. . . . .
107
1 10
1 12
Lime-aluminium-Ehosphate interactions
in soils and plants
7.4
102
Effect of lime and Ehosphate additions
on the chemical composition of Leucaena leucocephala
7.3. 5
.
Effect of lime and Ehosphate additions
on root growth
7. 3.4
.
Effec t of lime and EhosEha te additions
on Elant growth
7. 3.3
. . . . . .
Conclusions . . . . . . . . . . . .
121
.
.
. .
123
CHAPTER 8
ASSESSMENT OF PLANT-AVAILABLE PHOSPHATE
USING SEVERAL SOIL- TESTING PROCEDURES
125
8. 1
Introduction
125
8.2
Materials and Methods .
126
8.2. 1
Soils . . .
1 26
8.2.2
Extractable Phosphate .
1 26
8.2.3
Determination of buffer capacity
127
xii
Page
8.3
Plant Growth Studies . .
128
8.4
128
8.4.1
Plant g rowth studies
128
8.4.2
Relationship between plant uptake
of phosphate and soil-test results
8.5
Conclusions
129
137
SUMMAR Y AND CONCLUSIONS
139
APPENDICES .
144
BIBLIOGRAPHY
150
xiii
LIST OF FIGURES
Figure
2. 1
Page
Solubility o f aluminium in water solution
7
as a f fected by pH o f solution (McLean , 1 976)
2.2a
Influence of pH on the phosphate sorbed by
goethite at a solution concentration of
0.2 mmol L- 1 together with the proportion
o f phosphate present as the divalent ion
(Bowden et al. , 1980b) . . . . . .
2.2b
12
.
Retention o f phosphate by hydrolytic reaction
products of aluminium formed in systems at
the initial aluminium concentration o f
1.10 mmol L- 1 and OH /Al molar ratio o f 3.0
12
as a f unction o f time (Kwong et-al. , 1978)
2.3
E f fect o f pH on the amount of phosphate in
soil solution and on the amount of labile
phosphate present in an Illinois soil
studied by Mu rrmann and Peech (1969) ,
13
(reproduced from Haynes , 1982)
3.1
Map o f the Fiji Islands showing the location
o f soils used for the present study
3.2
Amounts of extractable Al ( • ) released and pH
19
( 0 ) of the soil suspension during eight
sequential extractions _of the unlimed soils
with M KCl
3.3
.
•
.
•
.
.
.
•
•
.
.
.
32
.
Amounts of Al extracted from unlimed soils by
the M KCl extraction procedures , short-term
shaking ,
(A);
long-term shaking ,
long-term standing , .(C);
(B);
2 X 1 h shaking , (D);
and 2 h leaching o f soils , (E);relative to the
cUmulative amount removed during 8 successive
extractions ,
(S).
LSD (5%) of results for
comparison between extraction procedure within
a soil
=
0.06
.
.
•
.
•
•
.
.
.
.
•
.
.
•
35
xiv
Figure
3.4
Page
Amounts of Al in M KCl extracts of unlimed
soils estimated by the oxine , (P);
aluminon , ( Q) ;
titration ,
(R) ;
and
AAS,
LSD (5%) of results for
(S); techniques.
comparison between analytical techniques
within a soil
3.5
=
0.06
.
.
.
.
.
. .
. .
.
37
.
Effect of increasing the solution
concentration of K on the intensity
( absorbance) of colour develope d by the
aluminon reagent
4. 1
39
Effect of soil: solution ratio on the distribution
of (a) total negative .(Ca ads + A 1-�KN03),
(b) negative (Ca ads) , and (c) positive charge
( Cl ads) in the four soils . . .
4.2
50
Effect of soil: solution ratio on the pH
(in the sixth e quilibration) of the soil suspension
4.3
51
Effect of soil pH on the distribution of negative
and positive charge determined in O.O lM CaC12 and
0.03M NaCl ,
(a) negative charge determined in
(b) total negative charge (Na ads +
0.OlM CaCl2 ,
Ca (NH4N03 )) determined in 0.03M NaCl ,
(c) negative charge (Na ads) determined in 0.03M
NaCl ,
(d) positive charge determined in 0.03M
NaCl , and
(e) positive charge determined in
O.OlM CaC1 2
4.4
•
.
.
.
.
.
.
53
.
Effect of increasing amounts of Ca(OH)2
on the pH of four contrasting soils
4. 5
56
Effect of soil pH and added P on the distribution
of negative and positive charge in soils.
(a)
negative charge in P-treated (16.1 mmol P kg- 1
(b) negative charge in untreated
so il) soils ,
(0 mmol P kg- 1 soil) soils ,
(c) positive charge
in P-treated ( 1 6. 1 mmol P kg- 1 soil) soils , and
(d) positive charge in untreated (0 mmol P kg- 1
soil) soils
•
•
•
•
•
.
.
•
•
•
•
58
XV
Page
Figure
5.1
E f fect of increasing pH on isotopically
exchangeable P in 4 soils in cubated with
(
3 rates of added P.
A.
5.2
=
8.1;
•
•
=
0;
16.1 mmol kg-1 soil) .
E f fe c t of increasing pH on Olsen P in 4 soils
(
incubated with 3 rates of added P.
A.
5.3
•
8.1;
e
•
0;
=
16.1 mmol kg-1 'soil) .
69
E f f e c t of in creasing pH on Mehlich P
and pH of the Mehlich extract ( A ) in 4 soils
A.
=
8.1;
•
=
•
(
incubated with 3 rates of added P.
6.1
66
.
0;
16.1 mmol kg-1 soil)
72
E f fe c t of soil pH on the amounts of Al
84
extracted from limed soils by M KCl solution
6.2
E f fe ct of soil pH on the amount of P sorbed
(
) at 2 initial solution P con centrations
e and P2
(P1
=
A. , Table 6.2) and the
con centration o f Ca (- - - ) in the control
samples ( •) and in the presence of added P
(P1
6.3
=
0 and P2= A , Table 6.2)
.
.
.
.
•
•
.
•
.
86
E f fe c t of pH on the amount of P sorbed by
untreated ( A. ) and P-incubated ( e )
Koronivia and Seqaqa soils at initial
solution P concentrations o f 0.69 and
2.15 mmol L-1 , respectively.
6.4
88
Values for pH and Ca and P concentrations from
the section on the pH-P sorption curve where
sorption in creases, plotted a ccording to the
method o f Clark and Peech (1955)
6.5
•
.
.
.
90
Effect of background electrolyte con centration
on the amount o f P sorbed by Nadroloulou soils
inc ubated with KOH at an initial solution P
concentration o f 1.61 mmol L-1
•
.
.
.
•
.
.
.
92
xvi
Figure
6.6
Page
Effect of background electrolyte concentration
on the amount of P sorbed by Nadroloulou soils
incubated with Ca (OH) at an initial solution
2
P concentration of 1.61 mmol L-1. (Hz O
• ;
=
0.01M KCl
1M KCl
6.7
.
0.1M
A
•
KCl
93
)
Effect of electrolyte concentration on pH
of the sorption medium in the Nadroloulou
so ils incubated with Ca (OH)z
6.8
95
Effect of increasing the pH by additions of
dilute NaOH to soils previously limed to
pH 6.9 on the amount of P sorbed by
Nadroloulou soil suspended in M KCl
7.1
Effect of increasing additions of Ca (OH)
2
on the pH of soils
.
7.2
96
.
.
.
.
•
.
103
.
Effect of soil pH on the amounts of Al
extracted by M KCl from soils treated with
3 rates of added P (Table 7.1).
( e
7.3
=
P1 ,
A
•
P2 , and
=
P3)
104
Effect of soil pH on the amount of P
extracted by the Olsen reagent from soils
treated with 3 rates of added P (Table 7.1).
(
7.4
P1 ,
.
A
=
P2 ,
and
a
P3)
105
Effect of soil pH on the amount of P extracted
by anion-exchange resin from soils treated
with 3 rates of added P (Table 7.1).
(
7.5
•
=
P1 ,
A
P2 ,
and
•
=
P3)
106
Effect of soil pH and added P (Table 7.1)
on dry matter top yield.
Vertical bars (I)
represent LSD (pH) at 5 , 1 , and 0.1% level
of significance.
e
7.6
=
(
•
P1 ,
A
P2 , and
P3) . . .
108
Effect of soil pH and added P (Table 7.1) on
dry matter root yield.
Vertical bars (I)
represent LSD (pH) at 5 , 1 , and 0.1% level of
significance.
( •
=
P1 ,
A
=
P2 , and
•
P3)
•
•
.
.
111
xvii
Figure
7.7
Page
Effect of soil pH and added P (Table 7. 1 ) on
the concentration of P in plant tops.
Vertical bars (I) represent LSD (pH) at 5, 1,
•
and 0. 1 % level of significance. (
.&
7.8
•
P2, and
=
=
=
P 1,
P3) . . . .
the concentration of P in roots.
113
Vertical
bars (I) represent LSD (pH) at 5, 1 , and 0. 1 %
(
level o f significance.
•
7.9
•
=
.&
P1,
P2 and
1 14
P3)
the concentration of Al in plant tops.
Vertical bars (I) represent LSD (pH) at 5, 1
(
and 0. 1 % level of significance.
.&
7. 1 0
P2, and
=
•
=
a
Pl,
1 15
P3)
Relationship between the concentration o f Al
in plant tops and
(a) dry matter top yield and
(b ) concentration of P in the tops . .
7. 1 1
1 16
Effect of soil pH on the concentration of Al in
roots of plants grown in soils treated with
4.84 mmol P kg- 1 .
Vertical bars (I) represent
LSD (pH ) at 5, 1 , and 0.1 % level of significance
7. 1 2
ll8
Effect o f soil p H and added P (Table 7. 1 ) on
the uptake of Al by plant tops.
Vertical
bars (I) represent LSD (pH ) at 5, 1 , and 0. 1 %
( •
level of significance.
and
7. 1 3
e
=
=
P 1,
.&
P2,
1 19
P3) . .
Effect of soil p H and added P (Table 7. 1 ) on
the uptake of Al by plant roots.
Vertical
b ars (I) represent LSD (pH) at 5, 1 , and 0. 1 %
level of significance (
and
•
=
P3) .
•
.
.
•
•
Pl,
.&
P2,
1 20
xviii
Figure
7. 14
Page
the ratio of P roots to P (roots +tops )
in pl ants.
Vertical bars (I) represent
LSD (pH) at 5, 1, and 0. 1 % level of
significance.
and
8.1
e
=
( •
P 1,
A
=
P2,
P3)
122
Effect of soil pH and added P (4.84 mmol kg- 1 )
on the dry weight of ryegrass tops.
Vertical bars (I) represent L SD (pH) at 5, 1 ,
130
and 0. 1 % level of significance.
8.2
Effect of soil pH and added P (4.84 mmol kg- 1 )
on the concentration of
in ryegrass tops.
(a) Al and (b) P
Vertical bars (I)
represent LSD (pH) at 5, 1, and 0.1% level
13 1
of significance . . .
8.3
Relationship between uptake of P by plants
(Leucaena leucocephala _-and ryegrass) and
amounts of isotopically-exchangeable P (a)
and, Meh1ich- (b),
Bray (II)-
(e),
resin- (c),
Bray (I)-
(d),
Olseh- (f), and Colwell-
extractable P (g)
8.4
132/ 133
Effect of soil pH and added P (4.84 mmol kg- 1)
on the amount of P extracted by the Bray (I) reagent
1 38
xix
LIST OF TABLES
Table
3.1
Page
USDA and Twyford and Wright ( 1 9 6 5 ) classification
of soils used for preliminary analyses .
3.2
. .
.
.
. ·,
22
.
Selective dissolution analyses and mineralogical
23
composition of the soils used
3.4
Amounts of Ca(OH)2 added to the soils during
the preliminary incubation studies
3.5
31
Percentage recovery of Al added to M KCl
extracts by four analytical techniques .
4.1
28
Cumulative amounts of Al removed during
eight seq uential extractions
3.7
25
pH of unlimed and limed soils used in the
standardisation of the M KCl extraction procedure
3.6
20
Some chemical parameters used to characterise
the soil·s
3.3
.
40
Effect of the concentration of prewash electrolytes
on the cumulative amount of Al extracted by the
saturating (CaC12) and extracting (KN0 ) electrolytes
3
and on the negative (Ca ads), total negative
(Ca ads + Al-KN0 3 ) , and positive charge (Cl ads)
determined in O.OlM CaCl2 in a range of unlimed
and limed soils
4.2
48
Correlation and linear regression coefficients
between negative charge (Na ads) and total negative
charge (Na ads + Ca(NH4N0 3)) determined in 0.03M
NaCl and negative charge (Ca ads) determined in
54
0.01M CaCl2 solution . .
4.3
Ratio of Fe: Ti and Fe: Mn in the citrate-dithionitebicarbonate extracts, positive permanent charge
estimated from Ti(IV) and from Ti(IV)+Mn(IV) substitution
in iron oxides and that determined by Cl adsorption
(Cl ads) from 0.01M CaC12 a bove pH 7
•
•
60
Table
5.1
Page
Amount of P extracted by the Mehlich reagent
( pH 1.3) from unlimed and limed and
-l
subsequently phosphate treated (8 . 1 mmol P kg
soil ) Batiri, Koronivia, and Nadroloulou soils .
5.2
74
Amounts of P extracted by the Mehlich and Olsen
reagents from unlimed but phosphate treated
(0, 8 . 1, 16. 1 mmol P kg- 1 soil) soils
6.1
pH values of the soils selected to
investigate the effect of pH on P sorption
6.2
80
Initial solution P concentration in the
background electrolyte during the investigation
of the effect of pH on P sorption
6. 3
76
81
Amounts of crystalline free Fe and Al, and short
range order Fe and A1 extracted by citrate
dithionite-bicarbonate and acid ammonium
oxalate reagents, respectively, and the ratio of
oxalate- to dithionite-extractable A1 and Fe . .
7.1
Amounts of P added to limed soils prior to
incuba t ion and growth of Leucaena leucocephala
8. 1
85
100
Proportion of variation in plant P uptake (R2 )
accounted for by extractable P alone and in
combination with various indices of buffer
capacity, as determined by multiple regression
analyses
•
.
130
CHAPTER 1
1
CHAPTER 1
INTRODUCTION
1. 1
Introduction
Low nutrient status (e . g . , P deficiency) and high concentrations of
certain elements (e . g., Al, Mn) are two of the most important factors
limiting crop production on highly-weathered, acid soils.
Such soils
are widespread in Fiji and occupy large areas of land which could
potentially be used for crop production if the nutrient status could be
improved.
It has been estimated (Twyford and Wright, 1965) that
production on these soils could be doubled by proper
management but
how this could be accomplished was not stated .
Phosphate deficiency results from an inadequate supply of available
P at the soil- root interface (Barber, 1962) .
This is compounded in
acid soils by a high P-sorption capacity which further limits the
diffusion of P in the plant root zone .
Soluble P compounds which are
added to correct P deficiency react with soil minerals by precipitation
(Kittrick and Jackson, 1956;
reactions (Bache, 1964;
1977;
Lindsay et al . , 1962) and/or sorption
Hsu, 1968;
Ryden et al . , 1977a,b);
P declines rapidly .
Syers et al. , 1971;
Ryden and Syers,
and the plant availability of the added
Also, high levels of exchangeable and soluble Al,
Mn, and Fe further limit plant P uptake by restricting root growth and
through chemical interactions with soluble P .
et al . , 1976;
Recent studies (Bolland
Gillman, 1984) showed that a considerable proportion of
negative sites in strongly-acid soils are occupied by Al3+ .
Aluminium
toxicity is often manifested as P deficiency (Foy and Brown, 1964;
Lowe and Bortner, 1973) .
Phosphate deficiency and Al toxicity problems in acid soils have
often been overcome by liming .
Liming acid soils can reduce P sorption
and Al toxicity by decreasing the concentration of soluble Al which
could otherwise interact with fertilizer P;
it also increases the net
cation-exchange capacity o f these soils (Wann and Uehara, 1978) .
However, considerable controversy exists in the literature as to the
effect of liming on P sorption .
It has been reported that liming
2
highly- weathered, acid soils can cause an increase (Griffin, 197 1;
Ryan and Smillie, 1 975), decrease (Griffin, 197 1) or not affect (Martini
et al., 1 9 74) the amount of P that can be extracted from acid soils.
Fur thermore, Sumner ( 1979) quoting the work of Amarasiri and Olsen ( 1973)
and Janghorbani e t al.
(1975) suggested that a similar controversy
exists in the literature on the effect of liming on plan t growth.
Par t of the problem appears to result from the diffic ulty of separating
direct and i ndirect effects of liming using e ither only plant data or
only laboratory-based data.
It is importan t to have an understanding
of the chemical processes governing the interac tion of Al and P when
acid soils are limed if increased agricultural production is to be
obtained .
This thesis describes studies designed t o develop a better under
standing of the impor tance of various mechanisms controlling plant
available P levels in selected, highly�eathered, acid soils from Fiji.
The s t udy was divided into two sections.
(i)
Section 1 was designed to
inve s t igate the effect of liming on the chemistry of selected acid soils
and (ii) examine how changes in pH, as influenced by liming, affect the
P-sorption capacity and the extractability of added P from these soils .
Sec tion 2 deals with glasshouse s tudies in which the interaction between
lime, Al , and P was examined in relation to plant growth, and the extent
to which this interac tion affec ts the availability of P as assessed by
plant uptake and a range of soil-tes ting procedures.
CHAPTER 2
3
CHAPTER 2
REVIEW OF LITERATURE
2.1
Introduction
The interaction between P and soil components and the influence that
this has on the concentration of P in the soil solution has been studied
e xtensively .
Several reviews relating to this subject have been published.
The more recent reviews include those by Parfitt ( 1 97�� Hingston (1981),
and White ( 1 98 1 ) .
These reviews have dealt with the interaction between
P and soil c omponents (e.g., gibbsite, goethite, calcite, clay minerals,
short -range order material, etc.) in considerable detail.
Because this thesis is concerned with lime -Al- P interactions, this
literature review will emphasise : (i) the mechanisms of P retention,
3
(ii) the chemistry of Al +, and (iii) lime-Al -P interactions.
The term retention is used to indicate both precipitation and
sorption processes.
2.2
Mechanism of Phosphate Retention
Retention of P by soils is characterised by an initial rapid reaction
between the substrate and solution P, followed by a slower reaction which
continues for a long time without reaching any true equilibrium (Barrow
&
Shaw, 1 975) .
The mechanisms involved in the retention of P have been
under investigation for many years.
Two different hypotheses have been
proposed, namely precipitation and sorption.
It is now generally
believed that P can be retained by either of these processes.
Chemical precipitation and sorption processes are similar in many
respects, the major difference being that sorption is a 2-dimensional
process whereas precipitation is 3 dimensional (Corey, 198 1).
generally occurs on
Sorption
surfaces of minerals that are themselves precipitates.
If an adsorbing species is identical to one of the component mineral ions,
sorption w111 contribute to crystal growth, making the sorption reaction an
4
integral part of the precipitation process (Bache, 1964).
Thus, sorption
and chemical precipitation theories were generally considered together by
many investigators in the 1 960's (e . g., Hsu and Rennie, 1962a) .
The chemical precipitation theory involves the removal of 2 or more
components from a solution by their mutual combination to form a new solidphase compound (Bache, 1964) .
This process can also involve the
dissolution of cations from the solid phase (Cole and Jac kson, 1950;
Kittrick and Jackson, 1956) and the subsequent stoichiometric precipitation
of these cations with P ions present in the soil solution .
Chemical precipitation processes are governed by the solubility
product principle, which for a solid electrolyte MaXb, in equilibrium with
its saturated solution at constant temperature and pressure is:
aq
3. 1
+
3.2
The most common cations responsible for the precipitation of P are
Fe3+ and Al3+ in acid solutions and ca2+ and Mg 2+ in neutral and alkaline
solutions .
All of these cations can form a range of compounds with P
which differ in their solubilities and are sensitive to the pH of the
system .
The precipitation of P compounds is not possible until the
ionic product of the ions exceeds the solubility product and therefore
in soils it occurs only under conditions of high P concentration .
The
formation of Al-P compounds by precipitation has been reported by
Kittrick and Jackson ( 1 956), Hsu and Rennie ( 1 962b) and more recently by
Van Riemsdijk et al . ( 1975) .
Van Riemsdijk et al . ( 1975) reported the
synthesis of a new P mineral when P ions were reacted with X-ray amorphous
Al hydroxide .
The formation of Ca-P through precipitation processes has
been reported by many investigators (e . g . Cole et al . , 1 953;
1 97 1;
Holford and Mattingly, 1975;
Kamprath,
Sanchez and Uehara, 1980) .
The retention of P through sorption processes involves the concentration
of P ions from solution at the surfaces of a solid phase in forms that are
exchangeable or replaceable (Kittrick and Jackson, 1 956).
is much older than the chemical precipitation theory .
Sorption theory
However, the
difficulty in explaining the observed decrease in exchangeability or
availability of P with time using simple sorption models and direct
observations of the role of P in crystal formation (Parfitt, 1978), led to
5
the development of the chemical precipitation theory.
Several workers
have attempted to explain the observed decrease in exchangeability with
t ime by postulating diffusion of P into soil particles (Cooke, 1966;
Evans and Syers, 1971;
Tambe and Savant, 1978).
More recently,
Barrow ( 1983b) proposed a model for describing the sorption and desorption
of P by soils and this model is believed to explain the decrease in
exchangeability with time.
(i)
The model includes 3 components:
the reaction between divalent P ions and soil constituents,
(ii)
the assumption that there is a range of values of surface properties,
and (iii)
the assumption that the initial sorption induces a diffusion
gradient towards the interior of the particle which begins a solid-state
diffusion process.
The important difference between this model and
other models involving diffusion (Cooke, 1966;
Evans and Syers, 1971;
Tambe and Savant, 1978) is that it attempts to take the change in surface
charge into account (Bolan, 1983).
Sorption processes can be separated broadly into 2 groJps, depending
on the affinity of the adsorbent for the adsorbates.
When anions are
bound by electrostatic forces, simply to balance the positive charge on
oxide surfaces, the process is classed as 'non-specific sorption' and
the magnitude of non-specific sorption equals the positive charge on the
surface.
Non-specifically sorbed ions include Cl , N0 3- , Clo -, etc.,
4
and are easily replaced by ions from neutral, monovalent salts, such as
NaCl.
In the second type of sorption, anions are bound tightly to the
adsorbent surface.
This process, termed specific sorption usually leads
to a net change in the surface charge (Hingston et al., 1967).
Specific
sorption ma y occur whether the surface is positive, neutral, or negative
(Bowden et al., 1980b).
332P0 4 , As0 3 , and Mo0 4
2.3
Ions which can be specifically sorbed include
Effect of Liming on Aluminium and
Phosphate Chemistry of Acid Soils
This section
(i)
is subdivided into 3 parts:
the solution chemistry of pure Al systems and its implication
to soils
(ii)
t he origin of exchangeable Al and its interaction with added P,
and
(iii)
the interaction between Al and P when soil is limed .
6
2.3.1
Solution chemistry of pure Aluminium systems
and its implication to soils
The solution chemistry of Al has been studied by numerous workers
(Magistad, 1925;
Smith, 1971;
Nair and Prenzel, 1978) who have reported
that Al is soluble at both low and high pH.
Magistad (1925) reported
that at pH 4 and below, A1 is present as alumino hydronium ions;
Al(H2 0) 6 3+·
As the solution pH increases (Fig. 2.1), the alumino
hydronium ions continually dissociate H + ions, leaving C)fC ions in place of
the OH groups.
According to McLean (1976), such a dissociation step
2
can be represented as follows:
2
Al (OH) +
+
H+
----l
Al(OH) +
2
+
H+
aq
___)
r--
+
H+
+
aq
�
Al(OH)
3
�
Al(OH) 4
+
H+
+
aq
�
�
Al(OH) 2
+
5
aq
�
�
2+
S
3Al(OH)
+
6
3
Al +
+
aq
2
Al(OH) +
+
aq
Al(OH) +
2
+
Al (OH)
3
Al'(OH)4
�
�
�
Al(OH)
H+
H+
3+
ions hydrolyse
As the pH of the solution increases, monomeric Al
to form a series of soluble Al-hydroxy compounds up to a pH of about 5;
between pH 5.0 and pH 7.5, insoluble A1 hydroxide is formed, above which,
soluble aluminate complexes (e.g., Al(OH) 4) are formed.
Recent studies by Rengasamy and Oades (1978; 1979) indicate that
hydroxy A1 polymers can form with increasing pH and these can vary in size
and chemical properties depending on the OH/Al ratio.
polymers can be formed;
when the OH/ A1 ratio is
<
Two
groups of
2. 0, most of the polymers
formed have a molecular weight< 50,000, but when the OH/Al is> 2.0, the
molecular weight is >100,000.
The smaller polymers are highly charged and
3
apparently possess a chain-like structure.
In the presence of Fe +,
3
copolycations can also form by bridging of A1 + and Fe3+
through hydroxyls
as follows:
3.3
7
(::' 11-1
u
-
�
-
-�
�
,
0
"'=""
et
)I>
�
+
�
...
"
...
c:
CD
V
c:
0
0
...
3-7
+
fWL
c
0
Fig. 2. 1
4
6
Solution pH
8
10
Solubility of aluminium in water solution
as affected by pH of solution (McLean,l976).
8
Aluminium can also eo-polymerize :at pH < 5 with Si (F�rmer et al,, 1979)
to form a colloidally stable, poorly-organized compound.
Such a compound
was named proto -imogolite as its structure and composition is close to
that of imogolite.
The course of the hydrolytic reactions discussed above and the nature
of the products formed depends on the rate of addition of base, temperature,
and the concentration of Al (Smith, 1971;
Farmer et al. , 1979).
Thus,
most of the work which has been conducted using pure systems has involved
-1
Al concentrations exceeding 1.0 mol L , whereas in soil systems and soil
-1
extracts the concentration is usually less than 0.001 mol L
Based on
these comparisons it would seem that polymerisation occurs less readily
in d ilute solutions.
According to Bache and Sharp (1976a) it would not
be easy to partition Al into monomeric and polymeric species because of
the extreme reactivity of the former species.
The implication from the studies conducted in pure systems is that,
in soils, Al can be present in a variety of forms which can range from
simple monomeric species to polymeric species of varying molecular weight,
either in combination with Fe or Si or both, depending on the pH of the
Such polymeric hydroxy species may also form when soils rich in
soil.
exchangeable A1 and Fe are limed (Amarasiri and Olsen, 1973).
2.3.2
Origin of exchang eable aluminium and its
interaction with added phosphate
In soils, Al is generally released from octahedral co-ordination in
minerals by weathering processes (McLean, 1976), the rate and extent of the
+
process being dependent on H ion concentration.
As the pH of the solution
3+
decreases to 4 or below, A1
ions dissolve from the edges of mineral
Upon release, these ions
structures (Jackson, 1963).
3+
to form Al(H o)
in solution and with an increase in
2 6
subsequent dissociation of the Al(H o) 3+ into various
2 6
in Section 2.3.1.
A large proportion of the released
combine with water
soil pH, there is a
species, as discussed
Al ions in acid
soils is sorbed on permanent cation-exchange sites (Brown and Newmann,
1973;
McLean, 1976;
Gillman, 1984) from which they can be removed by
unbufferred, neutral salt solutions (McLean, 1965).
Aluminium removed by
this method has been considered exchangeable (McLean, 1965).
However,
recent studies of the d isplacement of Al by neutral salt solutions have
shown that Al continues to be released from acid soils with successive salt
9
extractions and in amounts that vary considerably with the time of
extraction (Amedee and Peech, 1976;
1985).
Bache and Sharp, 1976a;
Lee et al.,
Such a continued release of Al is thought to be caused by a
gradual release of non-exchangeable forms of Al from polymeric hydroxy-A!
species, which may be strongly sorbed on clay surfaces (Bache and Sharp,
1976a) or present in interlayer positions in vermiculite (Lee et al.,
1985).
Slow release of Al can also result from dissociation of Al
organic complexes (Bloom et al., 1979).
In view of the possible release
of non-exchangeable Al in the salt extracts of soils, many investigators
(Amedee and Peech, 1976;
Bache and Sharp, 1976a,b;
Grove et al., 1982;
Lee et al., 1985) have suggested that it would be difficult to define a
precise fraction of exchangeable Al in soils.
A number of workers have shown that the P-sorption capacity of soils
could be related to exchangeable Al content.
For example, Syers et al.
(1971) reported that for a range of Brazilian soils, P-sorption capacity
was related to exchangeable Al content.
This supports the work of
Beckwith (1965) who showed that after leaching the exchangeable Al from
the subsoil of an acid, dark clay from Queensland, there was a large
decrease in P-sorption capacity.
In a series of papers, Muljadi et al.
(1966a,b,c) showed that if clay minerals have exchangeable Al 3+ or hydroxy
A! polymers as part of their exchangeable cations, P can be retained by
these forms of Al.
Although P-sorption capacity has been related to the
exchangeable Al content of soils, the results of such studies may not
be entirely reflective of the interaction between exchangeable Al and
added P.
This is because studies which involved removal of exchangeable
Al , for example by leaching (Beckwith, 1965), could result in a considerable
increase in the pH of the system (Bache and Sharp, 1976a) and therefore
the decrease in P-sorption capacity of the leached soils could be a pH effect
rather than due to removal of exchangeable Al, � se.
3
Cation-exchange resins saturated with Al + have been used to infer the
mechanism of the interaction between exchangeable Al and P.
Using an Al
saturated resin, Wild (1953) showed that "fixation" of P became larger as
the pH was increased and the Al hydrolysed more completely.
However, he
emphasised that the "fixation" of P depended on several factors, in
particular, on the type of reagent (NaOH, Ca(OH) ) used to increase the pH
2
and also on whether P was added together with the base or before increasing
the resin pH.
Thus, in the case of monovalent electrolytes, such as NaOH,
sorption of P was found to decrease consistently above pH 5.0.
Hsu and
10
Rennie (1962b) reacted P wl.th a cation-exchange resin saturated with
3
A1 + and presented e vidence that precipitation was taking pl ace even though
P retention conformed to Langmuir and Freundlich sorption isotherms.
Al though in the above discussions evidence is presented to show the
interaction between exchangeable A1 and solution P in both soil systems
and with Al-saturated resins, it has been suggested that in many acid soils
X-ray amorphous Al hydroxide or Al-hydroxy polymers attached to clay or
organic matter are responsible for most of the P sorption of the soils
(Amarasiri and Olsen, 1973 ;
Sims and El lis, 1983).
Using a cation
exchange resin, Robarge and Corey (1979) showed that the Al -hydroxy
species formed during neutralization of Al }+ provided the primary Psorbing surface.
As percentage neutralization of exchangeable A1 increased,
at constant total P concentrations, P in sol ution was found to initial l y
They suggested
}
that Al - hydroxy species formed during neutralization of Al + could be
decrease, pass through a minimum, and then increase.
This study has important implications to the
sorbed on clay surfaces.
interaction of A1 and P during the liming of soils.
2.3.3
Lime-a luminium-phosphate
interactions in acid soils
Al uminium toxicity and acidity problems in soils are usua l l y ame liorated
by liming.
Liming of acid soils increases the pH, thereby binding active
3
Al + ions in the form of Al -hydroxy compounds .
As discussed in Section
2.3.2., an increase in s oil pH results in the formation of a range of Al
hydroxy compounds, the solubilities of which are dependent on pH.
In
s oils, of pH > 4.5, Al-hydroxy polymers are dominant, the monomeric Al OH2+
arid
Al(OH)2�
tending to polymerise to species such as A16(0H)153+
(Hsu and Rich, 1960).
However, on clay surfaces there can be further
polymerization until "gibbsite islands" are formed (Rich, 1968).
of such "gibbsite isl ands" would be expected to occur at pH
>
Formation
5.0, and more
commonl y in interl ayered minerals, such as vermicu lite.
There are two interacting factors which influence the effect of pH
on the s orption of P by a substrate (Bowden et al., 1980b;
Haynes, 1982).
Firstly, as the pH increases, the surface becomes increasingly negative,
thereby resulting in a greater electrostatic repulsion and a decrease in
electrostatic potential.
at pH 2 and 7 .
Secondly, the pK1 and pK2 of the H3 P04 occur
Thus, as the pH increases from 2 to 7, the concentration
of HP04 2- increases 10-fold for each unit increase in pH.
This increase
11
in the concen tration of the divalent ion partially offsets the decrease
in electrostatic potential.
Thus, for pure systems , including goethite
(Bowden et al., 1 980g � b; Fig. 2.2a) and X- ray amorphous hydroxy-A!
polymers (Kwong et al., 1 978;
Fig. 2.2b ), P sorp tion would be expected to
decrease relatively slowly due to an increase in surface negative charge
until the pK2 of H Po is reached at 7.
Ab ove pH 7 the increase in
3 4
concentration of divalent ion slows to zero, whereas the decrease in surface
potential continues, hence sorption decreases mo re rapidly.
Although P sorption has been found to consis tently decrease with
increasing pH of pure systems, the effec t of liming soils on P-sorption
characteristics , as well as on the amoun t of extrac t able P, is less
consistent.
For example, Murrmann and Peech (1969) reported that as the
pH of acid soils was increased, the concentra tion of P in solution initially
decreased, passed through a minimum and then increased (Fig. 2.3 ) .
These
workers related the solubility minimum of P to the solubility minimum of
Al -hydroxy species.
They suggested that when limed soils were incubated ,
P was occluded during the forma tion of Al-hydroxy polymers in the pH
< 5.5 - 6.5.
range
As the pH increased above 6.5 to 7, the Al-hydroxy
species became increasingly soluble as the nega tively charged aluminate
formed, and previously sorbed P was released.
(Amarasiri and Olsen, 1973;
However, numerous workers
SoltanpQur e t al. , 1974 ;
Haynes and Ludecke,
1 98 1) h ave shown that , irrespective of whether P was added simultaneously
or after lime application , similar results were obtained.
This suggests
that an occlusion mechanism probably does not operate.
Recen t s tudies by Sorn- srivichai et al. (198 4 ) sugges t that some of
the inconsistencies reported on the amount of P extracted from limed soils
may be explained by careful consideration of the experimental parameters
involved and also the soil- testing procedure used.
Their studies showed
that the observed decrease in Olsen P with liming was possibly due to an
artefact in the Olsen procedure.
A survey of literature shows that whenever liming caused an increase
in sorption of P , highly-weathered acid soils were used (Lucas and Blue ,
1972;
Amarasiri and Olsen , 1973;
Mokwunye , 1 975 ) for s tudy.
Because
these soils contained high amounts of exchangeable Al, Amarasiri and Olsen
( 1 973) and Mokwunye (1975 ) suggested that increased sorpt ion when the soils
were limed was caused by the formation of active , X-ray amorphous Al
hydroxy polymers.
This hypothesis seems to be consisten t with the results
12
1·0
...
•
A.
0
E
1...
o-5
0·1
Fig.2. 2a
.-2
�
0
•
0
i
A.
0
A.
X
1111
u
�
-
I
C'll !lt
c:
0
5
4
7
•
pH
9
8
10
0
Q.
0
...
A.
Influence of pH on the phosphate sorbed by
goethite at a solution concentration of
0.2 mmol 1 -1 together with the proportion
of phosphate present as the divalent ion
( Bowden et al. , 1980b ) .
4
Fig. 2.2 b
S&Mpenaion pH
5
•
7
8
Retention of phosphate by hydrolytic reaction
products of aluminium formed in systems at
the initial aluminium concentration of
1 . 10 mrnol 1-l and OH/Al molar ratio of 3.0
as a function of time ( Kwong et al. (1978 ) .
13
4
Fig. 2 . 3
5
Soi l
•
pH
7
I
•
Ef fect of pH on the amount o f phosphate in
soil solution and on the amoun t o f labile
phosphate present in an Illinois soil
studied by Murrmann and Peech (1969) ,
(Reproduced from Haynes, 1982).
14
of the s t udy b y Robarge and Co rey (1 979) in which Al-hydroxy polymers were
shown to sorb P more actively than Al3+ .
Sims and Ellis (1983) suggested
that ac tive Al-hydroxy polymers formed during liming of soils could coa t
the surfaces of minerals thereby affecting surface charge charac teristics.
Such a change in surface charge would be expected to influence the P
sorption charac teristics of the soils.
St udies by Haynes and Ludecke (1981) indica te that the observed
increase in sorption with liming was largely an artefac t in the method
employed during the incubation of the soil.
They showed that if limed
soils were dried prior to incubation wi th P, liming decreased subsequen t
P sorp tion at any given equilibrium solu tion P concentration.
If the
soil was incubated with lime and the moist soil equilibra ted with P,
withou t intervening drying, the liming increased the P-sorption capaci ty
of the soil.
Based on these results it would appear that drying, occurring
under field conditions, can be impor tan t in relation to the effect of liming
on P availabilit y in acid soils.
By determining oxalate-ex tractable Al,
these workers suggested that the in tervening drying process increased the
ordering of X-ray amorphous Al-polyhydroxy species, thereby decreasing
net surface area and reac t ivity.
However, there is evidence in the
li tera t ure in favour of the stability of Al-hydroxy species on clays
(Brown and Newman, 1973) and therefore the drying process would have to
be extremely severe and prolonged for the activities of surfaces to
decrease t o the point where their capacity to sorb P is less than that of
the soils before liming (White, 1983).
Indeed, Westfall et al. (1973)
showed that only severe we t t ing and drying cycles had any significant
effect on the amount of extrac table A1 in soils.
Furthermore, Amarasiri
and Olsen ( 1973) and Friesen et al. ( 1 980a) dried and rewet ted their
soils several t imes following liming, yet obtained a decrease in extractable
P.
Working with a range of limed soils, Barrow (1984) concluded that many
of the inconsistencies reported in lime-P interaction studies may be
explained by consideration of 4 fac tors namely, the pH range over which
sorp tion is measured, the background electrolyte, the amoun t of desorbable
P in the soil, and the relationship between the pH and the poten tial on the
The soils used by Barrow ( 1984) were incubated with lime
soil surface.
°
a t 60 C to accelerate the reaction bet ween lime and soil components.
Such a high temperature may have had a significan t effec t on o ther physical
and chemical characteris t ics of the so ils such that there was some influence
on the resu lts of the sorp tion s tudy.
15
Although liroing of acid s oils has been shown to have a variable effect
on P-sorption capacity and the extractabili ty of P, it has generally been
reported to increase plant grow th up to pH 6 (Kamprath, 197 1 ).
This
positive growth response to lime h as commonly been at tribu ted to
a me lioration of Al-toxicity and/or increased P availability (Sanchez
and Uehara, 1 980).
Large applications of lime which increase the pH
values above 6 have been observed to cause a depression in plant growth
(e.g., Kamprath, 1971).
From the review of Kamprath (197 1), 3 possible
reasons can be listed for this decline in plan t growth at high pH values.
These are :
(i)
the t endency of liming to promote formation of smaller
aggregates and thus reduce the ra te of infiltration, making the soils
more susceptible to erosion in the field,
deficiencies (e.g., B , Mn , Zn), and
(iii)
(ii)
micronu trien t
P deficiency in some soils
could be induced by fo rmation of insoluble calcium phosphate (Ca-P)
compounds.
However, Sumner ( 1 979) recently related yield depressions at high pH
values to P deficiency and the initial exchangeable Al con ten t of soils.
He suggest ed that the level of exchangeable A1 at a given
pH reflec ted
the reac tivity of the aluminous surfaces which, in turn, governed the
solubilit y of P.
From his studies and those repor ted earlier he showed
-1
that when soils with exchangeable Al ranging from 1.4 to 10 mmol kg
were limed, there was a depression in plant growth at high pH values.
1
This con tras ted with s oils which contained 1 to 1.4 mmol Al kg- ;
these
soils showed a consisten t increase in plant growth even when limed to
high pH values.
In s ub sequent s t udies,Farina et al. (1980a,b) related
the depression in the growth of corn at high pH values to the presence
of a high concentration of A1 in the corn leaf .
It is evident from this brief review that there is some confusion in
the area o f lime-P interactions in soils.
The effec ts of liming on the
amounts of soil extrac table P are no t consistent, neither are the
explanations sugges ted by the inves tigators to explain s uch effects.
MO reover, there is s till a dilemma as to the actual effec t of liming on
the P-sorp t ion charac t eristics of highly�eathered, acid soils.
Although Sumner (1 979 } provided evidence for the impor tance of
exchangeable Al levels with regard to solubility of P during liming, the
work of Sol tanpour et al. ( 1974) and Haynes and Ludecke ( 1981) shows that
the method adopted during liming and P incubation of the soils is also
an important criterion de terming the amounts of solu tion P.
16
2 .4
Effects of Excess Soil Aluminium and
Low Phosphate on Plant Growth
When released by weathering processes, Al undergoes hydrolysis with
a resultant increase in soil acidity through the release of protons
(Jackson, 1967 ) .
In addition to being a source of soil acidity, Al is
a toxic element, particularly to leguminous plants grown in acid soils .
Toxic quantities of Al limit root growth and thereby reduce the uptake
of water and nutrients, thus decreasing plant growth (Fay et al . , 1978)
Levels of Al toxic to growth vary with plant species (Foy and Brown, 1964) .
It has
also been demonstrated, that in terms of molar activities, the
amoun t of Al toxic to plant growth is reasonably similar for different
soils (Adams and Lund, 1 966) .
The toxic effects of excess Al on the
growth of plants have been discussed by Fay ( 1 971 ) , Fay et al. (1978 ) ,
and Helyar, ( 1 978) .
Uptake of aluminium by plants
2 .4 . 1
Huett and Menary ( 1 979) reported that the uptake of Al by plants was
kinetically controlled and dependent on plant species .
uptake occurred in two steps;
They showed that
(i) a rapid initial phase which was more
p ronoun ced and more extensive for cabbage and lettuce than for Kikuyu
grass, and
(ii) a second phase which represented a linear (steady
state) uptake for cabbage and a slightly curvilinear uptake for lettuce .
Ac cording to Jac kson (1967 ) , Al taken up by roots is largely
precipitated in the root-free space , especially in the epidermis.
Clarkson (1967) has shown that much of this Al is exchangeable .
The latter
investigator presented data to show that in the initial stages of uptake ,
most of the Al becomes bound to the sorption sites in the cell wall .
Rasmussen ( 1 968) used electron microprobe techniques to determine the
mode of ent ry of Al and its distribution and localization in corn plants.
He found that Al precipitated on the su rface of the epidermal cells of the
root with no penetration into the c ortex so long as the root surface remained
intact .
The penetration of a lateral root through the endodermis cortex
and epidermis provided a channel of entry for Al into the cortex and conductive
tissues of both the lateral and the main root.
2 . 4. 2
Toxicity symptoms
The symptoms of Al toxicity are not easily identifiable .
Munns (1965a,b)
17
identified the effects of excess Al on lucerne and clover growth .
showed that, in both species,
a
He
concentration of 2(}) )..I M Al reduced root and
shoot weights in about equal proportions .
The observed symptoms were
inhibition of root elongation and lateral formation, roots becoming brown
and thickened, leaves of plants with higher Al becoming bluish green,
and the stems and petioles becoming red, as in P deficiency .
Often
foliar symptoms resemble those of P deficien cy and in clude overall stunted
growth;
small dark green leaves;
leaves and leaf veins;
late maturity;
purpling of stems,
and late yellowing and death of leaf tips .
In some plants, Al toxicity appears as an induced- ea deficiency or a
redu ced ea transport problem, as indic ated by curling or rolling of
young leaves, and collapse of growing points or petioles ( Foy et al . , 1978).
2.5
eon cl us ions
Several conclusions may be drawn from this brief literature review.
Chemical pre cipitation and sorption mechansms have been proposed to explain
the retention of P by soil components .
However, the chemical precipitation
theory probably becomes important only when very large concentrations of
either P or Al are present in the soil solution of acid soils or of P and
ea in weakly a cid soils or soils of neutral pH .
Liming has a significant effect on the charge characteristics of
soils, thereby affecting the interaction between added P and soil components .
Although liming is c ommonly practised to
ameliorate acidity and/ or
Al toxicity problems, it has been observed to have a variable effect on
not only the amount uf P extracted by soil-testing procedures, but also
on the P-sorption characteristics of soils .
Whenever liming was found to cause a depression in plant growth at high
pH values, it was always associated with soils in which exchangeable Al
-l
was higher than 1 . 4 mmol kg
soil.
CHAPTER 3
18
CHAPTER 3
GENERAL MATERIALS AND METHODS
3. 1
Soils
The Fij i archipelago rises from a shal low platform at the Northern end
of a suboceanic ridge extending south to New Zealand.
It has experienced
a large number of volcanic eruptions (Twyford and Wright, 1965) and most o f
the soil s are developed on parent materials which are mainly andesitic in
nature.
Six sampling sites (Fig. 3 . 1), representative o f important crop
production areas in the two major islands, were selected for prel iminary
investigation .
These soils (Table 3.1) represent the Seqaqa soil series
(Seqaqa Research Station), Batiri soil series (Seqaqa Research Station),
Koronivia soil series (Koronivia Research Station), Sote soil series
(Nadruloulou Research Station), Lega Lega soil series (Lega Lega Research
Station), and Drasa soil series (Lololo Pine Station).
Detailed
descriptions of these soils have been presented by Twyford and Wright
( 1 965) and Leslie (1984).
3. 2
Anal ytical �rocedures
Soil samples (0-300 � representative of the soils selected for study
were, in the first instance, subj ected to chemical and physical examination
by standard procedures.
Mechanical analyses to determine the amounts o f sand, silt, and clay
were performed by the sedimentation method (Jackson, 1956) after peroxide
treatment o f the soil.
Soil pH values were measured in water and M KCl
solution using a glass electrode and a 1 : 2 . 5 soil : solution ratio after
equilibration for 1 6 h.
After the initial characterisation of the soils,
all subsequent pH measurements in this study were determined in M KCl unless
stated otherwise.
Saunders (1 965).
Phosphate retention was determined using the method o f
19
FIJI
ISL AN DS
�
rolou lou RS
Fig . 3.1
10
sc a. le 1 :1,500PO"''!IIIIM'Ml
b . M 20 :10 40 50 111 1••
I
I
I
I
I I I
I
o
20
40
eo
eo K 111
Map of the Fiji Islands showing the location
of soils used for the present study.
20
Tab l e 3 . 1
USDA and Twy f o r d and Wr igh t ( 1 9 6 5 ) classifica t ion
o f s o i l s used for prel iminary analyses .
Class i f i ca t ion
Soil
Soil
s e r ies
samp l e
U . S . S o il Taxonomy
Twy ford and Wr i gh t ( 1 9 6 5 )
Koron iv i a
Koronivia
Humo xic Tropohumul t
Red yellow po d zo l i c
Sote
Nadro loulou
Typ i c Humi tro pep t
Humic l a toso l
Batiri
Bat i r i
Oxi c Pal eus tul t
Fer ruginous la t o s o l
S eqaqa
Seqaqa
Typ i c Haplus tox
Humic latosol
Drasa
Drasa
Us t i c Dyst ropep t
Ferruginous l a to so l
Lega Lega
Lega L ega
Aquic Pal eus tul t
Ferruginous l a t o s o l
21
Exchangeab l e cat ions were determined after ext ra c t ion w i th neutral
M ammonium a c et a t e ;
cal c ium ( Ca ) and magnes ium ( Mg ) in th e ex t ra c t wer e
meas ured u s ing a t omic ab sorpt ion spec trophotometry (AAS ) , and sodium ( Na )
and po tass ium ( K) by atomic emission spec tropho tometry (AES ) .
Exchangeb a l e
A l a n d H w e r e extracted f rom t h e s o i l s with M KCl a nd det ermined b y the
aluminon (Hsu , 1 96 3 ) and t i t ra t ion (Yuan , 1 9 59 ) methods , res pec tive ly .
To tal P wa s measured colo rimetrically af ter dry combus t ion o f s o il and
ext ract ion with 0 . 1M H 2 S 0 4 ( Syers et al . , 1 9 68) .
The organic carbon
con tent of the s o ils was det ermined u s ing the d ichroma te method (Black ,
1965) .
The r es ul ts of thes e analyses are presen t ed in Tab l e 3 . 2 .
Mine ralogical analysis o f the clay fraction ( < 2 � ) was performed by
X-ray d i f f ra c t ion .
Quan t i t a t ive est imates of gibb s i t e and kao l inite
were ob ta ined by different ial thermal analy sis .
Thes e resul t s are
pres en t ed in Tab l e 3 . 3 .
3. 3
S e l ec t ive Chemi cal Dis solution Techniques
Acco r d ing to Mit chel � et al ( 1 9 64 ) , po orly-ord ered ino rganic gel
mat er ial is the mo s t react ive component of the s o il sys tem .
A numb er o f
chemical extra c t ion pro c edur es are ava ilab l e for det ermining this ma terial .
In the present inve stiga t io n oxalate (Bl akemore et al . , 1 9 8 1 ) and c i tra te
d i thionit e-b icarbona te (Ja ckson , 1 9 5 6 ) ex tract ions were u s ed to es tima t e
this component o f the s o il .
The r esults o f th ese investig a t ions are
presented in Tab le 3 . 3 .
3. 3. 1
Det ermina t ion o f oxalat e-extrac tab le Tron and Aluminium
Soil samp l es ( < 2 mm ,
100-250 mg) were shaken with 40 mL ammonium
oxa l a t e ( 0 . 30 M , pH 3 . 0 ) in darkness (b ecaus e ammonium oxa l a t e removes
c ry s tal l in e i ro n oxides und er the inf luence of l ight , Schwertmann ( 1 9 64 ) )
°
end-over-end at 20 ± 2 C twice fo r 2 h , cen tr i f uged , and Al and Fe det ermined
us ing AAS with a ni trous oxide-ace tyl ene and a ir-a cetylene f l ame respe c t ively .
3.3.2
Det ermina t io n o f citrate-dithioni t eb i carbona te- ex t ra c t ab l e I ron and Al uminium
40 mL t r i so dium c i t r a t e ( 0 . 30M) and 5 mL sod ium bicarbona te ( 1M) wer e
°
a dded to s o il s amp l es ( 1 0 0 mg ) and the suspens ions heated to 8 0 C on a wat e r
b a th .
S o d ium dithioni t e ( 0 . 5 g ) was added and the suspens ion d ispersed
Tab l e 3 . 2
Some ch emi cal paramet er s us ed to chara c t e r i s e the s o ils .
Ext ra c tab le c a t ions
pH
So il
H20
M �4 0Ac
M KCl
Ca
Mg
Organ ic
ma t t er
M KCl
K
Na
Al
H
p
( 1 . 72 x %C )
mmol kg- 1
reten t ion
%
Ko ronivia
4.6
4.2
5.5
1.6
1.0
1.2
8.7
3.2
3. 3
62
Nadroloulou
4. 8
3.9
11.8
13.4
1 .9
1.9
35 . 6
13. 2
5.6
68
Batiri
4.8
4 .9
4.8
1.3
0.4
0.6
0. 3
0.0
4.2
88
S eqaqa
4.5
4.5
2.4
1.0
1 .9
1.4
9.3
3.0
16 . 6
95
Drasa
5.0
4.0
3. 1
6.2
0.8
1.2
25 . 0
7.2
1.9
68
Lega Lega
4.4
4.3
2 .0
0.5
0.5
0. 5
10 . 6
3.0
1.4
13
N
N
---- - - --------
Select ive d is so l ut ion anal yses and mineralog ical compo s it ion o f the so ils used . .
Tabl e 3 . 3
Dith ionite ext ractab l e
Oxa l a t e ext rac t ab l e
C l a y mineralogy *
Par t ic l e s i z e
So il
Al
Al
Fe
Sand
Fe
S il t
mrnol kg - 1
Clay
G ibb .
%
Kao l .
Verrn . **
% --:
49 . 5
68. 1
193
4 72
40 . 9
43. 3
15.8
0.8
56 . 6
tr.
111.9
62 . 9
288
95 1
24 . 1
31 . 6
44 . 3
0. 5
52. 7
+
Batiri
50. 0
10. 7
820
2399
19.8
37 . 9
42. 3
25 . 0
21 . 1
n. d
S eqaqa
64 3 . 4
94 . 4
1 029
1486
8. 3
49.4
42. 3
4.2
32 . 9
n.d
Drasa
82 . 6
9.7
211
946
13.8
40 . 2
46. 0
<0. 1
86 . 0
n. d
Lega L ega
22. 9
21.4
167
324
86 . 0
5. 0
9.0
Ko ronivia
Nadroloulou
*
no t determined
All minera lo gi cal ana lys es were performed on th e c lay f ra c t ion ( < 2 �m) .
n . d , t r . , and + ind i c a t e no t detected , t ra c e , and s ubord ina te amoun ts o f mineral , respec t ively .
**
gibb .
=
g ibb s i t e ;
kaol .
=
kao l in i t e ;
verm .
=
vermicul i t e .
N
w
24
ul trasonic a l l y , the heat produced b eing suff ic ient to mainta in th e
°
t emp erature a t 80 C (Mitchell et al . , 1 9 7 1 ) .
B o th Fe and Al were determined
us ing AAS as d e scr ibed in sect ion 3 . 3 . 1 .
3.4
Prel iminary L ime Incub a t ion S t udy
Bas ed on these chemical and miner alogical chara c t eris tics (Tables 3 . 2
and 3 . 3 ) , t h e Ko ronivia , Nadroloulou , B a t ir i , and S eqaqa s o i l s were cho s en
f o r detail e d l aboratory and gl asshouse s tudies .
Th es e soils varied
wi dely in c a t ion-exchange capa c i t y , oxal ate- and dithionite-ex t ract ab l e Al
a nd Fe , and M KCl - extrac tab le Al con t ent .
B ecause the obj ec t ives o f this
thesis were t o inves tigate th e chemi c al interac t ion b e tween Al and P , a
p r el iminary l ime s tudy was nec ess ary to inves tigate the in tera c t ion b e tween
these two e l ements in the soil .
The resul t s o f thes e inve s t iga tions were
used later in the planning of a glas shous e trial to a s sess the intera c t ion
b e tween Al and P and the growth o f p l ant s .
3. 4 . 1
Preparat ion of so il sampl e s
Lime a s Ca (OH) 2 wa s added at e i ther 9 o r 1 0 rates to ea ch soil ( Tab l e
3.4) .
The l ime was tho roughly mixed through the soil which was then
mo istened to approxima t ely f i eld capacity and incubated in polythene b a gs ,
±
w i th provis ion for aera t ion , at a t empe ra ture o f 20
0
2 C.
D i s t i l l ed
wa ter was a dd ed , as required , to c ompens a t e for evapo ra tive lo s s .
Af t e r
a period o f 6 weeks t h e soils were a ir dried , p a s sed through a 2 -mm s i eve
and subsampl es were taken for incub a t ion with P .
Pho spha t e was added as
a solut ion o f KH 2P0 4 a t 2 rates ( 8 . 1 a1rl l 6 . l mmo l kg - 1 soil ) and the s o i l s
were b rough t to approxima te fiel d capa c i ty a n d incub a ted a s des crib ed
above for a further 2 weeks .
Af t e r incubat ion , the soil s were air dried
°
at a tempera t ure o f 20 ± 2 C , pas s ed through a 2-mm s i eve , and s tored in
s ealed polyth ene b ags fo r further analys is .
3.5
Ext ra c t ion o f Aluminium f rom So ils us ing M KCl :
A Compa r ison of Me thods
3. 5.1
Int ro duc t ion
During the early s tages of th e overall s t udy i t was d e c ided to ext rac t
exchangeab l e Al from soil s us ing 2 s equential 1 -h extract ions with M KCl and
Table 3 . 4
Amo un ts o f Ca (OH)
2
added t o the soils during the prel iminary in cubation s t ud ies .
Level of Ca (OH) 2 added
Soil
LO
Ll
L2
L3
L4
L5
rruno l kg
-
L6
L7
L8
L9
1
21.6
27 . 0
32 . 4
46.0
54 . 1
64 . 9
81. 1
1 35 . 1
40. 6
46. 0
77. 3
81. 1
99 . 7
127 .6
148 . 5
1 59 . 5
1 89 . 0
0
5.4
21.6
27. 0
32 . 4
54 . 1
59 . 5
94 . 6
135. 1
0
33. 8
36. 5
67.6
73. 0
1 35 . 1
162 . 2
1 75 . 7
229 . 7
Koronivia
0
Nad rolo ulo u
0
Batiri
S eqaqa
4 . 86
N
\J1
26
then to det ermine this Al b y a modi fied a l uminon me tho d (H su , 1 9 6 3 ) .
As the work progr es s ed , howeve r , it became apparent tha t the na ture o f th e
Al ext rac t e d and det ermined b y this procedure wo uld greatly influence the
in terpreta t ion of the resul ts ob ta ined .
Thus i t was decided to c onduct a comparison o f methods for b o th
extract ing an d analys ing for Al so tha t the proc edures used in this thesis
could mo re eas ily be compared with o ther pub l i shed work and also to ens ure
t hat the resul ts obtained were no t the resul t of weakness in the analytical
pro c edure .
E s t imat es o f excha ngeab l e Al are usua lly made by ext ract ing s o i l s with
M KCl s o lut ion ( Co l eman et al . , 1 9 5 9 ) .
However , in recent years , inc reased
interes t in relat ing M KCl - exchangeabl e Al t o the l ime requ iremen t of s o i l s
h a s r es ul t ed i n a considerab l e divers ity i n the range o f extract ion
p ro cedures used .
Fo r exampl e , in the l a s t f ive y ears , exchangebale Al
ha s generally been extrac t ed e i ther by shaking so il s with M KCl solution
for a f ew s e conds (Pra t t and Bair , 1 9 6 1 ) , by overnight s tanding of s o i l s
i n M K C l solution (Yuan , 1 9 5 9 ) , o r by l ea ching soils with M KCl so lut ion
for 2 h ( B l ack , 1 9 6 5 ) .
I t is unl ikely that al l of these metho ds woul d give
a s imilar es t imate of exchangeable Al .
Such a diver s i ty in the ext rac t ion
procedures coul d also be one reason why there has b een inc reas ing d i f f i cu l ty
in quan t i t a t ively rela t ing exchangeab l e Al to p l ant growth s tudies .
Al though some M KCl-exchangeab le Al extract ion procedures have b een
comp ar e d in a numb er o f s tudi e s (McLean e t al . , 1 9 5 8 , 1 9 59 ;
1 96 1 ;
Pra t t and Bair ,
Bache and Sharp , 1 9 7 6 a ) , it is surpris ing tha t there is no publ ish ed
repo rt in which the l ea ching , shakin g , and s tand ing methods have b een
compared in the s ame s tudy .
Such a comparison is importan t , par ticul arly
in ensuring the s el ec t ion of a method which g ives the best es t ima te of
exchangeabl e Al .
Cons i derab l e d iversity a l s o exis ts in the analy t ical techniques which
have b e en used to det ermine Al in the M KCl ext racts of soil s .
Some o f
the c ommonly-us ed method s inc l u de AAS , t i tr a t ion , a n d calo r ime tric
pro c e dures involving aluminon o r oxine .
However , all of these a nal y t ical
t echniques d i f f er in th eir s ensi tivity to Al (Hes s e ,
1 9 7 1 ) and also suf f er
f rom varying degr ees o f in terf erenc e from the pres ence of o ther e l emen t s ,
such a s Ca , and Fe (Hes s e , 1 9 7 1 ) .
I n this s tudy sel ec t ed unl imed and corres ponding l imed s o i l s were
extra c t e d with M KCl us ing f ive different proc edur es and Al in the extra c ts
27
was d e t e rmined b y c a l o r ime t r i c procedures invo lving th e aluminon and oxine
r eagent s , and by t i trime t r ic and AAS techniques .
To inves tigate th e nature o f the l a rg e quanti ties of Al ext ra c ted f rom
the a c i d unl imed so il s , the resul ts above were compared wi th Al - release
c urves ob t ained from a sequen t ial extra c t ion pro c edure .
Metho d s
3. 5 . 2
3. 5 . 2 . 1
Aluminium release c urves
Soil s ampl es (4 g ) were shaken with 20 mL of M KCl fo r 1 h .
They were then c en t r ifuged and f il t ered through a Wh a tman No . 5 4 2 f i l ter
The extra c t ion procedure was r epea ted to give a to tal o f 8 x 1 h
p aper .
ext ra c t ions .
At the conc lus ion of each extrac t ion , the pH o f the soil
suspension was measured prior to centrifug ing and f il tering .
Al though
the s upernatant s o l u t ion f rom each ex trac t ion was always passed through the
original f il t er pap e r , the 8 extract ions w ere analys ed separa t ely , us ing
the oxine metho d .
3. 5 . 2 . 2
Det ermina tion o f M KCl - extrac tab le aluminium :
Ex trac t ion proc edur e s
Al wa s extra c t ed f rom unl imed and selec t ed l imed soils
( Tab l e 3 . 5 ) by 5 d i f ferent ext raction pro c e dures .
Each ext ract was
d iluted to 50 mL us ing M KCl and th e Al content determined us ing 4
d i f f e ren t analy t ical metho d s .
(a)
Al l ext rac t ions were done in dupl ica te .
S hort- term shaking ( P ra t t and Bair , 1 9 6 1 )
S o i l samp l es ( 4 g ) we re shaken by hand with 20 mL o f M KCl
solut ion in 50 mL polythene centr ifuge t ub es fo r 5 s .
The solut ions were
then f il te re d throu gh a Whatman No . 54 2 f i l ter paper .
The whol e ex tra c t ion
p rocess l a s t ed for approximat ely 2 min .
(b )
Long- t e rm shaking
Th is pro cedure was s imil ar to that describ e d in s e c t ion 3 . 5 . 2 . 2 ( a )
above excep t that the sampl es were shaken , end---o ver-end , for 16 h at 2 0
The s amp l es were then c entrifuged a t 3000
.rpm
±
0
2 C.
for 2 min and f i l t ered as
d e s c r ibed above .
(c)
Long- term s tand ing (Yuan , 1 9 5 9 )
S o il samp l es ( 4 g ) were placed i n separa te coni cal fla sks , 2 0 m L
28
Tab l e 3 . 5
pH of unl imed and l imed soil s used in the
s tandar d is a t ion of the M KCl ext r a c t ion pro c edur e .
pH
S o il
Unlimed
L imed
Ko ron ivia
4.2
4.4
Nadro l oulou
4.0
4.5
Batiri
4.9
5.3
S e qaqa
4.5
5. 1
29
M
KCl sol u t ions were added and the s ampl es swirled gently f o r approximately
5 s and a l l owe d to s tand fo r 1 6 h .
The suspens ion wa s then f il tered
through a Wha tman No . 54 2 f il ter paper and the res i due washed onto the
f il t er paper wi th 15 m1 of M KCl .
( d)
L each ing ( B l a ck , 1 9 6 5 )
Each s o i l s amp l e ( 4 g ) was p laced on doub l e Whatman No . 54 2
f il t e r papers and l each e d wi th 4 0
a t a rate o f 1 5 -20
( e)
mL
mL
o f M KCl solu t ion fo r a to tal o f 2 h
h- 1 .
S equent ial ex tract ion
S o il samples (4 g ) were shaken with 20 mL of M KCl for 1 h .
They were then centrifuged and f il tere d as d es c rib e d above .
p ro ce dure was t hen repea t e d .
The extra c t ion
Th e sup erna tant s o l u tion from the s econd
ex tract ion was passed thro ugh the o r ig inal fil t e r paper and A1 was
d e t ermined in the comb ined extra c t .
3. 5 . 2 . 3
Analy t i cal p ro c e dures
3. 5 . 2 . 3 . 1 .
(a)
Ca lorime tric metho ds
An a l i quo t o f the ex t ra c ts (0 . 2 to 3 mL) was
Oxine .
ana lysed for monomeric and total A1 by adap t in g th e mo dified ( Lee et al . ,
1 9 8 5 ) oxine metho d o f B a ch e and Sharp ( 1 9 7 6b ) .
This analys is was done
imme d iately a f t er extra c t ion in an a t t empt to l imi t any prob lems asso c i a t e d
wi th the d e polymer i zat ion o f A1 (Bach e and Sharp , 1 9 76b) .
Th is pro c edure
r equi res s p e c ia l i sed f low c e l l s in th e sp ec tropho tomet er to ensure s t ab l e
readings .
(b)
Aluminon .
Suitab l e aliquo t s ( 0 . 5 to 1 0 mL) of the
ext racts were analysed f o r A1 by the a l uminon me tho d describ ed by Hsu ( 19 6 3) .
°
However , ins t ea d o f hea t ing the s o l u t io ns at 80 C for 30 min to fac i l i ta t e
t h e b reakdown o f Al -organi c and Al-po lyhydroxy comp l exes (Hsu , 1 9 6 3 ) the
s o l u t ions were l ef t overnight fo r colour developmen t .
A comparison o f
t h e heat ing metho d with the overnigh t s tanding t echnique us ing M KCl extra c t s
o f s el ec ted s o i l s showed no dif ferenc e b etween t h e 2 methods .
3.5.2.3.2
Ti tra tion ( Yuan , 1 9 5 9 )
A s u itable al i quo t ( 1 0 t o 2 0 mL ) w a s t i trated with O . O lM NaOH ,
us ing pheno l p thal ein as an indicator , until a p ermanent pink colour was
ob t a ined .
One drop o f 0 . 0 1M HCl was a dded to r emove the pink coloration
and then 5 ml of 4 % NaF were added .
The return o f a p ink col ora tion
i n d i c at ed the p resence of extrac tab l e Al , and th e solution was back t i t ra t ed
30
unt il th e p ink col o ur d isappeared .
A s imilar t i t r a t ion o f M KCl was
used as the r eagent b l ank .
3.5.2.3.3
Atomic abs o rp t ion sp e c t ropho tome try
Al in the origi na l ext ract was d e t ermined by a s t andard AAS
t echnique ( B lakemo re e t al . ,
3 . 5 . 2 .4 .
1 9 8 1 ) us ing a n i t rous oxide-a c e tylene f lame .
Recovery tests
The r e covery of added Al in all procedures excep t oxine was
-1
inves tigated by a d d ing 1 mL of a solution containing 4 � g mL
Al to
suitab le vol umes of M KCl ex t ra c ts of soil ob ta ined us ing the short-term
shaking p ro c edure , des c rib ed above .
Fo r the oxine technique an al iquo t
-1
e quivalen t to 1 . 5 �g mL
Al wa s added to s ui tab l e vol umes of th e M KCl
extra c t s .
The p e r c entage r ecovery o f added Al was cal cul ated f rom th e
d i f f erence b e tween Al in the original M KCl ext ra c t and Al in th e extract
with added Al .
3. 5 . 3
Res ul t s and d i s cus s ion
3. 5 . 3 . 1
Different amounts of Al were r emoved with M KCl in 8-s equen tial
extrac t ions from the 4 unl imed s o ils ( Tab l e 3 . 6 ) .
In general , the larges t
amo unt o f Al was ex t ra c ted f r om the Nadroloulou s o il and th e smal l es t amount
f rom the Bat iri s o il .
The Koronivia and S e qaqa s o i l s released mo d era t e
amo un ts o f A l i n comp arison to t h e o ther s o i l s .
Cont ras t ing Al- r el eas e p a t t erns were a l s o ob tained (Fig . 3 . 2 ) .
General ly the curves showed a gradually decreas ing rel ease o f Al wi th
inc r eas ing extrac t io n numb er .
·
Th is sugges t ed a po s s ible concurrent
dis solut ion of exchangeab l e and non-exchangeab l e Al , rather than the sharp
dec � ease exp e c t e d with a wel l -de f ined exchangeab l e Al f ract ion (Bach e and
Sharp , 1 9 7 6 a ) .
In contras t , S i vasub ramanium and Tal ibudeen ( 1 9 7 2 ) ob t a ined
extrac tion curves cha racterised by a rapid initial rele ase fol lowed by a
s lower l inear releas e .
They sugges t ed tha t this al lowed them to dis t in guish
b etween exchangeab le and non-exchangeab l e Al .
However , th eir resul ts
coul d have b een influenced by the pH o f the M KCl s o l u t ion which was
adj usted to the pH o f the s o i l s uspension (Bache and Sharp , 1 9 76a) .
In
the s tudi es o f B a che and Sharp ( 1 9 7 6a ) , the pH o f the soil s uspens ion
inc reased f rom about 4 to about 5 during p ro gres s ive l ea ching of the s o il s .
In the present s tudy , the pH o f the soil sus p ens ion o f the Nadroloulou ,
31
Tab l e 3.6
Cumul at ive amoun t s o f Al removed dur ing
e igh t s equen t ial extra c t ions .
S o il
Koronivia
Nadroloulou
mmo l kg- 1
Al
9.3
45. 6
B a t iri
Seqaqa
---
0.6
8.2
32
30
-
NA�OLOULOU
10
0
KORONfVIA
I
0'\
�
5·0
0
E
E
<(
·�
a.J 4·0 �
0
QJ
-
--�.,__._____
-§
�
u
CS
'�
X
QJ
I
LJ
�
:1:::
0·2rt BATIRI
l : �
4 5 : ;
1
Fig . 3 . 2
2
3
6
Successive extractions
7
Amoun t s of extrac t ab l e Al (e) rel eas ed and
pH ( 0 ) of the s o il suspension during eigh t
s equent ial ext rac t ions o f the unl imed so i l s
wi th M KCl .
QJ
a.
V)
33
Koronivia , and S eqaqa soils a l so increased (Fig . 3 . 2 ) f rom ab out 4 to
ab out 5 during the ext rac tions , p resumab ly due to pro gress ive removal of
H+ and Al 3+ ions .
On avera ge , 60% o f total extrac table Al was rel eas ed in the first M KCl
extrac t ion of the Nad roloulou and Koronivia soils .
The B a t iri and Seqaqa
so ils , however , showed a mo re g ra dual rel ease wi th only 28 and 30 % ,
respect ive ly of the to tal amo unt o f Al ext racted b eing removed in the f irs t
ext ra c t ion .
The r e la t ively rapid release o f Al from the Nadroloulou and
Ko ronivia s o ils ( Fig . 3 . 2 ) is presumably due to a lowe r soil pH ( Tab le 3 . 5 )
and pos s ib ly to the pres ence o f vermicul i t e (Table 3 . 3 ) .
Aluminium
in terlayers in vermi cul ite have b een repo rted (Wilson , 1 9 7 3 ;
a s a so u r c e of ex t rac tab le Al .
McLean 1 9 7 6 )
The r el a t ively sharp decrease in the amount
o f Al removed during the co urs e o f the ext ract ions is probably a reflection
o f the col l apse o f the in terlayer spacing in vermi cul i t e (Lee e t al . , 1 9 8 5 ) .
Al though b o th of these soils have a s imil a r mineralogy , the reason why the
Nadroloulou soil re l eases l arger amo unts o f Al during the s equent ial
extract ion ( Tab l e 3 . 6 ) is probably due to i t s lower pH and s ignificantly
high e r vermi culite and clay content .
The Al-release curve for the gibb s i t i c Bat iri soil (Fi g . 3 . 2 ) is
s imil a r to that ob t a ined by Amedee and Peech ( 19 76 ) for s imilar soils with
a pH h i gh e r than 4 and wh ich were also domina ted by gibbs ite .
Th e h igh organic mat t er ( 1 6 . 6 % ) and diso rdered alumino s ilicate
( oxal a t e-ex t ractab l e Al , Tab l e 3 . 3 ) conten t o f the S eqaqa soil is probably
r espons ib l e fo r the slow re lease of Al in this soil as a cons i derab le
p roport ion of the Al may be p re s ent a s mono or mul tidentate o rganic
compl exes .
Both or ganic Al complexes and diso rdere d a lumino s i l i cates
( Mi t chell and Farme r , 1 9 6 2 ) have b een ci ted as sources of s lowly rel eased
Al .
B e caus e Al i s general ly cons idered to b e retained s t rongly by organic
mat t e r ( Co l eman and Thomas , 1 9 6 7 ;
B loom et a l . , 1 9 7 9 ; Lee et a l . , 1 9 85)
i t s extr a c t ion f rom an organ ic - rich soil could be exp e c ted to b e slower in
comparison w i th soils contain ing cons i derably l ess o rganic ma t t er .
3.5.3.2
Comparison o f extra c t ion procedures
There was a s ignif i cant (P < 0 . 0 1 ) varia t ion in the amounts o f
Al extra c t e d b y the 5 me tho ds , wi th s imilar trends b eing ob tained fo r b o th
unl imed and l imed s o ils .
B e cause the diff erences in the amounts of Al
extr a c t e d by the 5 d if f erent me thod s were generally s imilar , regardless
of the anl y t ical t echnique used to d e termine Al , only the resul ts fo r
34
unl imed s o i l s using th e o xine methods a r e presented ( Fig . 3 . 3) .
The amounts o f Al relea s e d by these methods ( Fi g . 3 . 3 ) increased in
the o r d e r :
short- term shaking < long- t e rm shaking < long- term s tanding
fol l owed by wash ing < 2 x 1 h shaking < 2 h l each ing .
However , the time
of shaking only b ec ame imp o r tant wh en there was a l a rge amoun t o f
extrac t ab le Al p r es ent i n th e s o il .
Fo r example , in the B a t iri s o il
which was l ow in extra c table Al , th ere was no signi f i cant d i f f erence
b etween the short- and long- t e rm shaking ext rac tion pro c edures , indicating
that the sho rt - t e rm extra c t io n was su f f i c ient to remove the b ul k o f the
ext ra c tab le Al f r om this s o i l .
However , such comparisons c o ul d par t ly
b e in f l uenced by the detec t i on l imits imposed by the analy t i c a l techniques .
Leaching of the soils usually relea s ed 2 to 3 t imes mo re Al than sho rt
term shaking and at l eas t 1 . 5 t imes mo re Al than the overnigh t s tanding
metho d s ( Fi g . 3 . 3 ) .
With the excep t ion o f the l eaching technique , the
c umul a t ive amount of Al removed during 8-sequential ext ra c t ions was always
h i gh e r than th at extracted by the o ther methods (Fig . 3 . 3) .
S urpris ingly ,
the l eaching te chnique ext rac t ed mo re Al than 8-s equen tial ext ra c tions
f rom the Ko ronivia and Nadro l oulou soils (Fig . 3 . 3 ) .
This i l l us t rates
the dras t i c a c tion of the l ea ch ing technique and the l ikeliho o d of i t
removing no n-exchangeab le forms o f Al .
The t rends obs erved for the extra c t ion o f Al ( Fig . 3 . 3 ) are po s s ib ly
exp e c t e d when the mo des o f a c t i on o f the extract ion pro cedures are
con s i dered .
Whi l e the shak ing methods represent a closed sys t em in which
equi l ib rium is es tab lished b etween solut ion and s o i l Al , the l ea ch ing
metho d is an open sys tem in which cont inual addi tion o f M KCl sol ution
dr ives the equi l ibr ium towa r ds further release of Al .
Mo reover , l eaching
of s o il s exposes the ion exchanger to a vir tual ly cons tant concentra tion
o f e l ec t rol y t e ( S ivasub ramani um and Tal ibudeen , 1 9 7 2 ) , in con t ra s t to the
o th e r me tho d s in which the c o n c entration decreases a f ter a p e riod o f
ext r a c tion , thus decreas ing the e f f i c i ency o f extra c tion .
S keen and S umne r
( 1 9 6 7 a , b ) a n d Amedee and P e e ch ( 1 9 76 ) have shown tha t the e f f ic iency o f
the extra c t ion depends on t h e conc entrat ion o f the elec troly t e .
The
long-term s t anding method r e l ea s ed mo re Al than shaking for the s ame perio d ,
p robably b ec ause of the wash ing s tep wh ich fol lowed the former extrac tion .
B ased o nly on the resul ts o f the p resent s tu dy , it wo ul d no t be po s s ib l e
to p ropo s e a method which woul d exclus ively remove exchangeab l e Al f rom the
s o i l s as the M KCl extra c ts would b e con tamina ted by varying amoun ts of
non- exchangeab l e Al .
Never th e l es s , a comparison o f the cumula t ive amount
35
��-
....
10
r-
��-
....
-
()3
02�
0.1 �
0
10
r-
2
S A B C D E
0
r-
r-
""r-
I-
S A B C D E
6
50
r-
,.....
8
6
4
�,--
Fig . 3.3
8
Nadruloulou
Koronivia
Seqaqa
Batiri
40
-�
,.....
rr-
30
f-
4
20
2
10
0
r-
r-
0
S A B C D E
-
S A B C 0 E
Amounts o f Al ex tra c ted f rom unl imed soils by the M KCl
extra c t io n pro c e dures ,
t e rm shaking , (B ) ;
2
short-term s haking , ( A) ;
x
1 h sh ak ing ,
long-
( C ) ; long-term
st an d i ng , ( D ) ; and 2 h l eaching o f s o il s, ( E ) ; r el a t ive to
the cumul a t ive amount removed during 8 s uc c e s s ive
extractio n , ( S ) .
LSD ( 5% ) of r es ul ts for comparison
b e tween ext rac tion p rocedure w i thin a soil
=
0 . 06 .
36
o f Al ext rac ted during 8 - s equent ial extra c t ions with the extrac t ion metho d s
showed that the l ea ching method pos s ib ly remov es s ome non- exchangeab le Al .
The long- term s tand ing metho d would also b e uns ui tab l e be cause o f the
sho r t washing ( i . e . l eaching) s t ep , which fol lows the extrac tion .
The
sho r t - term extra c t ion woul d p robably be ine f f i c i en t fo r soils h i gh in
ext r a c tab le Al .
In view o f these cons iderations , e i ther the l ong- term
shak ing or the 2 x 1 h shaking metho ds should provide reasonab l e e s t ima tes
o f ext rac table Al in the Fij i an soils .
However , for routine anal y s es
the l a t ter method is pref erab le due to the rap i d i ty with which samp l e s
can be analys ed .
3.5.3.3
Anal y t ical techniquec
B e cause the d i f f e rences b e tween the analy tical te chniques were
gene rally s imi l a r , regardl e s s o f the extract ion methods us ed , o nly the
2 x 1 h extra c t ion res ults of the unlimed soils are presented .
res u l ts in Fig . 3 . 4 show that there is a s igni f i cant ( P
in the amoun ts of Al measured by th ese m e tho ds .
<
The
0 . 0 1 ) d i f ference
In general , the aluminon
Al values are s omewhat larger than t i t r a t ion , whereas AAS and oxine
meas urements are in termedia te .
Th es e resul ts conf irm the obs ervations o f
Web b e r ( 1 9 74 ) who de termined Al us ing AAS , oxine ( 8-hydroxy quino l ine ) ,
and aluminon methods in M KCl extracts o f a range o f Cana dian s o i l s .
However , the s i gnif icant d i f f erence ob s e rved b e tween AAS and the ti tration
technique in the present s tudy is no t in agreement with the resul ts o f B ruce
and Lyons ( 1 9 84 ) who reco rded no signi f i c an t d i f fe renc e between th es e t e chniques .
The t i t ra t ion me thod failed to de t e c t Al in the unlimed Batiri s o i l
(Fi g . 3 . 4 ) al tho ugh very d i lute sol utions o f H C l ( O . OlM) and NaOH ( O . O lM)
-1
were used .
This s o il con tained less than 0 . 30 mmol A1 kg
soil .
In a l l
cases , the amoun t o f A1 measured b y the t i tra t ion method was l e s s than that
measured by the o ther techniques
al though there was good agre ement b e tween
the r ep l i cates in ext racts conta ining h i gh l evels of Al .
Th e calo rimetric (oxine and al uminon ) and AAS methods (Fig . 3 . 4 ) d e t e c ted
A1 in all the s ampl es , includ ing thos e o f the Batiri soil .
However , in the
AAS method there was a wi d e variat ion b e tween the replicates , _ parti cularly
tho s e l ow in Al .
A comp a rison o f the AAS technique with the oxine me tho d
ind i ca t e d that th ere was a t l east twic e the va riab i l ity among the r ep l i c a t e s
w i t h the AAS m e tho d (C . V .
=
8 . 3%) than wi th the oxine method ( C . V .
=
3 . 1%) .
Th i s variab i l i ty in AAS measured values may be a t t ributed to the rather l ow
l evels o f Al in the ext ra c t s and th e h i gh sal t concentrat ion which t ended to
37
0·6 r'
B atiri
12
:-g 0-��
Ill
I
Cl
..X
0
E
E
0·4�
r--
..0
0
u
8
r-1"�
2
0·1
P a R s
Fig .
8
4
:: 0·2
)(
UJ
�
10
6
(IJ
-J
J...
10
3.4
Nadruloulou
Koronivia
12
r--
<i (}3 1"
w
X:
Seqaqa
3 0 r'
r--r--
I-
25 �
....--
-
20"
-
�
15 �
4�
10 �
2
5�
6
..
__._....
o -.......
... __..
...
.....
o -.......__._
__._
P Q R S
P Q R S
P a R s
Amo un ts o f Al in M KCl ex tracts o f unl imed soils
es t imated by the oxine , ( P ) ;
t i tr a t ion , (R) ;
al umino n ,
and AAS t echniques ,
(Q) ;
(S) .
o f results for c omparison b e tween anal y t i cal
techniques within a soil
=
0 . 06 .
r--
LSD ( 5 % )
38
clog the b urner , the r eby varying th e solut io n up take ra te .
However , th e
o xine t e chnique gave go od rep roduc ib i lity in soil ex tracts containing b o th
Thi s is probab l y due to the so lvent ex t ra c t ion
l ow an d h i gh l evels o f Al .
s t ep whi ch s eparates the Al from the M KCl ex trac t prio r to development o f
colour using the oxine reagen t .
These resu l t s conf i rm the ob s e rva t ions
of � i en and Gj erd ingen ( 1 9 7 2 ) who repo rted a lmo s t identical variab i l i ty
among the rep l icates in the AAS ( C . V .
8 -hy droxy quinol ine ( C . V .
=
=
8 . 1 % ) method relative to th e
3 . 1 % ) method .
In agr eement with the present
ob s e rva t ions , th es e inv es tiga t o r s a l so found poor reproducibility at l ow
l evel s o f Al .
Moreover , they f o und that at an Al l evel of 1 . 8 5 mmol kg-
l
s o il , it wa s of advan tage to c a r ry out a solvent extra c t ion to conc ent rate
the Al an d to remove KCl , as is done in the oxine technique .
De t e rmination o f Al by the aluminon method was c ompl icated by the
p resence of h igh l evel s of Ca and K .
Interf erence prob lems due to the
pres en c e of Ca have long been known (Fr ink and Peech , 1 9 6 2 ;
Page and
Bingham, 1 9 6 2 ) and i t was ob served that Ca concentrations exceeding 0 . 0 2 M
supp re s s e d o p t i cal dens ity at low l evels o f Al and in tens ified i t a t h i gh
l evels o f Al .
Further s tudy s howed in terference from high levels o f K .
Th e curve in Fig . 3 . 5 shows tha t K conc en trat ions exc eeding 0 . 0 5 M
in tens i f i e d optical density at l ow l evels o f Al and supres sed i t at high
l evel s o f Al .
Turb i ty was a l s o ob s e rved a t h igh l evels o f Al .
These
prob l ems woul d be common in M KCl extracts of unlimed and l imed soils
which , i n add ition t o h igh l evel s of K, a l s o con ta in varying amo unts of Ca .
S u ch p r ob l ems may b e over come by appropriate d ilut ion , use o f a range o f
cal ib rat ion curves ( Skeen and Sumner , 1 9 6 7 a ) , o r by so lvent extract ion
(�ien and Gj erdingen , 1 9 7 2 ) of Al .
Howeve r , dilut ion of M KCl ext ra c t s
o f l imed s o il s , which contain l ow l evel s o f Al , may resul t in loss in
s ens i t iv i t y .
3.5.3.4
Re covery t e s t s
There was reasonab l e agr eemen t ( Table 3 . 7 ) between the amounts
of Al a d ded to the s o i l ext rac t s and the amo unt s recovered , part icularly in
the unl imed s o i l .
extra c t s .
Between 96- 1 00% o f a dd e d Al was re covered f rom the s o i l
However , the p ercentage recovery was variable in the extra c ts
o f th e l imed s o il s .
The highes t amount was recovered by the oxine me tho d
and th i s i llustrates the sens i t ivity o f th is metho d .
The rather l ow
recovery by the aluminon metho d f rom the M KCl extra c t s o f l imed soils may
be due t o interference p rob l ems asso ciated with high l evels o f
Ca
and K .
39
QJ
u
c
21
-a
jg
•
•
<(
•
o
0-74
Fig . 3 . 5
1·48
mmol
Al
distilled H20
O·OSM KCl
0· 1 OM KCl
0·20M KCl
2·22
2·96
E f f e c t of increas ing the solution
concen t ra t ion of K on the int ens ity
( ab so rb ance) of c o lour devel oped by
the a l uminon reagen t .
3·70
40
Tab l e 3 . 7
Percen tage recovery o f A1 added to M KCl
ext rac ts by four analyti cal t e chniques .
pH (M KCl )
Analy t i cal
t e chnique
Recovery
%
Na droloulou
4.0
Ti t ra t ion
96 . 2
( unl imed )
4 .0
AAS
98 . 1
4.0
Al uminon
98 . 1
4.0
Oxine
Ko ronivia
4.4
Titrat ion
96 . 0
( l imed)
4.4
AAS
95 . 2
4.4
Al uminon
92 . 7
4 .4
Oxine
98.5
5.1
T i t ra t ion
94 . 3
5. 1
AAS
94 . 8
5.1
Al uminon
88 . 4
5.1
Oxine
98 . 5
Soil
S eqaqa
( l imed )
1 00 . 0
41
Furth e rmore , s tud ies by H s u ( 1 96 3 ) h ave shown th a t the inten s i ty o f colour
an d the re covery of added Al f rom ext r acts by aluminon can vary with the
Trade Brand o f the a l uminon reagent .
Thus , the oxine me tho d woul d app ear to b e the mo s t sui t ab l e for
det erminat ion of Al in M KCl extrac ts o f bo th unl imed and l imed soils .
However , the o ther me thod s coul d equally wel l be used to measure Al in
M KCl extra c t s of unl imed so il s which contain high l evels of ext rac tab l e Al .
3.5.4
Conclus ions
Signi f ic an t l y d i f f erent amoun t s o f Al a re ex t racte d f rom th e s o i l s by
diffe rent ext ract ion proce du res .
General l y , the amount of Al removed by
M KCl was in the o rder of sho r t - t erm shaking
t erm s tanding fol lowed by washing
<
<
l ong-term shaking
2 x 1 h shaking
<
<
l ong
2 h l ea ch ing .
A reasonab le e s t ima t e of extra c tab l e Al may b e ob tained by shak ing
s o i l s twi c e f o r 1 h wi th
M
KCl .
Th is was dedu ce d by comparing th e amo un ts
o f Al ext r a c t e d by the d i f f erent extract ion p ro c e dures with th e cumula t ive
amo un t releas e d in 8 x 1 h - sequen t ia l extra c t ions .
Signifi can tly d i f feren t amount s o f Al were de tected by the t i t ra t ion ,
AAS , aluminon , and oxine methods .
The ti tra t ion procedure wa s found to
be th e l ea s t s ens i t ive , p a r t icularly for extra c t s low in Al .
The AAS and a l uminon va lues we re influenced by high s a l t concentrat ions
and the p re s en c e o f Ca and K , resp e c t ively .
The resu l ts o f the r e covery t e s t s howed tha t o f the 4 methods tes t ed ,
o xine was the mo s t s ens i t ive analy t i ca l technique for measuring Al in
ex trac ts of l imed and unl imed soi l s .
M
KCl
CHAPTER 4
42
CHAPTER 4
CHARGE CHARACTERI STICS OF ACID
SO ILS AS INFLUENCED BY L IMING
4. 1
Introduct ion
Mo st o f the reac t ions which c on trol nu t r i ent ava ilabil ity in so il s
are d ependent upon phy s iochemical pro c esses that o c cur a t the so il
solut ion-s o il particle interface (Bol t , 1 9 79 ) .
The charge chara c t e r i s t i c s
o f so il componen ts inf luence these processes and a r e thus impo r tant in
soil f ert i l ity.
At p r e s ent there i s very l i t t l e info rma t ion ava ilab l e on the surfac e
charge charac teri st ics o f Fij ian soil s .
The maj ority o f Fij ian so il s
are h ighly wea thered , s t rongly a c idic , and c o nsis t predominantly of
oxid ic and kaol ini t ic minerals .
Thus the s o i l s are predominant ly
var iab l e charge in nature .
Some F ij ian s o il s have been l imed to reduce soil acidity and Al
t oxicity p robl ems , but there is l i t t l e informa t ion on the e f f ec t tha t
l iming has had on the charge charac teristics o f these soils .
However ,
ther e is s uf f ic ient evi d ence f rom o ther s tud i es (Hings ton et a l . , 1 9 6 7 ;
Breeuswma and Lykl ema , 1 9 7 1 ;
S awhney , 1 9 74 ;
Wann and Uehara , 1 9 7 8 )
t o sugg e s t tha t sur f a c e charge d ensity and therefo re the c a t ion-exchange
capa c i ty o f so ils w ith var iab l e c harge may be al tered by l iming and P
addi tion .
Indeed , Wann and Uehara ( 1 9 7 8 ) r epor t ed a s igni f icant
correla t io n between the point of z ero charge (PZC) and the quantity o f
P appl ied t o a n oxiso l .
Al though th e s ur f a c e charge chara c t er i s t ic s o f var iab l e charge soils
have been a s sessed by a number of methods (B el l and Gillman , 1 9 7 8 ) , the
ion-reten t ion method ha s been used mo s t commo nl y .
two s t eps in the d e t ermina t ion of charge .
Th is method invo lves
The f ir s t s t ep cons i s t s o f
t h e r emova l o f ' na t iv e ' exchangeab l e ions by sa tura ting t he s o i l with
an index e l e c tro ly t e under specif i ed condit ions of ionic s t r ength and
the s econd s t ep involves r emoval and analy s i s of t he ions a d sorbed f rom
the index e l e c t rol y t e .
43
Th e s urface cha rge proper ties o f var iab l e charge soils a r e inf luenc ed
by s everal chara c t e r is t ics of the ind ex e l e c tro l y t e , includ ing ionic
spec i es , ionic s t r ength , and pH ( Van Ra ij and Peech , 1 9 7 2 ;
Harward , 1 9 74 ;
B e l l and Gil lman , 1 9 7 8 ;
Wada and
Bowden et al . , 19 80b ) .
Accor d in g to Bell and Gil lman ( 1 9 7 8 ) , the c a t ion and anion o f the s a turat ing
el ectrol y t e shoul d i d ea l ly be the s ame as the r espec t ive dominant ions in
the soil s olution .
From soil solution compos it ion s tudies o f wea thered
soils of t ropical and sub t ropical r egions , it has b een sugge s t ed (Le Roux
and S umner , 19 6 7 ;
G i llman and B el l , 1 9 7 6 ) tha t ei ther NaCl , KCl , CaC1 2
or MgCl 2 c o ul d b e s a t is factory a s sa tura t ing electroly t es .
However , a t
+
+
an ionic s t rength c l o s e t o that o f the so il so lution , both Na and K
would remo ve only part o f the exchangeable Al ( Van Ra ij and P e ech , 1 9 7 2 ;
Ga ll ez e t a l . , 1 9 7 6 ;
B el l and Gil lma n , 1 9 7 8 ) .
Failure to r emove
exchangeab l e Al , esp ecially f rom acid soil s , may resu l t in an under2+
2+
es t imation o f charge .
In view of this , e i ther Ca
or Mg
would a ppear
to b e mo r e suitable s a turat ing e l ec t roly t e s but al though saturat ion with
+
+
3+
2+ , it would s t ill
Ca 2+ would r emove mo re A1
than would Na , K , or Mg
ext rac t onl y part of the exchang eab l e Al a t the rela t ively low ionic
s t reng th o f the so il solution .
In view o f this , many inve s t igators have
opted to r emove exchangeab l e Al as well as o ther exchangeab l e ions b y
mul t ip l e , p r el imina ry washings w i th solu t io ns o f high ionic s t r ength
prior to equilibra t io n with an e l ec t ro lyte of low ionic strength .
Thus ,
Van Ra ij and Peech ( 1 9 7 2 ) removed exchangeab l e ions from s o i l s by f ive
washings wi t h M NaCl prior to s a tur a t ion with the appro pria t e elec t ro ly t e .
Al though r emoval o f exchang eab l e ions , part icularly Al , by p r el iminary
was hing o f so il s has b een cons id ered an int e gral par t o f the charge
mea sur ement proces s , the e f f ec t of this on t he subs equent measurement o f
charge is no t known .
Mo reover , el ectroly t es ranging in con c eri t ra t ion
f rom 0 . 2M (Gallez et al . , 19 7 6 ) to lM ( Van Ra ij and Peech , 1 9 7 2 ) have b een
u s ed for p r e l iminary wa shing prior to satura t ion of soils with solutions
o f low ioni c s t reng t h .
Widely d iffer en t soil : solut ion r a t ios have al so b e en used in the
measurement of cha r g e .
Fo r exampl e , Van Raij and P eech ( 1 9 7 2 ) used
s o il : sol u t io n ra t io s o f 1 : 20 and 1 : 1 0 , whereas Grove et al . ( 19 8 2 ) used a
ra t io o f 1 : 4 and 1 : 5 for th e satur a t ing and extract ing solut ions , resp e c t ivel y .
The sorp t ion o f anions b y soils i s influenced by the so il : s o l u t ion ratio
(Hope and Syers , 1 9 76 ;
B arrow and Shaw, 19 79a )
but the e f f e c t has b e en
a t t ributed s o l el y to a change in the rate o f r ea c t ion .
Because the
44
s o il : s o l ut io n ra t io a f f e c t s the pH o f th e s o i l s usp ens ion through
d ilutio n , a s wel l as c au s ing di ff erences in the amo un t s of H+ and Al 3+
r emoved f r om solut ion , i t woul d s eem l ikely that the ob served e f f e c t
on anion s o r p t ion is d u e to differences i n t h e surface charge o f the
sub s t r a t e , as wel l as to a change in the rate o f reac tion, per s e .
Ther e does n o t appear t o be any pub l i shed info rma tion on the e f f e c t o f
s o il : s ol u t io n ratio o n the surfac e charge charac ter i s t i c s o f soils .
Th is chap t er is d ivided into two sec t ions .
The f irst s e c t ion
d eals wi th the f a c to r s influencing the mea surement of charge and the
obj ec t ives o f this s e c tion .were :
(i )
to d e t ermine the effect o f the concentra t ion o f prewash
el ec t rolyt es on charge in a range of contras t ing soil s ;
( i i)
to evaluate the e f fec ts o f s o il : solu t ion ratio on surface
charge chara c t er i s t ic s ;
( i ii)
to c ompare the e f f ec ts o f the index c a t io ns , Na and Ca , on
the surface char g e charac t er i s t i c s of two soils a t a range
of pH va lues ( ob t a ined by l iming) .
The obj e c t ives o f the s econd s e c t ion o f th is cha pter were to use
a s tandar d is ed method to determine the e f f e c t of pH (as inf luenced by
l imin g ) and added P on the surf ace charge chara c t eristics of four acid
soils from F i j i .
4.2
Ma ter ial s and Methods
4 .2.1
Prepara t io n o f s o i l sampl es
A d e t a i l ed descript ion o f the p repara t ion of the soils has b een g iven
in Cha p t e r 3 (S e c t ion 3 . 4 . 1 ) .
Each o f the so ils was incub a t e d with
e ither 9 or 10 rates of Ca ( OH ) 2 und er the cond i t ions des crib ed in
Chap t e r 3 , for a period of 42 d .
At the end o f the incub a t ion perio d ,
s ub s amples o f the l imed soils were r e in cub a t e d with 2 ra tes ( 0 and
1 6 . 1 mmo l kg- 1 ) of added P (as s o l u t io n KH 2 P04 ) .
Af ter a p er io d of 1 4 d
°
the incub at e d s o il s were air dried a t a tempera ture of 20 ± 2 C and s to red
in s ea l e d po l y thene b ag s for further analys es .
45
4 . 2.2
Ch arge measurement proc edures
E f f e c t o f concen t r a t ion o f p r ewash
4.2.2.1
el e c t rolytes on cha rge sub sequently
d e t e rmined in 0 . 0 1M CaCl 2 so l u t ion
The amoun t s o f nega t ive and po s i t ive charge in an unl imed and
a l imed sampl e of each s o il were determined by mea suring the s o rp t ion o f
+
ca 2 and C l f rom 0 . 0 1M CaC1 2 solution a f ter r emoval of exchang eab l e ions
by ini t ia l wa shing of th e s o i l s wi th ei ther 0 . 5M, 0 . 1M , or 0 . 0 1M CaCl 2
solut ions .
The metho d was es sentially that o f Van Raij and P eech ( 1 9 7 2 )
excep t tha t th e p H and Al 3+ concentra t ion of each equil ibrat ion ext ract
were mea sured .
A 2-g ( < 2
mm )
s amp l e o f s o il was shaken with 4 0 mL of ei ther 0 . 5M,
O . lM , or O . O lM CaCl z solution in weighed cen t rifuge tub es for 1 h ,
c entri fuged ( 1 5 min , 3000 rpm) af ter measuring the pH o f the su sp ens ion ,
and f il tered into numb ered t ub es .
The extra c t ion was rep eated twic e .
Next , the s o i l s which had b e en suspended in 0 . 5M CaCl z were washed by
shaking once with deionized wa t er (5 min) and twi ce ( 30 min) w i th O . O lM
CaC1 2 .
The water wash reduced the concentra t ion o f CaCl z to about O . O lM ,
a s indicated b y the amo un ts o f ca 2+ and Cl in the extra ct .
This method
was d e s i gna ted
The s o il s ini t ia l ly washed wi th O . lM o r O . O lM CaCl z were equilib ra t e d
wi th O . O lM CaCl 2 solution f o r a fur ther 3 times for periods s imil a r to
t ho s e for the 0 . 5M Ca Cl z t r ea t ed soils .
These me thods were d es i gna ted
O . lM CaCl z I 0 . 0 1M CaCl z and O . OlM CaCl z , res p e c t ively .
Af t e r the f inal
equ i l ib r a t ion the tubes were c en tr i fuged , the solutio ns fil tered , and the
t ub es weighed to accoun t for en tra ined solution .
The residues were then
ext r a c t ed e i ther 4 or 5 t imes , depending on the concentrat ions of ca 2+
and Cl
in the f inal ext ra c t , with 20 mL of 0 . 5M KN0 3 .
Ext ra c ts 1 to 4
were comb ined whereas extra c t 5 was kep t separate to determine the amount
of ca 2+ and Cl extrac ted .
The pH o f a l l soil suspensions was determined fol lowing equil ib ra t ion .
Aluminium in al l CaCl 2 wash es wa s determined by AAS us ing a n i t rous-oxid e
a c e ty l ene f lame .
B ecaus e o f the l ow Al concen tra tion and h igh s al t
conc en tra t ion i n the KN0 3 extrac ts , Al in thes e extracts was d e te rmined
us ing the mo d ified aluminon reagen t (Hsu , 1 9 6 3 ) .
The ca 2+ and Cl in
the f inal equil ib ra t ion ext rac t and in O . SM KN0
extracts were d e t ermined
3
by AAS u s ing an air acety l ene f l ame and a t i t r imetric proc edure (Vo gel ,
1 9 6 2 ) , res pec t ively .
46
4 . 2. 2. 2
Ef f ec t o f so il : solut ion r a t io on nega t ive
and pos it ive charge d e t e rmined in 0 . 0 1M CaCl z
Th e surf ace charge o f unl imed s o i l s was measured a t 3 d i f f e rent
s o i 1 : solut ion ratios ( 1 g : 4 0 mL , 4 g : 40 mL , and 8 g : 4 0 mL) us ing the
0 . 0 1M CaCl z method ( S ec t ion 4 . 2 . 2 . 1 ) .
The pH and ca 2+ and Cl
concen trat ions o f the f inal equil ib ra t ion extra c t and the ca 2+ , Cl , and
3+
A1
c oncentra ti ons in the O . SM KN0 3 extra c t s were de termined as desc rib ed
above in S e c t ion 4 . 2 . 2 . 1 .
4.2.2.3
E f f e c t o f index ca t ion in the determina t ion
of nega tive and po s i t ive charge
Th is was examined for l imed Nadrolou1ou and S eqaqa soils by
2+
us ing e i th e r Na+ o r ca
a s the index ca t ion .
The method wa s e s s ent ially
the s ame as the 0 . 0 1M CaC1 2 method des c r ibed in S e c t ion 4 . 2 . 2 . 1 above ,
excep t that exchangeab l e ions were r emoved by washing the s o il s amp l es
twice ( for 1 h) with M NaCl prior to equil ib rat ion with either 0 . 0 1M
CaCl 2 or 0 . 0 3M NaCl s o l u t ion .
Adsorb e d ions were extrac ted 5 t imes with 20
mL
o f O . SM NH 4 N0 3
(Van Raij and Peech , 1 9 7 2 ) in the case o f Na+ and wi th 2 0 mL o f O . SM KN0 3
Nli4 N0 3 was used as the ext ra c t ing
in the case o f Ca satur a t e d s o il s .
+
elec t ro ly t e in the case o f Na be cause it is a b e t ter exchanger ( Tan ,
1 9 82) for Na+ than is K+ .
The Na+ in the f inal equi l ib ra t ion solutions
and O . SM � N0 3 extrac t s was det ermined us ing a tomic emi ssion s pec tro
pho tomet ry and Ca 2+ and C l in the O . SM KN0 3 and NH4 No 3 ext rac t s were
det ermined as des crib ed in S ec t ion 4 . 2 . 2 . 1 .
The concentra t ion of Al 3+
in O . SM NH4 No 3 and O . SM KN0 3 ext rac ts was d e t e rmined as in S ec t ion 4 . 2 . 2 . 1 .
4 . 2. 3
Effect o f s o il pH and added P on n ega t ive and
po s i t ive cha rge det ermined in O . O lM CaCl 2
Surf a c e charge in l imed and s e l ec t ed l imed and s ub s equen t l y P - treated
( 1 6 . 1 mmol P kg- 1 s o i l ) soils was determined u s ing the O . O lM CaC1 2 method
+
( S ec t ion 4 . 2 . 2 . 1 ) .
The ca 2 and Cl in the f inal equil ibrat ion ext ract
+
and ca 2 ,
Cl , and A1 3+ in the O . SM KN0 3 ext r a c t s were determined as in
S e c t ion 4 . 2 . 2 . 1 .
47
4.3
Res ul t s and Dis cus s ion
Charge measurement proc edures
4.3.1
4 . 3. 1 . 1
Effec t o f conc entra t ion of prewash elec troly t es
The wa sh ing of unl imed soils with electro l yt es o f d i f f er ent
concen trat ions ha d a variab l e e f f e c t ( Tab le 4 . 1 ) on charge sub s eq uently
determined in O . O lM CaC1 2 .
In general , the magnitude of b o th n ega t ive and po s i tive charge
inc reased with the conc entra t ion of th e prewash elec tro lyte .
However ,
the incr eas e in nega t ive charge was dependent on pH and the M KCl-ex tra c table
Al cont ent o f the s o ils ( Tab l e 4 . 1 ) .
Fo r exampl e , the Ba t ir i and
Nadro l o ulou s o il s had pH values of 4 . 9 and 3 . 9 , r es p e c t ively and contained
-1
0 . 3 and 3 5 . 6 mmo l kg
s o i l of M KCl - extrac tab le Al , respec tively .
Mea sured nega t ive charge varied from 1 . 9 to 2 . 4 and f rom 1 6 . 0 to 2 2 . 7
�mol ( p ) kg- 1 , respectively , d ep ending on the concen tra t ion o f the prewash
elec t ro l yt e .
Moreove r , in the l imed s o i l s ( Tab le 4 . 1 ) , where M KCl-
ext rac tab l e Al wa s r educed to zero , the r e was only a small effect of the
concentra t ion of prewash el e c trolyte on negat ive charge .
Th es e resul t s
sugg e s t t h a t t h e varia t ion in nega t iv e charge i s rela ted t o the amo unt o f
Al extra c t ed during the prewashing s tep .
The amount o f Al extrac ted
increased with the concen t r a t ion of the prewash elec trolyte and this was
a s so c iated with a correspond ing d e c r ea s e in O . SM KN03 - extrac tab l e Al .
Neverth el es s ,
th e cumul a t iv e amo unt s o f Al ext rac ted dur ing the prewash ,
sa turat ing , and extract ion s t eps were essential ly the s ame for the
3 methods .
I f Al in the O . SM KN0 3 extra c t bal anced the nega t ive charge as Al 3 +
then when it was included in the cal c ul a t ion o f charge as a counterion
the adj u s t e d negative charge was a lmo s t independent of the concentra tion
o f t h e prewa sh e l ec trolyt e .
This i s probab ly the reason why th ere was
vir tual l y no ef f ec t o f the concent r a t io n of the p r ewash solution on
s urface nega t ive charge of l imed s o i l s when M KCl-extractab l e Al was
r educed to z ero .
S l ight d i f f er ences in the n ega t ive charge a f ter
a dj u s tmen t s for Al in O . SM KN 0 3 were probab l y d ue t o the diff e rences in
the pH of t h e s o il s uspens ion .
The use o f electrolyte solut ions o f high ionic s tr ength for
measurement of charge has o f t en b een c r i t ic ised (During , 1 9 7 3 ;
Harward , 19 74 ;
B el l and G i l lman , 1 9 78 ;
Wada an d
Uehara and Gil lman, 1 9 8 1 ;
Bl ack
Effect o f the concentrat ion o f prewash el ectro lytes on t he cumul a t ive amo unt o f Al ex t racted by the
sa tura t ion ( Ca C 1 2 ) and extrac t ing ( KN0 3 ) elec tro lytes and on the negat ive (Ca ad s ) , t o t a l nega t ive
( Ca ads + Al-KN0 3 ) and posit ive charge (Cl ad s ) d e t ermined in 0 . 0 1M CaC1 2 in a range o f unlimed
and l imed so il s .
Tab l e 4 . 1
pH 11
Method /Ill
0 . 0 1M
cac1 2
Soil
Al extra c t e d
by
cac1 2
ea ads
To tal negat ive
charge
Po s i t ive
cha rge
( Ca ads + Al-KN0 3 )
KN0 3
(Cl ad s )
cmo l ( p ) kg- 1 soil
Koronivia
4.9
4.9
4. 7
1
2
3
2.0
1.7
0.8
0. 2
0. 2
1.0
3. 6
3. 3
2. 7
3. 8
3. 5
3. 7
1.3
1. 1
1.3
Koronivia
( l imed )
5. 7
5. 7
5. 8
1
2
3
n. d
n.d
n. d
n. d
n. d
n. d
8.0
8.0
7.6
8.0
8.0
7.6
1.3
1. 3
0.9
Nadrol oulou
4.8
4.7
4. 3
1
2
3
7.6
6. 7
3. 2
0. 8
1.5
5. 7
22. 7
20 . 8
16 . 0
23 . 5
22 . 3
21. 7
1.9
1.5
1.2
Nad roloulou
( l imed )
6.0
6.2
6. 2
1
2
3
n.d
n.d
n.d
n.d
n. d
n. d
30 . 9
30 . 7
28 . 9
30 . 9
30. 7
28 . 9
1.3
1.2
0.5
Bat iri
4.7
4.6
4.5
1
2
3
0.4
0.4
0. 1
n. d
n.d
0. 3
2.4
2. 3
1.9
2.4
2. 3
2.2
4.7
3. 2
Batiri
( l imed )
5.2
5. 2
5. 2
1
2
3
n.d
n. d
n. d
n.d
n. d
n. d
2. 9
2.8
2.7
2.9
2.8
2. 7
4. 1
3.0
2 .6
Seqaqa
4.7
4.6
4.6
1
2
3
2.4
1.8
0.8
0. 5
1.2
1.7
8. 2
6. 8
6. 5
8. 7
8.0
8. 2
3. 2
2.8
2.6
S eqaqa
( l imed)
5.7
5.8
5. 7
1
2
3
n.d
n. d
n.d
n.d
n. d
n. d
17. 2
16. 9
17. 2
17.2
16. 9
17. 2
3.0
2.7
2.4
11
11 11
pH in s ixth equil ibrat ion ext ra c t .
Metho d 1
=
0 . 5M CaCl 2 l H 2 0 I 0 . 0 1M CaCl 2 .
Me t hod 2
=
0 . 1 M CaC1 2 I 0 . 0 1M CaC1 2 .
Metho d 3
3.0
=
O . O l M CaC1 2 .
..,..
CXl
49
and Camb el l , 1983) .
Recently , Grove et a l . ( 1 982) r ecommended an unbuf f e red
solutio n , having a n ionic s trength which was suf f i c i ent to remove
exchang eab l e Al , a s b e ing th e mos t suitab l e .
The p re s ent s t udy ind i ca t es th a t th e r emova l o f Al by e l e c t rolytes o f
d i f ferent ionic s t r ength has l i t t l e e f f ec t o n the nega tive charge o f s o i l s
de termined in O . O lM CaCl2 provided Al removed b y O . SM KN0 3 is included i n
th e cal cul a t ion o f charge .
However , a h igh concentration o f prewash
electro lyte has a s igni f i cant effect on the pH of the soil suspension
measured in the s ixth equil ib ra t ion , which may be the reason for the
s l igh t ly h igher nega t ive char g e which was ob s erved even after inc l us ion
of Al in KN0 3 ext r a c t s ( Tab le 4 .1) .
In view o f the s e observa tions , the
O . O lM CaCl 2 metho d , wi th inc l us ion in the charge ca l c ulation of Al r emoved
by KN0 3 is p robab l y the mo s t s a t is fac tory p ro cedure .
4.3.1.2
Effec t o f soil : s o l u t ion ra t io
S o i l : solution r a t io had a variab l e e f f e c t on th e surface
charge o f unl imed soils ( Fig . 4 . 1 ) .
As expec ted , the pH o f the soil
suspen s ion was a l so a f f e c t e d ( Fig . 4 . 2) .
These d i f f erences were general l y
r el a t ed to the amount o f Al extracted during the s a turation pro cess .
Thus the h igh nega t ive charge and h igh pH observed a t a wide soil : solution
ra t io we re prob ab ly due to the r emoval of r el a t ivel y large amoun ts o f Al
and exchangeable H ions in comparison to tho s e removed at a narrow
so il : s o l u t ion r a t io .
The inc reased pH ( Fig . 4 . 2) a t a wide s o il : solution
ra t io is due , a t l ea s t in part , to a d i l u t ion e f f ec t .
These resul t s
f urth e r confirm the need to a c count for ext rac tab l e Al i n charge
measurement pro cedures wi th a c id s o il s .
Appre c i ab l e amo unts o f Al were d e t e c ted in the O . SM KN0 3 extrac ts o f
the so il s b u t the quantit i es d epended o n the s o il : s o l ut ion ra t io .
At a
narrow s o i l : s o l ut ion ra t io , r el a t ively l a rge amo un t s of A1 were present
in the O . SM KN0 3 extrac t s , s ugges t ing incomplete exchange during
Inclus ion o f this Al in the charge cal cul a t io n e l imina t e d
equilibratio n .
t h e d i f f e r ences i n nega t ive charge determined a t d i f f erent so il : solution
ratio s .
Diff erences in po s i t iv e charge a r e p robab ly due to the incomplete
exchange o f Cl
-
for anions su ch as N0 3
- and S0 2- on the s o il surfa c e ,
4
p ar t icularly a t a narrow s o i l : solution r a t io .
Th e implic a t ion o f th is s tudy is that p re l iminary washing o f soils w i th
so
-20
-·
N A DROLOU L O U
· - - -- - - -
-
·
-12
J
en
�
-
a.
-
-4
-2
b
-a
�
-8
a
+I
0
4
- - - �
•
-
-
-
•
•
+2
•
+4
c
+4
•
8
b
0
-4
0
S E QAQA
+8
0
8
4
c
I
0
E
u
Gl
B AT I R I
E'
1'0
.&:.
u
..
-
...
-
- lt•
-
-
-
-
·
•
-8
a
b
.
..
c
+4
KORON I V I A
-
-
-
·-
-
•
+4
So i l : Sol u t i on (X: 40) ratio
E f f e c t o f soil : solut ion ratio on the d is t r ibut ion
of
( a) to tal nega t ive ( Ca ads + A1-KN0 3) ,
( b ) negat ive ( Ca a ds ) ,
-
-
·
•
+ 8�-----�
0
4
8
+8�-------�--�
4
0
8
Fi g . 4 . 1
-
and
( Cl ads ) in the four so il s .
( c ) po s i t ive charge
8
b
c
51
o
•
�
•
Batiri
Koronivia
Seqaqa
Nadroloulou
4 0 �------�------�--------�------�
2
4
6
0
Soil : solution ( X : 40 ) ratio
Fig . 4 . 2
E f f ec t o f s o i l : solution ra t io on the pH ( in
th e s ix th equil ib ra t ion) o f the s o i l susp en s ion .
52
el ectrolyt es o f high ionic s t rength can b e d ispensed w i th p rovided charge
measurement is condu c t ed at a wi d e s o i l : solution rat io with inclus ion o f
Al in the 0 . 5M KN0 3 extra c t in the cal cul a t ion o f charge .
I
I
I
I
I
Fur thermore ,
th e previously reported e f fe cts o f s o il : solut ion ra tio on P sorption may
s a t isfacto r ily b e exp l ained us in g the present resul t s .
P sorp t ion obs erved (Hope and Syers , 1 9 76 ;
Thus , the lower
B arrow and Shaw , 1 9 79a ) 9t
a wide soil : solu t ion ratio is pos s ib ly due to the large nega tive charge
on soil p ar t ic le sur f aces relat ive to that at a narrow soil : solution ra tio .
Th is wo uld a f f e ct the rate o f reaction (Hope and Syers , 1 9 7 6 ) .
4 . 3. 1 . 3
Ef f ec t o f the index ca tion
Negativ e and po s i t ive charge measured in O . OlM CaCl 2 were
b o th high e r than va lu es d e t ermined in 0 . 03M NaCl (Fig . 4 . 3) .
S imil ar
resul t s were obtain ed by Van Ra ij and Peech ( 1 9 7 2 ) and Mo rais e t al . ( 1 9 7 6 )
and they were explained by the ion-valence ef fect on the po t ential-de termining
ions .
Ryden and S y e rs ( 1 9 7 5 ) sugges ted that increas ing the val ence of the
s upport ing electrol y t e increa s es the screening effect on surface charg e ,
thus al lowing increased sorpt ion o f H+ and OHl ions t o ma int ain a cons tant
surface po t ential .
Corre l a t ion s tudies ind i ca t e a s trong l inear relat ionship between the
n egative charge measured in NaCl and in CaCl z (Tab le 4 . 2 ) .
Howeve� nega tive
charge mea sured in NaCl was always lower than that measured in CaCl 2 , as
indica t ed by the slope co e f f icien t .
This d i fference was mo r e pronounced
for l imed s o ils (Fig . 4 . 3) , an d may be explained by the st rong retent ion of
ea 2+ comp ared to Na+ by soil col l o ids (Breeuswma and Lykl ema , 1 9 7 1 , 1 9 7 3 ;
Kinniburgh e t al . , 1 9 7 5 ) .
S to o p ( 1 9 80 ) reported that s o ils co ntaining
mo s t ly hy drous Fe and Al oxid es , and thus col loids of the variab l e charge
type r e t a in ions through various types of adsorp tion mechanisms .
He
found tha t adsorbed P incr eased the adsorpt ion o f ca t ions and lowered the
In view o f thi s , ca 2+ in the 0 . 5M NH 4 N0 3 extra c t s of b o th
Nadroloul o u and S eqaqa soils was measured , in addition to Na+ .
Appreciab l e
2
amo unt s o f ca + were detected i n the extrac ts at pH values exceeding 5
PZC .
and when the se were included in the cal culat ion of charge ( l ine b in
Fig . 4 . 3)
decreas ed .
the d if ference b e tween the NaCl- and CaCl 2 -measured values was
When the adj u s t ed charge was included in the correl ation
analys es b o th an improved correla t ion coef f i cient (Tab l e 4 . 2) and a s lope
approaching uni ty wer e ob t a ined .
These resul ts sugges t that mo s t o f the
ob s e rved var iat ion in the NaCl- and Ca Cl 2 - measured nega tive charge in
53
N A DRO L O U L O U
b
a
c
- 20
� -· -
-
- 10
0
�
I
0"1
.JiC
-
+ 5
�
d
4
•
6
5
-
E
0
u
QJ
0"1
'-
,
.c
w
-40
S E Q AQ A
a
b
"
,A
,. _,.._ ..
/
-
---
- -
..
"'
.
c
d
•
pH
Fig . 4 . 3
E f f e c t o f s o il pH on the dis tribut ion o f negat ive and pos i t ive
charge d e t e rmined in O . O lM CaCl z and 0 . 0 3M NaCl .
(a) negat ive charge determined in O . O IM CaCl z ,
(b ) to tal nega t ive charge (Na ads + Ca (NH 4 N0 3 )) determined in
0 . 0 3M NaCl ,
(c) negative charge (Na ads ) determined in
0 . 0 3M NaCl ,
( d) po s itive charge determined in 0 . 03M NaCl ,
and
(e) po s i t ive charge determined in O . O IM CaC1 2 .
54
Tabl e
4.2
Co rrelat ion and l inear regres s ion c o e f f ic ients b etween
nega t ive charge ( Na ads) and total negative charge
(Na ads + Ca ( NH 4 N0 3 )) determined in 0 . 0 3M NaCl and
negative charge (Ca ads) d et ermined in O . O lM CaC1 2
solution .
Equa t ion o f l ine
So il
Nadrol oulou
Na ads
=
=
S eqaqa
Na ads
=
r
6 . 7 0 + 0 . 6 1 Ca ads
0. 98
-0 . 68 + 0 . 9 3 Ca ads
0 . 99
3 . 2 6 - 0 . 88 Ca ads
0. 85
- 1 . 9 1 + 0 . 9 2 Ca ads
0 . 99
55
l imed soils is due to the inab il ity o f Na+ to exchange for ea 2+ a t high
pH values .
The .resul ts further imply that fo� variab le charge s o il s ,
s uch as thos e used in the pres ent study , CaCl 2 should be used to measure
charge .
4 . 3. 2
Effect o f soil pH on charge charac teristics of soil s
Th e amount o f l ime required t o increase the p H varied between soils
(Fig . 4 . 4 ) , indica ting differences in their pH buffering capacities .
The
buffering capacity of Batiri s o il is low .
Only approximately 400 kg
Ca (OH) 2 ha-1 woul d b e required to increas e the pH to a depth of 1 00 mm
from 4 . 9 to about 5 . 2 , at which point M KCl-extrac table Al would be zero .
The low buffe ring capaci ty reflects the highly weathered nature o f this
soil .
Much larger amounts of lime (Fig . 4 . 4 ) were r equired to adj us t
the pH o f Koronivia , Nadroloul ou , and S eqaqa soil s .
These soils required
4 to 14 times more Ca (OH) 2 than the Batiri soil to increase pH to about
5 . 2 , p resumab ly due to the high buffering capac ity resulting from large
amount s of M KCl-extractab le Al ( 9 . 3 - 3 5 . 6 mmo l kg- 1 soil) , organic ma tter
( 5 . 6 - 16 . 6% ) , and oxalate-extractable Fe ( 6 3 - 94 mmol kg- 1 soil ) and Al
( 50 - 643 mmo l kg- 1 soil) in these soils .
S imilar l arge amounts of l ime
(800 - 5000 kg ha- 1 ) were required to increase the pH of s trongly acid
Hawaiian and Nigerian tropical soils to about 5 . 0 (Fox , 1 980) .
Manganese
toxic i ties may also pose a problem in these s o ils b ecause the pH required
to decrease s olub le Mn to a s ufficiently low l evel is relatively higher
than t hat for Al (Fox , 1 9 80) .
All s o il s pos sessed both pos itive and negative charges , the amounts
o f which changed s ignificantly with an increa s e in s o il pH (Fig , 4 . 5) .
This apparent co -existence o f positive and negative charge is consistent
with the sugg es t ion of Van Raij and P eech ( 1 9 7 2 ) , Espinoza et al . ( 19 75 ) ,
and Black and Waring , ( 1 976) that charges are arranged on soil colloids
s uch that they do no t completely cancel each o ther .
The presence of negative charge at low pH values in all the so il s
was p robably a resul t o f different amounts o f permanent nega tive charge
aris ing from isomorphous sub s titution within the lattices of clay mineral s
(Robertson e t al . , 1 954 ;
Holland et al . , 19 76) .
Mineralogical
examination indicated that moderate amounts of kaolinite were pres ent in
all the soil s , appreciable amounts of vermiculi te were present in the
Nadroloulou s oil , and trace amounts of vermicul i t e were present in the
Koronivia soil .
The high n egative charge at low pH values in the S eqaqa
56
9
8
7
•
•
•
•
25
0
Fig .
4.4
50
75
1 00
Ca(OH)2
125
( mmo l
Batiri
Koronivia
Seqaqa
�rolouiOu
�0
kg-'}
1
E f f e c t o f incr ea s ing amounts o f Ca(OH) 2
on the pH o f four contras t ing s o ils .
57
soils is probab ly due to the high organic mat ter cont ent (16 . 6%) o f this
soil .
Liming o f all soils caused a rapid increase in the negative charge
(Fi g . 4 . 5 ) .
However , the magni tude o f the increas e varied with the soil s .
For example , the negative charge in the S eqaqa soil increased f rom 8 to
over 3 8 cmol (p) kg- 1 compared to only a small increase o f 2 to 10 cmol (p) kg- 1
in the Batiri soil over the same pH range .
Thes e resul ts sugges t tha t
the ability o f these soils to retain cat ions shoul d vary cons iderably .
The
Batiri and Koronivia soil s , even if limed to pH 6 , have a much lower cat ion
reten t ion capaci ty than the S eqaqa and Nadroloulou soils , sugges ting that
losses o f added ca tions wo ul d oe substantially larger in the first 2 s o il s .
The e f f ec t o f pH on negative and pos itive charge is wel l es tablished
and cons idered t o b e due to proton trans fer mechanisms involving o rganic
mat ter , Fe and Al oxides , and the edges of kao l inite (Parks , 1965 ;
Hings ton et al . , 1 9 72 ;
Van Raij and Peech , 1 9 72 ;
Ryden et al . , 1 9 7 7a , b )
and o ther clay minerals such a s illite (Hendersho t and Lavkul ich , 1983) .
The p resence of pos i t ive charge at low pH values is cons is tent with
the mineralogical compo s i t ion of these soil s .
All soils contained an
abundance of either gibbsite or goethi t e , or both ( Table 3 . 3) and
appreciable quanti ties of poorly-ordered mat erial in the clay f rac tion .
Gibb s i te and go ethite minerals carry net pos i t ive charge (Parks , 1 96 5 )
i n the normal s o il pH range .
However , the magni tude of pos i t ive charge
can vary , depending on organic mat ter , which can mask positively-charged
s i tes (Bell and Gillman , 1 9 78) .
Although pos i t ive charge decreas ed with increas ing pH , detec table
amoun ts were s t ill present in all s o il s a t pH values higher than 7 .
For example , the Batiri and S eqaqa soil s contained in excess o f 0 . 9 cmo l
(p) kg-1 positive charge wh ereas the Nadroloulou and Koronivia soils
contained between 0 . 3 and 0 . 4 cmol (p) kg- 1 .
S imilar observations were
reported by MOrais et al . ( 1976) and Tess ens and Zauyah (1982) in a range
of h ighly�athered , Brazilian and Malays ian soils , respec tively .
The
lat t er workers a t tributed this to permanent posi t ive charge result ing
from isomorphous sub s t itut ion of Ti 4+ (Quirk , 1 9 6 0 ) and/or Mn4+ in the
Fe oxide lat tice (Sumner and Davidtz , 1965) .
Using the rat io of Fe to
Ti and/or Mn present in citrate-di thionite-b icarbonate (CDB) extracts ,
the Malaysian workers were able to cal culate the formula of the subs tituted
Fe oxide mineral in the Malaysian oxisols .
58
N ADRO L O U LOU
-40
a
b
... ..
SEQAQA
b
a
-30
-20
�
I
Ol
..X:
0..
�
-10
0
+5
3
4
5
7
6
0
�
8
+5
4
5
c
d
7
6
0
E
u
QJ
Cl
�
,
.L::.
LJ
B AT I R I
-16
KORON I V I A
-16
-12
b
a
b
a
-8
-4
0
+4
4
Fig .
4 .5
�
Ef f e c t
pH
of
p o s i t ive
c h arge
( 1 6 . 1 mmol
in
P kg-
mm o l P kg-
un t r e at e d
8
s o i l pH and adde d
( 1 6 . 1 mmol
(0
7
6
5
c
d
1
soil )
soil )
P kg-
(0
1
soil s .
1
mmo l
P on t h e
( a)
soil )
(c)
soils ,
1
(b)
soil )
pH
charge
negat ive
p o s i t iv e
and
6
7
8
d i s t ribut ion o f n e g at i v e
negat ive
soil s ,
soils ,
P kg-
5
(d)
soils .
c
d
and
in P - t r e at e d
c h arge
charge
p o s i t ive
in untre at e d
in P - t r e at e d
ch arge
in
59
Appreciable amounts (Table 4 . 3 ) o f Fe , Ti , and Mn were detected in
Us ing these
the CDB extracts of the Fij ian soils used in this s tudy .
data and calculations s imilar to tho s e o f Tess ens and Zauyah , ( 1982) ,
a theoretical, permanent positive charge was calculated ( Table 4 . 3) .
A comparison b etween ob served and calculated resul ts shows that it was
necessary to include b o th Ti and Mn substitution to ac count fo r the
ob served pos i tive charge in all s oils except Koronivia .
In the
Koronivia s o il , the ob s erved charge was somewhat higher than the
theoretical charge .
In general , the point of zero charge (PZC) varied considerably
b etween the soils .
Three o f the four soils were always negat ively charged
and therefore no direct measurement of the PZC could be made .
excep tion was the Batiri soil which had a PZC o f 5 . 3 .
The
However , inspec tion
of the pH-charge curves (Fig . 4 . 5 ) suggests that PZC ' s were in the
fol lowing approximate range :
>
Batiri ( 5 . 3) > Koronivia ( - 4) > S eqaqa (- 3)
Nadroloulou wh ich did no t have a PZC in the normal pH range .
The
l itera ture indicates tha t the PZC o f s trongly-weathered soils , such as
oxisols , ult iso l s and al fisols , can vary from 2 . 7 to 6 . 5 (Van Raij and
Peech , 19 7 2 ;
El-Swaify and Sayegh , 1 9 75 ;
Gal l e z et al . , 1 9 7 6 ) , the
lower values being ob tained for surface horizons due to higher amounts of
organic mat ter .
The present values ob tained are in agreement with
published val ues .
4 . 3. 3
Effect o f added phosphate on charge
The changes in surface negat ive and pos i t ive charge resulting from
P additions were small compared to tho s e resul ting from lime additions ,
describ ed above .
However , the magnitude o f reduction in positive
charge induced by P addit ion was no t consis tent at all pH values .
A
sub stantially larger ef fect of P on surface positive charge was obs erved
at low pH values .
These obs ervat ions are in agreement with those o f
o thers (Hings ton et al • • 1 96 7 ;
Raj an et al . , 1 9 74 ;
Ryden e t al . , 1 9 7 7a) .
The decreased effect o f P on pos i tive charge a t high pH was interpreted
as b eing due to a higher tendency for P to s o rb on neutral and negative
s ites (Breeuswma and Leuklema , 1 9 7 3 ;
1 9 7 7a} .
Raj an e t al . , 1 9 74 ;
In thes e s tudies the ratio of a+ consumed or OH
-
Ryden et al . ,
released mol - l
o f P sorb ed increased with pH , sugges ting that P was sorb ed by ligand

exchange with surface • �Hi+ groups in the low pH range , but disp laced OH
from neutral and negat ive sites a t high pH .
If such a ligand exchange
mechanism coul d be accepted for P then the r el ease of OH
can be used to
60
Table 4 . 3
Rat io o f Fe : Ti and Fe : Mn in the c itrate-dith ionit eb icarb onate ext racts , pos i t ive permanent charge
e s t ima t ed f rom Ti ( IV) and f rom Ti (IV) + Mn ( IV)
sub s t i tution in i ron oxides a nd that determined by
Cl ads or p t ion ( Cl ads) from 0 . 0 1M CaCl 2 above pH 7 .
Positive permanent charge
Rat io
Soil
Fe : Ti
Fe : Mn
Till
(Ti+Mn ) ff
substitut ion
Cl ads/Ill
cmo l (p ) kg- 1
Koronivia
1 2 20
829
0 . 04
0 . 10
0 . 36
Nadrolo ulou
1 00 1
5 60
0 . 05
0 . 38
0 . 33
Batiri
398
752
0 . 62
0 . 92
0.91
S eqaqa
1 1 18
229
0 . 13
0 . 70
1 .05
ff
ffff
Charge cal cul ated by us ing the metho d o f Tes s ens and Zauyah ( 1 9 8 2 ) .
Experimentally ob s e rved charge .
61
infe r changes i n surface charge i n the soils .
Add i tion of P had a larger e f f e c t on negative charge a t h igh pH
values (Fig . 4 . 5 ) .
Thes e ob servat ions are in a c co rd wi th the l i gand
exchange mechanism dis cus s e d above .
The small incr ease in n ega t ive
charge , with a sub sequent d e cr eas e in po s i t ive charge , p rob ably have
resul t e d in an overal l decreas e in the P ZC fol lowing addi tion of P .
S imi l ar sh if ts in PZC were r epo rted by Parks ( 19 6 5 ) , Mekaru and Uehara
( 1 9 7 2 ) , and Raj an ( 19 7 8) .
Wann and Uehara ( 1 9 7 8) us ed this decreas e in
P ZC as an indica t ion of the increased capacity of so ils to r e t a in cations .
Th ey showed that appl icat io n o f about 50 mmo l P kg- 1 to an oxisol lowered
the P ZC f rom near 5 . 0 to b e l ow 3 . 5 in the presence of Ca , and recommended
the appl i ca t ion of P as a means of increas ing the nega tive charge
( ca t ion retention) of soils .
However , they emphas is ed tha t , in many
c as es , i t would not b e ec onomi cal to apply P mer ely to incr eas e the
negat ive charge , but as variab l e charge so ils generally requir e large
appl i cat ions of P for plant growth , the increas e in nega t ive charge comes
as an added b ene f it .
Al though the amount o f P added to the s o ils in
the present s tudy was one third of that used by Wann and Uehar a ( 19 78 ) ,
the r esul t s ind i ca te that a very l arge amount of P would be required to
ob t a i n an increase in negative charge which would have ,any long-term ,
b enef ic ial effec t on plant growth .
4.4
Conclus ions
The e f f e c t s of the conc en tra t ion of prewash el e c t rolyt es , soil :
solut ion rat io and index cat ion on the magnitude o f surface charge were
inves t igat e d using s el ec t ed unl imed a nd l imed s o i l s f rom Fij i .
Preliminary washing o f s o il s , with elec t ro l y t es o f varying
concentrat ion
and s o il : solu t ion r a t io , had l it t l e e f f e c t on surface
negative Charge provided Al in the 0 . 5M KN0 3 extrac t was included in the
calculation o f chkrge .
Surface charge measurements ob tained in O . O lM
CaCl z were a lways larger than tho s e i n 0 . 03M NaCl .
MO s t o f the differenc e
+
+
was due to the inab il ity o f Na to exchange with ca 2 in so il s a t high pH .
Based on these obs erva t ions i t would appear tha t the b e s t method for
measuring Charge in l imed Fij ian s o il s is to use 0 . 0 1M CaCl z w ith inclus ion
of Al in 0 . 5M KN0 3 , without a prel iminary washing with an el ec troly te o f
high conc entrat ion .
62
L iming o f soils caused a si gnif icant increas e i n negative charge and
a smalle r decrease in po s i tive charge .
The latter was detec ted in all
soils even a t pH values h igher than 7 .
Presence o f pos itive charge at
high pH values was explained by isomorphous sub s titution of Ti 4+ and/or Mn4 +
in the Fe oxide l a t t ice .
The ef fect o f a dded P ( 1 6 . 1 mmol kg- 1 s o il ) on s urface charge was
d if f erent a t di f ferent pH values but was a lways smal l relative to that
due to liming .
The Batiri and Ko ronivia s o ils had a relatively low negat ive charge
at field pH values , sugges t ing tha t losses of added cations would be
s ub s ta n t ia l ly larger th an tho s e exper ienced with the l ess wea thered soil s .
I
I
I
I
(
I
I
'
CHAPTER 5
63
CHAPTER 5
EFFECT OF LIMING ON PHOSPHATE
EXTRACTED BY TWO SO IL-TES TING PROCEDURES
5.1
Introduct ion
I t has generally b een ob s e rved that l iming o f hi ghly-wea thered , acid
soils increas es plant growth and this has b een a t t ributed to an allevia t ion
of Al toxicity and/ o r an increase in the ava ilab il ity of P and o ther nut rients
( Sanchez and Uehara , 1 98 0 ) .
However , there have b een a numb er o f reports
( e . g . , Sumner , 1 9 79 ) , based on plant P data , where l iming acid soil s to near
neutrality appeared to decreas e P availab i li ty a f ter an ini tial increas e in
availability up to s o il pH values of approxima tely 6 .
In contras t to
these ob servations , considerable controversy exi s t s in the l i terature
r egarding the effects o f l iming on the amount of P extrac ted by various
so il-t est ing pro cedures .
For examp l e , whereas Rhue and Hensel ( 1 9 8 3)
r eported an increase in Mehl ich-extractab l e P with increasing l ime rates ,
Grif f in ( 1 9 7 1 ) recorded both an increas ing and a decr eas ing trend in
Mehl ich-extractabl e P with increasing pH d ep ending on s o il typ e .
Furthermore , Lamb ert and Grant ( 1 9 80 ) and Sorn-sr ivichai e t a l . ( 19 8 4 )
repo rted a cons is tent decrease i n Olsen-extractab l e P with increasing lime
addi tions .
The lat t er investiga tors a t tr ibuted the decrease in Ols en P
with increas ing pH to an artefact in the O l s en metho d .
Th is suggests
that some of the conf l ic t ing report s in the l i t erature may b e explained by
care ful cons iderat ion o f the soil-tes ting pro cedures .
Wi th the exception o f the s t udy on the influence of pH on Olsen
extractable P by Sorn-srivichai et al . ( 19 84 ) and the suggest ion by
Kamprath and Wat son ( 1 980) that l i �ng may decreas e the eff iciency o f weak
acid extractants for the r emoval o f P , there is l i t t l e information in the
l i t erature on the ef f ect of l iming on the amount of P extracted by soil
tes t ing procedures used to assess the availab l e P s tatus of soils .
The
present s tudy was des igned to inves t igat e the effects of liming on the
amount of P extrac t ed f rom acid soils by two contras t ing soil-tes t ing
pro cedur es and to relate these to the amoun ts of iso topically-exchangeabl e P .
64
B ecaus e the soils used in this s tudy were ini tially s t rongly acid
and were sub s equently l imed , the two so il-tes t ing proc edures s elected were
thos e developed by Olsen ( Ol sen et a l . , 1954 ) and Mehlich ( Nel son et a l . ,
1 9 5 3) .
The O l s en procedure was cho s en b ecaus e it was ini tially designed
(Ols en et al . , 1 9 54 ) for cal careous so ils and sub s equently shown to b e
s uccessful with a c i d soils (Ba rrow and Shaw , 19 76) .
The Mehl ich
pro c edure was s e l ec t ed becau se it is r elatively suc c essful with highly
weathered , acid soils ( Thomas and Peas l e e , 1 9 73) .
The determina t ion o f
exchangeabl e P i s l ess emp irical and provides a goo d measure o f po tent ially
ava ilab l e P (Barrow , 1983a) .
5. 2
Ma terials and Methods
5 . 2. 1
Preparat ion of s o il sampl es
Deta i l ed descript ions of the so il s , and the methods us ed for their
pr eparation are given in Chap ter 3 , but a brief de s c rip t ion is included
b el ow .
L ime a s Ca ( OH) 2 was added to the 4 soils (Bat iri , Koronivia ,
Nadro l o ulo u , and Seqaqa ) at either 9 or 10 rates and thoroughly mixed and
inc uba ted under the condi t io ns spec ifi e d in Chap ter 3 .
At the end of the
incub a t ion period , the soils were air d ried and subsampl es were
reincub ated with 3 ra tes of added P .
Fo l lowing this second incubation ,
the s o ils were air dried and s tored in polythene bags for chemical analysi s .
5.2.2
Chemical analyses
Exchangeab l e P:
'
I so topic-exchange measurements were done by shaking
0
soil ( 1 g) with distilled wat er ( 39 mL ) end-over-end for 16 h , a t 20 C .
Carrier-free 3 2p ( 1 mi , 4 � C i ) was then added and the tub es shaken for a
fur ther 4 h .
This meth�d was essen t ial ly the same as that used by Le Ma�e
Z
( 19 8 1 ) , except that after the addition of 3 p the s o il s were shaken for 4 h
rather than 24 h .
The tub es were c ent r ifuged , the solution pas sed through
a memb rane f il te r ( < 0 . 4 5 � m) , and 3 2 p activity and 3 1 p determined in the
f il tra t e .
The a c t ivity of 3 2p was det ermined by l iquid scintillat ion
coun t i ng u sing a t riton-toluene s c int illation cocktail (Pat terson and Green ?
1965) .
The method o f Murphy and Ril ey ( 1 9 6 2) was used to measure
s o il extra c t s .
31
P in a l l
65
�lse�-extractable P ( O l s en e t al . , 1954) :
30 min with 20
1 g o f soil was shaken fo r
mL
of 0 . 5M NaHC0 3 (pH 8 . 5) solut io n in polypropylene tub es ,
centri fuged , and f il tered through Wha tman No . 5 f il ter paper .
Inorganic
P was d e termined in the ext rac t by the method of Murphy and Riley ( 1 9 6 2 ) .
Mehl ich-ext_rac table P (Nelson e t al . , 1 9 5 3 ) :
5 g of soil was shaken
for 5 min with 20 mL o f Mehli ch r eagent ( 0 . 0 1 3M H 2 S 0 4
+
0 . 0 5M HCl , pH 1 . 25 )
in pol ypro pylene tub es , centrifuged , and f il tered through Wha tman No . 5
f il ter paper .
Ino rganic P in the ext racts was det ermined as des crib ed
above for Ol sen-extractab l e P .
The pH o f the Mehl ich extract a f t er
f il tration was also measured .
During this study it was ob served that the pH o f the Mehl ich r ea gent,
fol lowing ext ra c t ion of l imed so ils , had increased considerably above i ts
ini t ial value of 1 . 25 .
To determine the effec t o f pH increase on the
amounts of P ext racted by the Mehl ich reagent , ext rac tions were carried
out on s el ec t ed l imed and unl imed samp l es of the Bat iri , Koronivia , and
Nadro loulou soils in a Rad iometer Copenhagen autoburette .
Th is maintained
t he pH o f the extrac tant close to 1 . 25 by the automatic addit ion o f
0 . 1 3M H 2 S 0 4 + 0 . 5M HCl , which was 1 0 t imes mo re concentrated than the
Mehl ich reagent .
The use of such a s t rong reagent to control pH ensured
only small changes in the volume of the extractant .
All resul t s are presented as the means of dupl icates .
5.3
Results and Discuss ion
Add i tion of l ime to each s o il resul t ed in samp les covering a range o f
pH f rom 4 . 0 - 5 . 0 to over 7 .
5 . 3. 1
Effect of l ime and phosphate addit ions on
iso topically-exchangeab le phospha te
Lime and P addi tions had a marked e ffec t on the amounts o f exchangeab le
P (Fig . 5 . 1) .
I n the abs ence o f added P the B a tiri , S eqaqa , and Nadroloulou
s o i l s generally showed an increase in exchangeable P up to pH values b e tween
6 and 7 . 4 , above which it decreased .
In contras t , in the Koronivia soil ,
there was a small decreas e in exchangeabl e P between pH values 4 to 5 . 5 ,
above which i t increas ed s l ightly .
In the high P-so rbing Ba tiri and S eqaqa
so i l s , trea t ed with l ow l evels of l ime , s o lution P was nondetec tab l e and 3 2p
66
SEOAQA
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BATIRI
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pH
Effe c t of increas ing pH on iso topically-exchangeab l e P
in 4 so il s incub ated wi th 3 rates of added P .
C
•
=
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.A
=
8.1;
•
•
1 6 . 1 mmo l kg -1 s o i l ) .
67
counts were very low . so it was no t pos s ib l e to ob ta in exchangeable P values .
even in so il samples t r eated wi th medium ( 8 . 1 mmol kg- 1 ) levels o f P .
In s o ils incub ated with added P , exchang eab le P increase d several fold ,
maintaining the trend with increas ing pH obs erved for the untreated so il s .
Howeve r , the ext ent o f the increase in exchangeab le P with in creas ing l ime
and P a d d i t ions varied between the soil s .
For exampl e , in the Bat iri
soil , _ exchangeable P inc reased rather rap idly above pH 6 . 0 , rel ative to
the increase recorded for the o ther soil s .
Furthermo r e , whereas
exchangeab l e P increas ed within each l ime l evel in the Koronivia ,
Nadrol oulo u , and S eqaqa soils t r eated with increas ing amounts o f added P ,
it was no t measureab l e at low pH values in the Batiri so il even when it
was trea te d with med ium amounts of a dded P (Fig . 5 . 1 ) .
In the Batiri
soil , a marked increas e in exchangeable P wi thin each l ime l evel at low
pH val ues was only recorded at the highes t- level of added P .
These r e sul ts s ugges t that the Ba tiri soil contains a small number o f very
active P-so rbing si tes which are rapidly neutrali zed by l iming up to
pH 6 , o r by addition o f a high l evel of P .
Sub s equent to neutral ization
of the s e s i tes there was a rapid increase in exchangeable P .
In cons idering these resul t s , the firs t point to no te is that more
than one physio-chemical process may be invo lved in the exchange reaction .
Thus , when P is sorb ed by a soil , several reaction mechanisms are po ssible ,
depending on the pH and the charge on the surface (Mo t t , 1 9 7 0 ) .
Moreover ,
�so top ic -exchange is a continuing process in soil sys t ems (Barrow , 1 9 7 5 ) .
Rather than having a definite end po int , the added 3 2 P exchanges at a
decreas ing ra te wi th an increas ing propor tion of the s o il P , the rate
of exchange b eing dependent in p art on the s trength of the bond between sorbed
P and the sub s t rate (Ryden and Syers , 1 9 7 7b ) .
Thus , the ob served init ial
increa s e in exchangeab l e P with s o il pH may be due to an increase in net
s urface negative charge ( Chapter 4 , S ec t ion 4 . 3 . 2) o f the s o il s with l iming .
As a r es u l t o f this the potential in the plane o f sorption woul d b ecome
increa s ingly negative (Bowden et al . , 19 80b) , thereby decreas ing the b inding
energy b etween sorbed P and the surfac e .
At high pH values , surface
r eact ions may be comp l icated by p recipitation reac tions invo lving
solution P .
Ca
and
Thus , the decreas e in exchangeable P a t high pH values in the
Batir i , Nadroloulou, and Seqaqa soil s sugges ts the formation o f a new phas e ,
which i n this case i s probably Ca -P (Wi l d , 1 9 53 ;
and Peech , 1 9 6 9 } . .
Ma t t ingly , 1 9 7 5 ;
Mll rrmaun
However , the pH a t which precipi ta t ion r eactions o c cur
wil l vary with the conc en tra t ion of Ca and P in solution (Wild , 195 3 ;
68
Barrow et al . , 1980 ) .
Th i s is possibly the r eason why Koronivia soil ,
to wh ich the lowest amoun t of l ime was added , did no t show a maximum in
exchang eable P over the pH range s tudied , whereas in the Seqaqa soil ,
wh ich r ec e ived the larges t amount o f l ime , exchangeab le P reached a
maximum a t approximately pH 7 .
5.3.2
Effect of l ime and phosphate additio ns
on Olsen-extractab le pho sphate
Bo th l ime and P add i t ions had a variable effect o n Olsen-extrac table
-l
P (Fi g . 5 . 2 ) .
In genera l , less than 0 . 1 mmol P kg
soil was extracted
f r om the unlimed s o ils which had no t b een incubated wi th added P .
When
the soils we re incubated wi th added P , the propo rt ion of added P extracted
varied from l ess than 1 % i n the Batiri and Seqaqa soils to be tween 12 and
35% in the Nadroloulou and Koronivia soils , r espectively .
Th ese resul ts
are cons is tent with the mineralo gical compo s i tion o f the soils in that
the Batiri and Seqaqa s o il s , which contain large amounts of either acid
o xalate- or di thioni te-extrac tab le Fe and Al ( Tab le 3 . 3) , so rb ed the
largest amounts o f added P .
In contras t to the r es ults for exchangeabl e P , l iming had little effect
on the amount o f P extrac t ed by the Olsen reagent from soils which had no t
b een incubated with added P .
However , in soils treated wi th the medium
l evel o f added P th er e was a t endency fo r Ol s en P to dec rease initially
with inc r easing soil pH and this was followed by an increase a t high pH
values .
Th is trend was more p ronounced in soils trea t ed with high l evels
o f added P s uch tha t there was a d ef ini te minimum in Ol sen P between pH
values 5 . 5 and 6 . 5 (Fig . 5 . 2) .
A decreas e in Ol s en P with increas ing pH has b een repor ted recently
(Lambert and Grant , 1980 ;
Sorn-srivichai et al . , 1 9 84 ) and this was
a t tributed ( S o rn-srivichai et al . , 1 9 84 ) to an artefact in the Olsen
p ro cedur e .
The latter inves tigators provided good s upport ing evidence to
s ugges t that P coprec ipi tated as a Ca-P compound in the Ols en extract .
They sugges t e d that s uch coprec ipita t ion b ecame mo re pronounc ed as so il pH
increased due to the higher qUan t it i es o f Ca present .
Cop recipita t ion
s ho uld also increase wi th an incr eas e in the amo unt o f added P and this was
no t ed in the present s tudy .
The incr ease in 01 s en P above pH 5 . 5 - 6 . 5 does not appear to have
b een reported by previous wo rkers , possibly b ecause the soil s in these
s tudies were no t l imed to pH values as high as those in the present s tudy .
69
BAT IRI
SEQAQA
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6
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5
I
25
5
�
'-
0
�·
�
Effect o f increasing pH on Olsen P in 4 soils incubated
with 3 ra t es of added P .
( •
=
0;
&
=
8.1;
•
=
1 6 . 1 mmo l kg-1 so i l) .
I
9
70
Howeve r , a s imil ar trend in exchangeab le P i n limed so ils was r eported by
Mur rmann aod Peech ( 1 969) and they a t t r ibuted the increase in exchangeab le
P above pH 5 . 5 to increased solub i l i ty of Al-hydroxy po lyme rs and the
rel eas e o f o c c lu de d P .
Such a mechanism would be less likely in the
presen t soi l s because , unl ike in the s tudy of Murrmann and Peech ( 1969) ,
P was added to s o ils subs equent to incub ation with l ime .
Al though , it is
no t poss ib l e to p rovide a definite exp lanat ion for the increas e in Olsen P
above pH 5 . 5 - 6 . 5 , the iso topic-exchange data ( S ec t ion 5 . 3 . 1 ) suggest
that the in crea s e was rel ated to the r a te at which P was rel eas ed in the
Olsen extr ac t .
Two competing solub i l i ty equ ilibria, presumabl� operate
dur ing the O l s en extrac t ion .
There is an intera c t ion be tween Ca and
C0 4 2- , and also b e tween Ca .and
et a l .
re)-�ased P .
As
shown by Sorn-s rivichai
( 1 984 ) , the rapid rel ease of s orb ed P in Olsen extracts at low and
medium pH values increas es the concen t r ation of P to l evels which cause
p recipi tation of insolub le Ca-P compounds .
no t operat e at h igh pH val ues .
Presumably� this p rocess does
Resul ts from the iso topic-exchange s tudy
(S ect ion 5 . 3 . 1 ) indica te tha t mo s t of the added P would be pres ent as
insolub le Ca-P c ompounds a t h igh pH values .
Rel a t ive to sorb ed P, the
rel ease o f P from precipitated compounds during the Olsen extraction woul d
b e s low , s o that the only p recipitation reac tion woul d be b etween Ca and
C0 3 2- .
Th is fo rmation of CaC0 3 would considerably reduce the concentra tion
of Ca in the O l s en extract s uch that the interac t ion be tween Ca and P is
minimi sed .
Bas ed on this mechanism, Olsen e t a l . ( 1 954 ) recommended the
Olsen test for c al careous s oils .
Al though Ol s en P inc reased above pH 5 . 5 - 6 . 5 , s o il s normally would
no t usuall y be l imed above this pH in view of the ob served deleterious
e f f e c ts on plant growth of l iming acid s o ils to pH values in excess of
6 . 0 (Sumner , 1 9 7 9 ) .
Al so , although c r i tical Olsen P l evels are not
ava i l able for the range of c rops grown in Fij i , a comparison with
reconnnended O l s en P values for acid s o il s on which crops of med ium P
-1
requirement ar e grown i n the United S ta tes o f America ( 0 . 2 - 0 . 3 mmo l kg ;
Thomas and P eas l ee , 1 9 7 3) indicates that at the med ium o r high levels o f
added P used in this study , all unlimed and l imed so ils had higher Olsen P
values than tho s e required for normal c rop growth .
Thus , the observed
decrease in Ols en P wi th increas ing pH may no t b e o f much prac t ical
s ignif icance to plant growth in the sho r t term .
71
5.3. 3
Ef fect o f l ime and phosphate additions
on Mehl ich- ext ractab l e pho sphate
I n g eneral , Mehl i ch P dec r ea s ed with incr ea s ing s o il pH for all so ils
except Koronivia , in which i t b ehaved rather s imilarly to Ols en P ( F ig .
5 . 3) .
However , the extent o f the decreas e in Mehl ich P varied with b o th
the s o il and the level o f added P .
Fo r exampl e , in the ab sence o f added
P , Mehl ich P wa s nea r ly cons tant with increas ing pH in the Ba tiri ,
Koronivia , and Nadro l o ulou s o il s .
In the S eqaqa s o il , Mehl ich P decreased
s l ightly with increas ing s o il pH .
As was the case with the Ol s en extrac tion , the amount of a dded P which
was extra c t ed by the Mehl ich r eagent varied with the P-sorption capac i ty
o f the s o il s .
Fo r exampl e , in t he high P - s o rb ing , unlimed S eqaqa s o il ,
l es s than 1 % o f the added P ( 1 6 . 1 mmol kg- 1 ) was extra c t ed in compa rison
30 ,
to the unl imed Batiri , Koro nivia and Nadro loulou soils f rom which 3,
and 1 5% o f the added P ( 1 6 . 1 mmol kg- 1 ) was extrac ted , r espec t ively .
Thes e resul ts are s imilar to tho se repo r ted by Grif fin ( 1 9 7 1 ) who
also ob tained a va riab l e t rend in Mehl ich P with inc r eas ing pH .
Ac cord ing
to Kamprath and Watson ( 1 9 8 0 ) such trends are due to var iat ions in s o il
type .
I n the present s tu dy , it is apparent f rom the pH of the Mehl ich
extract (Fig . 5 . 3 ) tha t in all except the Ko ronivia so il there was a
s igni f icant effect o f s o il pH on the pH of the extrac tant and this may
c ontribu t e to the r educed efficiency o f the Mehl ich r eagent wi th inc reas ing
s o il pH .
This increase in pH of the Mehl ich ex trac tant coul d be a r es ul t
o f 2 facto r s , namely
( i ) s o il mineralogical and chemical chara c t er i s t i c s
and their e f f ect on P-buff ering c apacity o f the s o i l and
l iming .
( ii ) the rate o f
For examp l e , there was a marked d i f ference in the pH of the
Mehl ich reagent b efor e and af t er extraction of the unl imed so il s .
Thi s
d i f f erence was mo re p ronounced i n the Nadroloulou and S eqaqa so ils which
were also high in Al and Fe components , and o r ganic ma t t er and clay con t ent .
Altho ugh
the Batiri s o il also contains very large amounts o f Al and Fe
components , there was no marked e f fect o f thes e on the pH o f Mehlich reagent ,
p resumably b ecaus e o f the lower o rganic mat ter content .
The effect o f
increas ing l ime additions o n the p H of the Mehl ich extract is well shown by
the resul t s for the S eqaqa , Nadroloulou, and Kornivia s o ils .
Becaus e very
l arge amoun ts of lime were added to the Nadroloulou and Seqaqa s o il s , a larger
e f f ec t o n the pH o f the Mehl ich extrac tant was ob s erved and associated wi th
this was a much sha rper d ecrea s e in Mehlich P with increas ing pH (Fig . 5 . 3) .
In contras t , r ela t ively small amounts of l ime were added to the Koronivia
72
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2
1
Fig . 5 . 3
Effect o f increas ing pH on Mehl ich P and pH of the Mehl ich
ext ract ( .6. ) in 4 s o ils incubated with 3 rates of added P .
( ·•
=
�;
.A
=
8.1;
e
=
1 6 . 1 mmol kg- 1 soil) .
73
s o il and cons equently l i t t l e change i n pH of the Meh lich extra c t was
r eco rded with increasing soil pH .
This also corresponded to small changes
in Mehl i ch P with increas ing s o i l pH .
To f ur ther inves t igate the e f f ec t of pH on the amount o f P extrac t ed
by th e Mehl ich reagent , selected unlimed and l imed Ba tiri , Koronivia ,
and Nadro loulou so ils were extracted a t a constant pH us ing an autobure t te
(see S e c t ion 5 . 2 . 2 ) .
The r esu l t s of this inves t iga t ion (Tab l e 5 . 1 ) showed
that wh en the pH of the ex trac tant was cons tant , the amount o f P extracted
was independent of the ini t ia l soil pH .
These resul ts conf irm the
sugges tion o f Thoma s and P easlee ( 1 9 7 3 ) and Kamprath and Wa tson ( 1 980)
that so ils dominant in Fe oxid e , CaC0 3 and high clay content rap idly
neutra l i z e the acid o f the Meh l i ch r eagent , thereby decreas ing its
e f f i c i ency .
5. 3.4
Comparison o f soil - t es t ing proc edures
The amounts of P ex tracted by the soil-testing proc edures generally
increas ed in the order Mehl ich P < Olsen P
<
exchangeab le P .
The only
excep t ion to this was the Ko ronivia soil whi ch had been incubated with
added P .
val ues .
In this s o il, Olsen P values were h igher than the exchangeable P
The soil- t es t ing pro c edures showed con tras t ing trends wi th pH .
To expl a in these contras ting e f fec ts of pH on the extrac tion o f P , the
mechanisms involved during the extrac t ion o f P by each soil - tes t ing
pro cedure and also the nature o f P cons tituting the exchangeab l e pool mus t
b e considered .
The Mehlich r eagent (pH 1 . 25 ) is expec t ed to princ ipal ly dissolve P
r eactive surfaces (Al and Fe hydrous oxides ) and Ca-P (Thomas and P eas l ee ,
1 9 7 3) .
The Olsen r eagent probably desorb s loosely-hel d P , and some
chemiso rb e d P (Ryden and Syers , 1 9 7 7a) , and disso lves P from cer tain Ca-P
compound s (Olsen et al . , 1 9 54 ) .
Exchangeab l e P is believed to b e mainly P
wh ich i s loo sely held on or in s urfac es of P-reactive compounds and P
mineral s (Ryden and Syers , 1 9 7 7a ) and some chemisorbed P , d epending on the
I
p er iod o f equ ilib ra t ion (Bolan , 1: 9 83) .
Given these mechanisms and the fact
tha t exchangeability genera l ly increas es wi th an increase in nega t ive charge
(Ryden and Syers , 1 9 J 7b ) , all the so il- tes t ing p rocedures examined in this
s tudy would be expec ted to remove an increas ing amount o f P wi th an increase
in pH .
Such a trend in Mehlich P with pH for l imed s o il s was repor ted by
Ho l ford ( 1 9 83) and Rhue and Hensel ( 1 9 83) .
Moreover , b ecause the Mehl ich
reagent o perates by a di s s ol ution mechanism it would be expec t ed to remove
Tabl e 5 . 1
Amount of P ex tra c t ed by the Mehl ich r eagent (pH 1 . 3 ) from unl imed and
l imed and subsequently pho spha t e treated (8 . 1 mmol P kg - 1 soil )
Batir i , Koronivia , and Nad roloulou soils
• .
pH of soil
Soil
Treatment
Before ext ra c t ion
Mehl ich P
Dur ing extra c t ion
(mmo l kg
-1
)
-
Batiri
Koronivia
Nadro l oulou
unl imed
4.9
1.3
0.6
l imed
8.0
1.3
0. 5
unl imed
4.2
1.3
2.3
l imed
7.9
1.3
2. 3
unl imed
4.0
1.3
1.3
l imed
7.4
1. 3
1.2
....,
�
75
the larges t amount o f P .
However , in the pres ent inves tigation the amount
of P extra c t ed by the Mehlich r ea gent , relat iv e to that extrac ted by th e
Olsen r eagen t , depended on soil type as wel l as on whether the soils were
l imed ( Tab l e 5 . � ) .
Fo r exampl e , in all the l imed soils Mehl ich P was
l es s than Ol s en P and th is mus t have been due to the neutralizing effec t
o f l ime on the Mehl ich extrac tant ( S ec tion 5 . 3 . 3) .
In the unl imed
soils , Mehl i ch P generally depended on the ini t ial P-sorpt ion capacity o f
the s o ils .
Thus , wher eas i n the l ow to mo dera te P-so rbing Koronivia and
Nadrol oulou s o ils , Mehl ich P was identical to Ol s en P , i t was generally
l ess than O l s en P in the high P-so rb ing Batiri and Seqaqa soil s .
Thes e
d i f ferences are probably due to d i f f erent degrees o f se condary sorp t ion
( Caj uste and Kussow , 1 9 7 4 ) of ex t ra c ted P by the so il constituents .
Although . exchangeab le P was generally h igher than Olsen and Mehlich
P , it does no t necessarily indicate extensive exchange with chemi sorb ed
P.
Some o f the differences in P ex trac ted by these methods mus t be due
to the arte f a c t in the Ol sen method and the effects of l iming on the
efficiency o f the Mehl i ch extrac t an t .
With the excep t ion o f exchangeable P , which generally increased wi th
increas ing pH , the Mehl ich and O l s en ex trac tants showed rather contrast ing
t rends wi th increasing pH ;
thes e were at tributed to a decrease in th e
e f f ic iency o f the Mehl ich reagent b ecaus e of the inc reas e in pH during
extraction and to an artefact (Sorn-srivichai et al . , 1 9 84 ) in the Olsen
method .
In v iew o f th ese prob l ems it is no t practicable to correlate
the resul ts o f the 3 so il-tes ting procedures .
Although lime addit ions a f f e c t e d the amount of P extracted by b o th
s o il-tes t ing p roc edures , only in the case of the Mehlich pro cedure were
the changes s uf f iciently large to b e of concern .
Thus , in 3 of the 4
so ils, an incr ease in pH f rom 4 . 5 to 5 . 5 r esulted in a dec rease in the amount
o f P, suf f i c i ent to change the ranking o f the soils f rom medium to low ,
particularly in the P - treated soils , based on cri tical P concentratio ns
provided by Thomas and P easlee ( 1 9 7 3 ) .
5 .4
Conclus ions
Liming o f soils can have a variable effect on the amount o f P ext r a c t ed
by the Olsen and Mehl ich metho ds and on the amount o f exchangeable P .
Isotopically-exchangeab l e P generally increas ed wi th pH , Mehl ich P general ly
76
Tabl e 5 . 2
Amoun t s o f P ext racted by the Mechlich and O l sen
r eagents f rom unl imed but pho spha te trea t ed
-1
( 0 , 8 . 1 , 1 6 . 1 mmo l P kg
s o i l ) so ils .
So il
Treatment
Mehl ich P
O l s en P
mmo l P kg- 1
Koronivia
Na droloulou
Bat iri
Seqaqa
0.0
0. 1
o. 1
8. 1
2. 1
2.2
16 . 1
4.9
4.9
0.0
0. 1
0. 1
8. 1
0. 9
0.9
16. 1
2.4
2. 1
0.0
<0. 1
0. 1
8.1
0. 1
0. 5
16. 1
2.4
2. 1
0.0
<0. 1
0. 1
8. 1
0. 1
0.5
16. 1
0.2
1.3
77
decreased wi th increas ing pH and Olsen P decreas ed up to about pH 5 . 5
to 6 . 5 , above which i t increas ed .
The ini t ial decrease in Olsen P with increas ing pH was a t t ributed to
an ar t e fact in th e O l s en me thod ( Sorn-s r ivichai et al . , 1984 ) , whereas
the decrease in Mehl ich P wi th increas ing pH was attributed to a decreas e
in the ef f iciency of extra c t ion caused by an incr ease in pH o f the extrac tant .
In view of these problems associated with the Mehl ich and Ol sen
p ro cedu res and the fa c t that plant growth general ly increases in the pH
range up to
- 6 . 0 i t may b e d if ficul t to relate th ese soil P tes t resul ts
to th e up take of P by plants .
wil l b e cons idered in Chap ter 8 .
This aspec t of soil-tes ting p ro c edures
CHAP TER 6
78
CHAPTER 6
EFFECT OF LIMING ON PHOSPHATE S ORPTION BY ACID SOILS
6. 1
Introduc t ion
I n addi t ion to r educ ing Al toxic ity , l iming a f fects the amount of P
sorb ed by a c id s o il s , and th es e effects a re var iabl e .
For example ,
l iming of s o ils has b een shown to d ecr eas e (Woodruff and Kamprath , 1 9 6 5 ;
Lop ez-Hernandez and Burnham , 1 9 74 ;
(Lucas and B lu e , 1 9 7 2 ;
Ryan and Smill i e , 1 9 7 5 ) , increase
Mokwunye , 1 9 75 ;
affect (Woodruff and Kampra th,
S ims and Ell is , 1983) or no t
1 9 6 5 ) the sorption o f added P by s o il s .
Various reasons have b een advanced to exp lain these con tra s t ing
e f f ec t s .
Fo r exampl e , Haynes ( 19 8 2 ) attributed the contrast ing effects to
incons is t encies in the s o il incubat ion procedures , sugges ting that the
interv ening drying process b e tween the addition of l ime and P could have a
s ignif icant effec t on the P-s o rption charac t er is t ics of soil s .
However ,
Ba rrow ( 1 9 84 ) attributed the variable effects of l iming to 4 fur ther
facto r s , v i z :
the pH over which sorp t ion is measured , the background
el ectrolyte , the amount o f desorbable P in the s o il , and the r el a t io nship
betwe e en pH and the po ten t ia l on the soil surfac e .
From these s tud ies it
is appar ent tha t the experimental cond i t ions u s ed may substant ial ly
inf luence the resul ts ob taine d .
A s d is cussed in Chapter 1 , h ighly-weathered , a c id so il s are relatively
common in Fij i and large areas have low p roduc t ion potential mainly due to
acid i t y , Al toxic ity , and P d ef iciency prob l ems .
A clear unders tand ing
o f the eff ec t of so il ac idity and l iming on P ava ilab il ity is required prior
to the development of thes e infer t il e so il s .
To this end the effec t of
soil pH on the s o rption o f P by four soils from Fij i was inves t igated .
S orpt ion was also s tud ied us ing sel ec ted l imed s o i l s which had also
p reviously been treated with added P .
Attemp t s were made to explain the
t rend s ob tained in terms of the interact ions of Al , Ca , and P .
79
6. 2
Ma t e r ia l s and Me t hods
6.2. 1
Preparat ion and prel iminary analysis o f s o i l s
Detailed descrip t ions o f the s o ils used and their preparation have
·
b een g iven in Chapter 3 .
Each o f the � 4 - - so ils., B a t ir i , Koronivia ,
Nad roloulou , and S eqaqa were inc ub ated with either 9 o r 10 ra t es o f
l ime to ob tain a range o f pH va lues u p t o 7 (F ig . 4 . 4) .
Af ter the
incub a t io n perio d , wh ich l as t ed 4 2 d , the l imed soils were air dried
at a t emp era ture o f 2 0
±
0
2 C and subsamp l es of the Ko ronivia and Seqaqa
I
-1
soils were reincubated with 8 . 1 and 1 6 . 1 mmo l kg
s o il , resp ec t ively o f
added P .
These soil s were s el ec ted for incubat ion with P b ecause they
repres ented the lowe s t and the h ighes t P-re t ention capac i ties o f the
group of s o i l s in this s tudy .
S el ec t ed samples of th e l imed so il s in the pH range - 4 to - 7 , were
subs equently chosen for this s t udy (Tab le 6 . 1 ) .
6.2.2
Incubation o f s o il s with KOH
Dur ing the cours e o f the sorption s tud ies it was ob served that in
all the s o il s the equilibrium s o l u t ion P conc entra t ion t ended to decreas e
at h igh pH values .
To inve s t iga t e whether this was due to the presence
of Ca o r due to the format ion of X-ray amorphous Al -hydroxy po lymers of
h igh r ea c t ivity (Amarasiri and O l s en , 1 9 7 3) , samples of Nadroloulou soil s
were incub ated with KOH as d e s c r ib ed b elow .
The Nadrol oulou s o il was
chosen b ecause it contained very l arge amounts of M KCl -extrac table Al .
Sub samp l es of the Nadroloulou soil were thoroughly mixed with
varying rates of KOH and incub at ed for 28 d under condi t ions s imilar to
t hose used for l ime add i t ion ( Chapter 3 , S e c t ion 3 . 4 . 1 ) .
At the end o f
28 d , the incubated soils were a i r dried , passed through a 2 -mm s ieve ,
and s to red in polythene bags f o r further analy ses .
The incub a t ion p er io d
was d ec r ea s ed from 4 2 d t o 2 8 d b ecaus e o f the mo r e r eac tive nature o f
KOH .
6.2.3
Phosphate sorptio n s t ud ies
Sorp t ion was inves t iga ted a t ; 2
ra tes of P addit ion (Tab l e 6 . 2 ) ;
one rate was common to each o f the 4 l imed soils whereas the s econd ra t e
varied depending on the P-re tent ion capacity o f the s o il .
Sorp t ion o f
P was d e termined b y weighing dup l icate 1 . 0-g samples o f soil into c entr ifuge
80
Tab le 6 . 1
pH values of the soils s elec ted to investigate
the effect of pH on P sorption.
S o il
LO
Ll
L2
L3
L4
L5
Koronivia
4.2
4 .4
5.2
5.8
6.6
7.5
Nadroloulou
4.0
4 .4
5.4
6 .0
6.8
7.4
Bat iri
4.9
5.3
6.5
6.6
7.4
S eqaqa
4.5
5.2
5.6
6.5
7. 1
81
Table 6 . 2
Init ial s olut ion P conc ent rat ion in the background
electrolyte during the inves t igat ion of the
ef fec t of pH on P sorp t io n .
Ini t ial solut ion P concentration
Soil
P2
Pl
mmol L - 1
K.oronivia
0. 43
1 . 08
Nadroloulou
1 . 08
1 . 61
Batiri
1 . 08
1 .47
Seqaqa
1 . 08
3 . 23
82
tubes togeth er with 15
mL
o f 0 . 0 2M CaC1
2
of toluene to prevent microb ial a c tion .
solution containing a few d rops
Af t er the add i t ion o f an
appropriate quantity o f P (as KH Po solutio n ) the solut ions were made up
2 4
to 30 mL with deioni z ed wa ter to give a f inal CaC1 conc entra t ion o f O . O lM .
2
- °
The tubes were then s haken for 7 2 h a t 2 0 ± 2 C .
A s eparate set o f
control t ub e s containing the soil s and the b ackground e l e c tro ly te , b u t with
no added P , were shaken at the same t ime .
At the end o f the shaking
per io d , the pH of the susp ens ion was measur ed .
The t ub es were then
c entrif uged and the �upernatant solution f i l t ered through a Whatman No . 5
f il t er paper .
The Ca and P concentrations in the solution were determined
us ing AAS and the calo rimetric me thod of Murphy and Ri l ey ( 1 9 6 2 )
respec t iv ely .
The above pro c edure was rep eated to inves tigate the effec t of pH on
the so rpt io n o f P a t 1
ra te o f added P by samp l es o f the Ko ronivia and
Seqaqa so il s which had b een l imed and sub sequently incubated with P .
The rates o f added P used in the so rption s tu dy were 0 . 2 7 and 2 . 1 5 mmol L- 1
fo r the Ko ronivia and S eqaqa soils
r espect ively .
The e f f ec t o f pH on the so rption o f P by Nadrol oulo u soil , incubated
with l ime o r KOH , was also inves tiga ted using either H o , 0 . 0 1 , 0 . 05 , 0 . 1 ,
2
or lM KCl a s the ba ckground e l ec trolyte .
The method us ed was as
-1
describ ed above , except that only one rate ( 1 . 6 1 mmol L ) o f added P was
used .
At the end o f the shaking per io d , the pH of the suspens ion was
measured .
The Ca and P concentrations o f the supernatant solut ions were
det ermined as d es c r ib ed abov e .
The concen tra t ion o f K in the superna tant
solu tions of KOH-inc ub ated soils was also d e termined us ing a Gal l enkamp
f lame photomet er .
6.3
Res ul ts and Discuss ion
6.3.1
Ef f ec t o f l iming o n pH, extra c tab l e aluminium,
and O l sen-ext rac tab l e pho sphate
Al l soils were init ially s trongly acid (Tab l e 6 . 1 ) , with pH (M KCl )
values varying in the rel atively narrow range o f 4 . 0 to 4 . 9 .
The amoun ts
o f A1 extrac ted by M KCl varied cons iderab ly b etween the so il s , ranging
-1
-1
from 0 . 3 mmol kg
in the Ba tiri soil to 3 5 . 6 mmol kg
in the Nadroloulou
soil .
The ra tio o f oxalate- to d i th ioni t e-extrac tab l e Al and Fe varied
83
from 0 . 06 to 0 . 6 3 and
< 0 . 0 1 to 0 . 14 � respe c t ively (Tab le 6 . 3) ,
r ef lec t ing the wide varia t ion in the nature o f the metal oxides present
in the s o il s .
Incub a t ion wi th lime genera ted a s er ies o f s ampl es for ea ch so il with
pH values ranging from abou t 4 to 7 . 5 (Tab le 6 . 1 ) .
As soil pH incr eased
there was a sharp decrease in M KCl - ext rac table Al values (Fig . 6 . 1 ) .
The amoun ts o f P extrac ted by the Ol s en reagent were low (0 . 0 1 - 0 . 1
-1
mmol kg ) in the soils which had no t b een incub a t ed with add ed P
(Chapt er 5 , S ec t ion 5 . 3 . 2 ) .
When the Ko ronivia and Seqaqa s o ils were incubated with added P there
was a smal l change in the pH of the soils and al so the amount o f M KCl
ext rac tab le Al .
However , Ol sen- extrac table P increased at equivalent
l ime l evel s in each soil ( Chap t er 5 , S e c t ion 5 . 3 . 2) .
6 . 3. 2
Effec t of pH on phospha te sorption
The s o ils d iffered ma rkedly in the ir ab ility to sorb P at all pH
values (Fig . 6 . 2 ) .
In general , sorp tion decrea s ed in the o rder Seqaqa >
Ba tiri > > Nadroloulou > Koronivia .
These differences are mo s t l ikely
due to the large varia t ion in the amo unt and na ture of Fe and A1 components
present in th e soils (Table 6 . 3) .
The high sorp t ion observed in the
S eqaqa so il was probably due to the pres ence of large amounts of oxal ate
and d i thionite- extractabl e Fe and Al .
The ac tive component in the
Batiri s o il is l ikely to be free c rys tal l ine Fe oxide minerals which a r e
p resent i n abundance ( > 1 3%) and wh i c h r esul t i n a n e t surfac e pos i t ive
charge (Chap ter 4 , S ec tion 4 . 3 . 2 ) at the natural soil pH .
There was l i t tle effect of pH o n the amo unt of P sorbed a t low
ini tial s o lu t ion P concentrat ions particularly in the high P-sorb ing
Batiri and S eqaqa so ils (Fig . 6 . 2 ) .
This was surpris ing because even
at the lowes t P additions the amo unt added during sorption s t udies was a t
l eas t 1 0 times h igher than that usually added to thes e highly-weathered
soil s in the f ield .
However , in all the s o ils , at very h igh initial
s olu t ion P concentrations (Table 6 . 2 ) the amo un t of P sorbed decreased ,
pas s e d through a minimum b etween pH 5 . 5 - 6 . 5 , and then incr eased again
(Fig . 6 . 2) .
This trend was mos t p ronounced in the low P-sorb ing
Koronivia s o il .
the soils .
The pH at which minimum sorp t ion o ccurred varied be tween
A similar effect o f pH on P s o rp tion was repo rted by Marsh
( 1 9 8 3) for a range o f l imed vol cani c ash s o ils from N� Zealand and mo re
84
NADRCl.CXJL.ClJ
- 0
·o
Cl)
-
I
p
� 10
0
E
E
<(
-
15
C1l
tj
�
u
KORONIVIA
5
0
tj
�
10
�
L&.J
u
�
-
:l:
SEQAQA
5
0
�J
Fig . 6 . 1
BATIRI
4
�
5
pH M KCl
6
•
•
7
Ef fect o f s o il pH on the amounts o f Al
extra c t e d from l imed soil s by M KCl solution .
•
85
Tab l e 6 . 3
Amounts o f c rys talline f ree Fe and Al , and sho r t-range
o rder Fe and Al ext rac ted by c i t rate -dithionit e-b icarbonat e
and acid ammonium oxal a t e reagents , respectively , a n d the
ratio o f o xalate- to d i thionite-extrac tab le Al and Fe .
So il
Oxala t eext ractab l e
A1
Fe
Dith ioni t eext rac tab le
A1
Fe
Oxala t e- : di th ioniteextrac tab l e
A1
Fe
IIDilO l kg- 1 soil
49 . 5
68. 1
193
472
0 . 26
0 . 14
111 .9
62 . 9
288
951
0 . 39
0 . 07
Batiri
50 . 0
10. 7
820
2 39 9
0 . 06
<0 . 0 1
S e q aqa
643 . 4
94 . 4
10 2 5
1487
0 .63
0 . 06
Ko ronivia
Nadro loulou
86
Wri
96·
0
SE AQ A
Q
0
95·5
l
•
....
I
I
10
Fig . 6 . 2
Effect of soil pH on the amount of P sorbed ( ---- ) a t
2 initial sol u t ion P concentrat ions ( P l
P2
& ,
=
•
and
Tab l e 6 . 2) and the concentra t ion of Ca ( -
- -)
in the con tro l samples C • ) and in the presence o f added P
(Pl
=
=
0
and P 2
=
ll.
,
Table 6 . 2) .
�
0
-
87
recen tly b y Barrow ( 1 9 84 ) for CaC03 -trea t ed Aus tral ian soils .
Al though the d if f er ences in P sorp t ion were smal l , they were
sufficien t to produce a s ignificant increas e in equil ibrium P concentration
with increasing pH .
For examp l e , a t an ini t ial solution P concentra t ion
o f 1 . 08 mmol L - 1 , the equilib rium solution P concentra tion increa s ed f rom
0 . 00 3 mmol L- 1 in the unl imed , h igh P - so rbing S eqaqa soil to 0 . 00 5 mmol L- 1
a t pH 5 . 2 whe re minimum P was s orbed .
Be cause the concentra t ion o f P
in the s o il solution is usually very low , th is
- 7 0% inc rease in
equil ib rium solution P may have a b eneficial e f f ec t on plant growth .
Resul ts relat ing to this effect are repo r t e d in Chap ter 7 .
A s imil ar
bu t somewha t larger increase in equil ibrium solu t ion P concentra tion was
ob s e rved for the o ther s o il s .
The variat ion in the amount of P sorb ed wi th incr eas ing pH was apparent
regard less of wheth er the soil s had been incubated with added P (Fig . 6 . 3 )
after the ini tial incub at ion wi th l ime .
However , the amoun t o f P sorbed
by soil s which had b een incub ated wi th bo th l ime and P was lower than
that sorb ed by so il s incubated wi th l ime only .
These resul ts indicate
tha t some of the previous ly added P was p robab ly o ccupying P-sorp tion
sites , b locking th em from further react ion (Barrow , 1 9 Sg ) .
The ob served ini t ial decrease (Fig . 6 . 2 and 6 . 3 ) in the sorp t ion o f P
was probably due to a comb ina tion of factors .
These would include the
neutra l i zat ion o f Al 3+ and hydroxy Al as A1 hydroxid e during l iming
r es ul t ing in a decrea s e in the numb er of P-s o rp t ion s i tes (Syers et al . ,
19 7 1 ;
Sanchez and Ueha ra , 1 9 80 ) .
Bowden e t a l . ( 1 9 7 7 ) have sugges ted
that an inc r ease in s o il pH , such as occurs on l iming , caus es the sur fac e
to b ecome increas ingly negative , resul t in g in a decrease in the elec tros tatic potent ial in the plane of adsorp t io n .
This would resul t in a
decrease in the s tr ength of sorp tion of P wi th inc reas ing pH .
The increas es in sorp tion on the h igher pH s id e o f the observed minima ,
al though repo rted by s everal wo rkers , are more d if f icul t to explain .
Bowden e t al . ( 1 9 7 7 ) s ugges ted that wi th increas ing pH , the concentration
o f HP04 2 - would inc r ease 10 fold with each unit rise in pH and this ion may
be sorb ed p referentially , bu t in a later s tudy (Bowden et a l . , 1 9 80b ) i t
was suggested that this increase in concentration o f the divalent HP0 4 2 ion with increas ing pH would only be suffi c ient . to s low down the rate o f
dec rease i n sorp t ion and no t cause an inc r eas e .
In a recent review ,
Haynes ( 1 985) has s ugges ted that the add i t ion o f h igh rates o f l ime may
increase the ionic s trength and the conc entration o f Ca in the sorp tion
88
66 {a) Seqaqa
65
•
i
�
0 63
1 9 (b) Koronivia
••
•
•
•
�
"0
�
'-
� 18
�
'E
�
§E
<(
17
I
I
16
5
Fig . 6 . 3
6
pH 0·01MCaCl2
Effect o f pH on the amount of P s o rbed by unt reated ( .& )
7
and P-incub ated ( e ) Koronivia and S eqaqa soils at init ial
solution P concentrat ions o f 0 . 69 and 2 . 15 mmol L-1 ,
r espec tively .
!I
89
medium and tha t this , in turn , may have an effect on P s o rp tion .
The
r esul t s s hown in Fi g . 6 . 2 indicate tha t the conc en tra t ion o f Ca in the
control s amples (no a dded P) increased by a factor of about 1 . 1 when
s o il pH was incr eas ed from 4 to about 7 , indicat ing a small inc reas e in
th e ionic s trength of the sorp tio n med ium .
A s imilar increase in Ca
conc entrat ion was no t ed for the systems containing added P .
A survey o f th e l i terature sugges t s that whenever a n inc rease in P
sorption has b een ob s e rved at h igh pH val ues , either Ca was used as the
l iming material or was present in background elec trolyte .
l ed autho rs (Fox et a l . , 1 9 7 4 ;
Parf i t t , 1 9 7 9 ;
This had
Bar row e t al . , 1 9 80 ;
Sanchez and Uehara , 1 9 80) to propose the precipi ta tion o f cal c ium
phosphate ( Ca-P ) as the mechanism respons ib l e for the de crease in
solution P concentrat ion at h igh pH valu es .
Thus , us ing the method o f
Cl ark a n d Peech ( 1 9 5 5 ) , Barrow e t al . ( 19 80) showed that the decrease in
P concentra t ion at h igh pH values during th e so rp t ion o f P by goethi te
coul d be due to the f o rmation o f insolub le Ca-P compound s .
However ,
in a mo re r ecent s tudy , Barrow ( 1 984) foun d that sorp tion increased at
high pH values even wh en the concentra t ion of Ca was too low for
precip i t a t ion to occur .
Helyar e t a l . ( 1 9 7 6 a , b ) rel ated the increas e
in sorpti on by gibb s i t e at h igh pH val ues to the pres enc e of Ca and
p ropo s ed surface compl ex format ion b e tween s o rb ed P and solution Ca and P .
When the values fo r pH , ·ea concen trat ion , and P concentra t ion from
the.
s e c t ion o f the graph where sorption increased (Fig . 6 . 2 ) were plotted
(Fig . 6 . 4 ) us ing the method o f Cl ark and P ee ch ( 1955) and Barrow e t al .
( 1 9 80 ) , they fell cl o s e to the l ines representing bo th a c ta-cal cium
phosphate and hydroxyapatite .
F rom these r esul ts , it . seems possible
that the increased so rption above about pH 5 . 5 - 6 . 0 was probably due to
the f o rmat ion o f insoluble Ca-P c ompounds .
However , confo rmity o f
solution concentration data t o sol ubility equil ibria do e s no t neces sarily
prove that precipitation has o ccurred becaus e kinetics , including the time
requi r ed for nucl ea tion , are igno red .
·
Nevertheles s , further evidence
fo r the format ion of insolub l e Ca-P compounds at high pH values was
presen ted in the iso topic-exchange s tudy repo.rted in Chapter 5 .
6 . 3. 3
Effect o f backgound electrolyt e on pho spha t e sorption by
Nadroloulou soil in cubated wi th cal cium o r po tassium hydroxide
To further inves t iga t e the apparent increase in P sorp t ion at h igh pH
describ ed above , sampl es o f the Nadroloulou soil were incubated wi th KOH to
90
•
•
•
•
Nadroloulou
Seqaqa
Koronivia
Batiri
6
Cl
u
CL.
......
+
DicalcilJTl
phos�e
8
..
0
�
"
I
a.
Octo -calcilJn
phOSJflate
10
4
Fig .
6.4
6
pH - i pCa
8
10
Values for pH and Ca and P concentrat ions f rom
the sec t ion on the pH-P sorp tion curve where
sorption increas es , plot ted according to the
method of Clark and Peech ( 1955) .
91
give a range o f pH values comparable to that ob tained earlier by incubation
with Ca (OH) 2 (Section 6 . 2 . 1 ) .
and KOH-treated soils were
2
then u s ed to inves tigate P sorption from a variety o f background electrolytes .
Bo th the Ca (OH )
When P sorp tion by the KOH- incubated soils was s tudied , us ing KCl
as the background el ectrolyte ( Fig. 6 . 5 ) , the amount o f P sorb ed decreased
s t eadily with increas ing pH , irrespec tive of the concentrat ion of the
elec tro lyte ;
there was no evidence o f a tendency for sorpt ion to increase
at h igh pH .
The lowest amo unt o f P was sorb ed when wa ter was used as
the suppo rt medium (background electrolyte) and the highest amount from
the 0 . 05M KCl medium.
In contras t , when sorp t ion was conducted with soils which had been
incubated wi th Ca (OH)
(Fi g . 6 . 6) and H o was used a s the background
2
2
el ectroly t e , the amount o f P sorbed pass ed thro ugh a minimum at about
pH 6 . 9 above wh ich it increa s e d again .
Interes t ingly, the rate o f
increase i n sorpt ion o f P above p H 6 . 9 decreased consistently with the
increase in the conc entrat ion of the b ackground elec tro lyte (Fig . 6 . 6 ) ,
such that this effect was not observed in M KCl .
This apparent increase in sorption above pH 6 . 9 may be due to one or
more o f the three reasons advanced previously , viz : the precipi ta tion of
inso lub le Ca-P compounds (Barrow et al . , 1980 ;
Sanchez and Uehara , 1 980) ;
the formation of a surface P-Ga-P complex (Helyar et al . , 1 9 76a , b ) or an
increase in ionic s trength a t h igh rates of l ime addition (Haynes , 1 9 85) .
To inves tigate the las t pos s ib ility , K and Ca concen trations were measured ,
in the H o extracts o f the KOH- and Ca (OH) -incubated soils , respectively .
2
2
When sorpt ion was conducted in H o using the Ca (OH) -incubated soils , the
2
2
-1
concentration o f Ca ranged f rom undetec table amounts to 0 . 3 mmol L
In contras t the concentration o f K in the KOH-incubated soils ranged
-1
from <0 . 01 t o over 4 . 5 mmo l L
a t the highest pH .
Thus there was a
much l arger increase in ionic s trength in the presence o f KOH than there
was in the presence of Ca (OH ) •
Therefore the increas e in P sorption
2
at high pH values is unlikely to be due to an ionic s trength e f f ec t .
I t would appear that the inc rease in sorp tion is due either to insoluble
Ca-P o r surface complex formation , as sugges ted by Helyar et al . ( 19 76a , b ) .
However , Parfitt ( 1978) has c ommented that the surface complex mechanism
of Helyar et al . ( 1 9 76 a , b ) would sugges t interac t ion on the (001 ) face o f
g ibbs ite which was no t likely from infra-red s tudies .
Furthermore ,
i f there was a complex forma t ion o f the type propo sed by Helyar et al .
then exchangeab le P would b e expected to increase s teadily even at the
92
45
'?
40
0
V)
iO'\
=0 35
E
E
-,:::]
CLJ
.J:l
'-
0
V)
Cl..
�
0
+-
c:
�
0
E
<(
15
10
4
Fig . 6 . 5
5
6
pH of sorption medium
7
Effect of b ackground elect rolyte concentration
on the amount of P sorb ed by Nadroloulou soils
incubated with KOH at an initial solution P
concentration o f 1 . 61 mmol L-1 .
8
93
5
4
Fig . 6 . 6
7
6
p H of sorption medium
Effect of b ackground electrolyte concentrat ions
on th e amount o f P sorb ed by Nadroloulou soils
incub ated wi th Ca(OH ) z at an ini tial solution
P concentration of 1 . 6 1 mmol L-1 .
(HzO
=
1M KCl
e
=
;
0 . 0 1M KCl
• )
•
=
&
;
0 . 1M KCl
=
•
,
•
7·5
94
high es t pH , and not decrease as reported for the soils in the p resent s tudy .
I nspec t ion o f Fig . 6 . 7 , shows that the pH o f the sorption medium
differs cons iderably b etween the various b ackground e lec trolytes .
Interes tingly fo r the cases where a minimum in sorp t ion was no t observed ,
the pH of the sorption medium for the soil with the highest ra t e o f
Ca ( OH ) 2 addition was less than 6 . 9 .
To verify whether the decrease in solution P concentration at high
pH values was a pH effect , a separa te sorp tion s tudy was conducted by
inc reasing the pH o f the background el ectro lyte in which a minimum in
sorpt ion was no t observed to values higher than 6 . 9 .
by
�-
add ition
of dilute NaOH solut ion .
This was achieved
The results of this s tudy
showed ( dot ted l ine , Fig . 6 . 8) an in crease in sorp t ion even wh en M KCl
,
was u sed as the background el ec trolyte , provided the pH was higher than
6.9.
This investiga tion to gether wi th those repor ted earlier on KOH-
and Ca ( OH ) 2 -incubated Nadro loulou soils suggest that the observed decreas e
in solut ion P concentration a t high pH values was probably due to an
interaction b etween pH , Ca , and P , such that an insoluble Ca-P compound
was fo rmed .
Th is is consis tent with the results o f the iso topic-exchange
s tudy repo rted in Chapter 5 ( Section 5 . 3 . 1 ) , which showed an init ial
in crease in exchangeab le P a t low pH which was followed by a sharp
decrease a t high pH values .
Such a sharp decreas e in isotopic exchange
at h igh pH was interpreted (Murrmann and Peech , 1 9 6 9 ) as being due to
the format ion o f insolub le Ca-P compounds .
6.4
Conclus ions
Liming of soils had a variab le effec t on P-sorption capac i ty .
In
general , P sorpt ion decreas ed with an increase in the rate of l iming up
to about pH 5 . 5 - 6 . 0 , above which sorp tion increased .
However , the rate
o f initial decrease in sorp t ion varied markedly b etween soils , such tha t
in the high P-sorbing Batiri and Seqaqa soils , the decrease was insignificant
relat ive to the amount of P added .
Nevertheless , an appreciable increase
in equil ib rium solution P concentration was ob s erved with all o f the soils
up t o the pH of minimum sorp tion .
As would b e expected , incubation o f soils wi th added P fo llowing
l iming resul ted in a larger decrease , with increas ing pH , in the amounts of
P sub sequently sorbed as compared to the soils with no previously added P .
95
7-5
6· 5
E
::::J
·-
-o
Ql
E
c
.o
5·5
�
a.
L.
0
V)
0
:J:
a.
•
A
•
4�5
•
4
Fig . 6 . J.
I
5
pH of SOil
H20
0·01M KCl
0·1 M l<l
1M �
I
6
7
Effect o f electroly te concentrat ion on pH o f
the sorp t ion medium in the Nadroloulou s o il s
incub ated with Ca (OH) 2 .
96
4
Fi g . 6 . 8
6
7 7-5
5
pH of sorption medium
Effect o f increas ing the pH by addi tions of
dilut e NaOH to s o il s previous ly l imed to pH 6 . 9
on the .amount o f P sorb ed by Nadroloulou s o il
s uspended in M KCl .
97
Th e obse rve d ini t ial decrease : in sorp t io n was probably related to the
decrease in reac tive Al and also to an increase in surface nega tive charge
r esul t ing f rom increa s ed s o il pH .
The in c r ease in surface negative charge
would result in a dec rease in the el ec tro s tatic poten tial in the plane
o f ads o rp tion , thereby decreasing the sorpt ion of P ( Bowden et al . , 1 9 7 7 ) .
From a s o rp tion s tudy o f soils incubated with KOH and Ca ( OH ) z and
u s ing a vari ety of e l e c trolyte con cen tra t ions , it was conc luded that the
p re s ence o f ea was a s sociated with the inc rease in P s o rp tion at high
pH values .
These investi ga tions also showed that , at the solu tion P
and ea concentrat ions s imilar to thos e encoun tered in the pr esen t s tudy ,
it was e s s en t ial for the pH of the sorp t ion medium to be higher than
6 . 9 for a decreas e in solution P concentration to be ob tained .
CHAP TER 7
98
CHAPTER 7
L IME-ALUMINIUM-PHOSPHATE INTERACTION AND
THE GROWTH OF LEUCAENA LEUCOCEPHALA
7.1
Introduc tion
Low c rop yields i n much o f the humid tropics are a s soc ia ted with
high s o il a c idity and / o r acute P deficiency .
These two factors are no t
independent and one o f the prima ry reasons commonly propo sed for l iming
acid s o il s is to inc r ea s e P ava ilab il i ty to plants ( S anchez and Uehara ,
1 9 80) .
The mechanisms by which an increa s e in P avai labil ity occurs
are far f rom clear but are known to involve the so il and plant components
of the s ys t em .
A c i d soils f r e quent ly contain phyto toxic l evels o f solub l e and
exchang eab l e Al (McLean , 1 9 76 ;
1 9 82) .
Foy and Fleming , 1 9 7 8 ;
Webb er et al . ,
A maj or chara cteristic o f Al toxic ity is an inhibi tion o f the
upt ake and t r ansloca t ion o f P by plants (Foy and Fl eming , 1 9 7 8 ;
and Fox , 1 9 7 8 ) .
Jones
Thus , l iming acid soils o f t en increa s es P uptake by
plants by decreas ing Al toxicity rather than by an e f f ect on soil P
availab i l i ty , per s e
(Vickers and Zak , 19 7 8 ;
Haynes and Ludecke , 1 9 8 1 ) .
Con f l i c t ing views are held as to the e f f e c t s o f l iming on P
availab i l i t y in highly-weathered , acid soils and the s e have been d iscus sed
in Chap ters 5 and 6 .
Neverthel ess , l iming has general ly b een found to
increa s e p lant growth up to pH values where M KCl-extrac table Al is reduced
to very low l evel s .
Lime in excess of this amount has been reported to
decrea s e p lant growth (Amarasiri and Ol sen , 1 9 7 3 ;
1975 ;
Juo and Uzu , 1 9 7 7 ;
Sumner , 1979 ;
Janghorbani et al . ,
Far ina e t al . , 19 80a , b ) .
S everal cont ras t ing r easons have b een put fo rward to explain the
d ecreas e i n p lan t growth a t high pH values .
For exampl e , Fox e t al .
( 1 964)
attribu ted the decrea s e to a reduc t ion in ava il ab l e P caused by the
format ion of ins o l ub l e Ca-P compounds in the s o il .
e t al .
In con tras t , Friesen
( 1 9 80b ) a t t r ibut ed the decrease to induced Zn def ic iency .
Far ina et a l . ( 1 9 80a , b ) related the decrea s e to the presence o f high l evel s
99
o f Al in the plant t is s ue at soil pH val u es where growth was d epres sed .
In view o f this, i t appears tha t there is a need for f urther s tudies o f :
( i)
the intera c t ion b etween l ime , Al and P in acid so il s and
( i i) how this interaction affects the ava ilab i l i ty of P as a s s es sed by
p lant up take .
I n this s tudy , four Fij ian soils wer e incubated with different amoun ts
o f .. l ime and P, in a factorial des ign , to produce soils covering a range
of pH and P s tatus , and the effec t of th es e trea tmen ts on the growth o f
the tropical l egume Leucaena leucocephal a was inves t iga t ed .
7.2
Ma ter ial s and Me thods
7 . 2. 1
So ils
The 4 so il s u sed in this s tudy have b een describ ed previously
( Chap t er 3 ) .
Each s o il was incuba ted with a range o f l ime (Fig . 7 . 1 )
and P ( Tab l e 7 . 1 ) addi t ions .
The exper imental des ign us ed with 3 o f the
4 soil s was a repl icated 5 x 3 l ime ( Ca (OH) 2 ) x P complete fac to rial .
With the f o urth so i l (Koronivia ) only 4 rates o f l ime were app l i ed b ecaus e
of the l imi t ed s upply o f this s o il .
Th e l ime rates were cho s en so as to
ob tain app ro ximat ely 0 , 3 0 , 7 0 , and 1 0 0% redu c t ions in the amo unts o f
M KCl-extrac table Al o r ig inal ly present i n the s o il .
were ·added :
LOne
Three rates o f P
was common to all s o il s and , o f the o ther 2 r a t es ,
1
wa s very low and the o ther d epended on the P-sorpt ion capac ity o f the
soil ( Tab l e 7 . 1 ) .
L ime wa s incorporated into r eplicate (4 ) s amp l es of so il by tho roughly
mixing in Analyt ical Grade Ca (OH) 2 : and incub a t ing a t field capacity
at 20
±
0
2 C for a period o f 4 2 d .
D i s t il l ed water was added every
second day to compensate for evaporative lo s s es .
Af ter each addition o f
wate r , the so il s were r emixed tho roughly .
At the end o f the incubation
°
perio d , the s o il s amp l es were air dried a t a temp erature of 20 ± 2 C and
passed through a 2-mm sieve .
The repl i ca tes a t each l ime l evel were then
bulked , thoroughly mixed , and reweighed .
Subsampl es of the l imed soils
were reincub a ted wi th various l evels of P (Tab l e 7 . 1 ) under the same
condit ions .
Af ter 14 d , the l ime-P trea ted s o il s were air dried a t a
t emperature o f 20
±
0
2 C , pass ed through a 2-mm s i eve and sampl es at each
treatment level were bulked and mixed thoroughly .
Subsampl es o f thes e
soils were used f o r chemi cal analyses and plant growth s tudies .
MA SSEY U N lV�R SITY
J.IJ�B.AR'(
100
Tab l e 7 . 1
Amounts of P added to l imed so ils prio r to
incub at ion and growth of Leuca ena l eucocephal a .
p
So il
P1
app l ied
P2
p3
mmo l kg - 1 soil
Koronivia
0 . 40
2.42
4 . 84
Nadro l oulou
0 . 80
4 . 84
9 . 68
Bat ir i
1. 61
4 . 84
14 . 52
Seqaqa
1 . 61
4 . 84
1 4 . 52
101
7.2.2
Plant growth s t udy
Th e plant growth s t udy was conduc ted in a contro ll ed-c l imat e
0
l ab o ratory , at average day and nigh t t empera tur es o f 30 and 20 C ,
respe c t ively , and r el a t ive humidities o f 7 0 and 8 5%, respectively .
Lime- and P-t reated s o il s ( 500 g ) were placed in 100mm x lOOmm-plas tic
conta iners , wi th polythene b ag liners no contain any l eacha t e .
trea tment was rep l i cated 4 t imes .
Ea ch
The po t t ed soils were mo is tened to
f ield c apacity with di s t il l ed water : Three · s eeds o f the tropical l egume
Leucaena leucocephala , wh ich had b een soaked in dis til l ed wa ter for 24 h ,
were planted i n each po t .
The po ts were covered t o minimi ze evapo rat ion
and the s eeds al lowed to germina te .
Af ter 14 d the pots con taining mo re
than one plan t were wa tered and then thinned to one plant per po t .
Where no s eeds germina ted , s eedl ings w ere trans planted from a s epara te
po t containing 1 4 - d old plants .
Where pos s ib l e , s eedlings were trans
planted from repl icate po t s o f the s ame trea tment .
were wa tered to f ield capac i ty daily .
Thereaf ter the po ts
Solu t ions ( 20mL ) cont aining all
maj o r and mino r nutrients except P (Middl eton and Toxopeus , 1 9 7 3 ) were
added every s e cond day to each po t , p r io r to addition of dis t illed wa ter .
Plants wer e checked da ily and intac t , dying l eaves were examined , removed ,
and s to red in lab el l ed bags (Jones and Jones , 1 9 7 9 ) .
l ater inclu ded in the yiel d determina t ions .
germina t io n the plants were harves ted .
These l eaves were
Twelve weeks a f ter
The roo ts were carefully
s eparated from the s o il , washed free of a dhering s o il , and the herbage
°
(tops and roo t s ) oven dried for 48 h a t 6 5 C , weighed , and ground fo r
s ub sequent chemical analysis .
7.2.3
Chemical analys es
7 . 2 . 3. 1
So i l s
The pH (Appendix 2) o f incub ated s o il s b efore and af ter cropp ing
was d e termined in M KCl a t a soil : s o l u t ion ra tio o f 1 : 2 . 5 a f t er equil ib rat ion
fo r 16 h .
Prio r to c ropping , sub samp 1 es o f the s o il s were extrac ted with
M KCl us ing the method des crib ed in Cha p t er 3 (Sec tion 3 . 5 . 2 . 3 , b ) .
A1 in
the extrac t was det ermined by the mo d i f ied aluminon method (Hs u , 1 9 6 3) , as
des c r ib ed in Chapt er 3 ,
( S ec t ion 3 . 5 . 2 . 2 , b ) .
Available P was det ermined by the O l sen- ( Chap ter 5 , Sec t ion 5 . 2 . 2 )
and r es in-ext ra c t ion procedures ( Cooke and Hislo p , 1 9 6 3) .
The r es in-extrac t ion
102
procedure was chosen b ecause it i s general ly bel ieved to a c t l ike a
p lant
roo t , in that it provi des a con t inuing s ink fo r available P (Amer et al . ,
1955) .
I n this pro c edure s o il s amp l es were shaken wi th a Cl-exchange
r es in ( ! Dowex � 2-XB , 20-50 US mesh) and 35 m1 o f deionized wa ter for
1 6 h, follow ing wh ich th e b ags containing resin were washed free o f
adher ing so i l us ing deio n i z e d wa ter .
The r es in bags were then shaken
for 1 h with 2 5 m1 of M NaC l solut ion and P was determined in s uitab l e
al iquo t s u sing the calor ime tric proc edure o f Murphy and Ril ey ( 1 9 6 2 ) .
7.2.3.2
P l an t s
Plant ma ter ial ( 20- 100 mg of roo t s or tops) was
ashed a t
4 80 C for a period o f 4 h i n boros il ica te tub es ( 1 5mm x 70mm) , coo l e d , and
dissolved wi th 7 m1 o f 2M HCl (prepared from redistil l ed Analy tical Grade
acid) .
The s amp l es were then analys ed for maj o r and mino r el ements by
P lasma Emis s ion S p ec t ros copy (ICP , Lee , 1 9 8 1 ) .
7.3
Resul t s and Dis cus s ion
7 . 3. 1
Effect o f l ipdng on pH, ex trac tab l e
aluminium , a n d ext rac t ab le phosphate
As d i s cussed previous ly ( Cha p t er 4 , 5 , and 6 ) , all the soils were
s trongly a c i d ( Tab l e 3 . 3 ) , with the init ial pH fall ing in a r el a t ively
narrow rang e o f 4 . 0 - 4 . 9 .
cons iderabl y
among
However , M KCl-extractable A1 val ues varied
th e soils ( Tab le 3 . 3) .
Availab le P , meas ured us ing
-1
the Ol sen-extrac t ion pro cedures , ranged from < 0 . 1 mmol kg
in the B a t iri ,
-l
Na dro l oulo u , and S eqaqa soils to approximately 0 . 2 mmol kg
s o il in the
Ko ronivia s o il (Fig . 7 . 3 ) .
In cont rast to Olsen P , res in P was
-1
mmo l kg
i n all 4 s o i l s (Fi g . 7 . 4 ) .
<0 . 1
With increasing addi tions o f l ime , pH values increased from 4 . 0 - 4 . 9
to over 6 . 0 (Fig . 7 . 1) .
There was a corresponding decrease in M KCl
extractab l e Al ( Fi g . 7 . 2 ) such tha t in each so il it was und etec table a t
or above pH 5 . 2 .
Incubat ion o f l imed s o il s wi th added P resul ted in a s everal-fol d
incr ease i n b o th Ols en P (Fig . 7 . 3) and res in P (Fig . 7 . 4 ) a t equivalent
lime l eve l s in all s o il s .
However , there was
ext ractab le P wi th inc reas ing s o il pH .
'1
cons iderab l e varia t ion in
Thus , in each o f the s o il s ,
103
5
Fig . 7 . 1
I
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200
100
10
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I
15
300
c
BATIRI
•
KORONIVIA
I
20
I
25
mmol Ca(OH)2 kg-1 soil
Effect of increas ing additions o f
C a (OH) 2 o n the pH o f soils .
30
35
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Fig . 7 . 2
BAT IRI
1
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4
7
6
pH
Effect of soil pH on the amounts of Al extracted by
M KCl from soils treat ed wi th 3 rates of added P
(Table 7 . 1 ) .
( e
=
Pl ,
.&
=
P 2 , and
•
=
P3) .
105
BATIRI
SEQ A Q A
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••
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6
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pH
4
...
&
...
•
I
I
6
5
Effe c t o f s o il pH on the amount o f P extracted by the
Ol sen reagent f rom so ils treated with 3 rates of added P
( Tabl e 7 . 1 ) .
( •
=
Pl ,
&
=
P 2 , and
e
=
P3) .
I
7
1 06
BATIRI
06
-
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•
•
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Fig . 7 . 4
..
•
6
5
4
6
5
I
Effect o f soil pH on th e amount o f P extrac ted by anion
exchange res in from soils treated wi th 3 ra tes of added P
(Table 7 . 1 ) .
(
•
=
Pl,
•
=
P 2 , and
.&
=
P3) .
7
10 7
r e s in P increased to a maximum value b etween pH 4 . 4 to 5 . 5 , above which
it d ecreased .
This trend was mo s t pronounced in the S eqaqa s o il which
had b een incubat ed with e i the r medium or high l evels of a dded P .
In contras t to resin P , Ol sen-extrac table P g enerally decreas ed in
s oil s t re a t e d w i th either low or medium amounts of added P (Fi g . 7 . 3) .
At the h ighes t l evel o f added P there was a tendency fo r Ols en P to
incr ea s e at h igh pH values ( Fi g . 7 . 3) .
M KCl-extractabl e A1 d ecreased sl ightly with each l evel o f added P
(Fig . 7 . 2) .
Ef f ect o f l ime and phosphate
7.3.2
addi tions on plant growth
Fo r al l 4 soil s , inc r easing amounts o f added l ime caused an inc rease
in d ry mat ter yield to a maximum value and then a decline (Fig . 7 . 5 ) .
The pH a t which maximum yiel d oc curred varied b etween the soils (Fig . 7 . 5 ) .
For the B a t i ri and S eqaqa s o ils maximum yields were ob tained at about
pH 5 . 2 and for the Koronivia and Nad ro loulou soils they o ccurred a t about
pH 4 . 4 .
However , the l imi t ed numb er of l ime l evels in this s tudy , do es
no t p e rmi t an accurat e definition of the pH a t which maximum growth is
pos s ib l e .
The magni tude of these changes in d ry mat ter yiel d was
s trong ly d ep endent on P sta tus .
Thus , at the lowes t l evel o f added P ,
the e ff ec ts o f l ime additions were small and in each o f Batiri , Nadro loul o u ,
and S eqaqa so i ls , the only s ignif ican t (P < 0 . 0 1 ) inc r ease i n yield
occurred at L 2 .
In con tras t , increas ing l ime a dditions to the Ko ro nivia
soil had a larger effect on crop yield even a t the lowes t l evel o f added P
(Fi g . 7 . 5 ) .
Further additions o f P accentuated the e f f e c t o f l ime on crop growth
(F i g . 7 . 5) .
However , the magnitude o f this e f f ec t varied b etween so i l s .
For examp l e , l iming had a ma rked effect on crop yield a t the medium l evel
o f added P in the Koronivia and S eqaqa soils but i t showed a pronounc ed
e f f ec t on yield only a t t he highes t l evel o f a dded P in the B a tiri s o i l . .
Interes t ingly , al though both the Batiri and S eqaqa soils were incub a t e d
w i t h the s ame amounts o f P and h a d a n identical pH at which maximum growth
was recorde d , l iming had a much mo r e pronounced e f f ec t on crop growth in
t h e latte r s o il , part icularly a t L l and L 2 .
This increased effec t o f
l iming on crop growth in S eqaqa soil was a lso s imilar to the increased
availab il i ty of P as measured by the res in-extrac tion procedure (Fig . 7 . 4 ) .
1 08
20
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Fig . 7 . 5
z
I
I
3
I
5
4
6
pH
7
Effe c t o f soil pH and added P (Table 7 . 1 ) on
d ry mat ter top yiel d .
Ve rtical bars ( I )
repres ent LSD (pH) a t 5 , 1 , and 0 . 1 % l evel
of s ignificanc e .
e
=
P3) .
( •
=
Pl ,
A.
=
P 2 , and
109
In contras t , liming did no t have any major effect on crop growth in th e
Nadroloulou soil (Fig . 7 . 5 ) , even with th e h igh est l evel of added P .
Al though the pH a t which maximum yield o c curred varied b etween soils
( Fig . 7 . 5 ) ,
th e M KCl - ex t ra c tab le Al content o f th e soils was either zero
o r ra ther small (Fi g . 7 . 2 ) at l ime l evels a t which maximum growth o c curred .
L ime in excess o f this amo unt caus ed a depres s ion in plant growth , as
r epo rted abo ve .
Th es e obs ervations confirm th e f ind ings of Woodruff
and Kampra th ( 1 9 6 5 ) , Fries en e t al . ( 1 9 80b ) , and Farina et al . ( 19 80 a , b ) ,
all o f whom obs erved a po s i t ive res pons e o f plants to l ime at o r l es s
t han p H values a t which exchangeab l e Al was reduced to zero .
In these
s tudies , l ime in exc ess o f this amo unt cau sed a depres s ion in plant growth .
Al though crop growth increas ed to a maximum with all 4 soils , there
is some doub t as to wh e ther the rates of P addition were suf ficient to
To inves tiga te this further ,
e nsure that opt imum growth was a chieved .
the conc en t r ation of P in pl ant tops wa s compared with that repo rted in
p revious s t udies ( Gonzal e z et al . , 1 9 80 ;
Maniboo l , 1 9 8 2 ) .
From th is
c omparis on , it appears tha t P was no t l imi t ing in the Koronivia and
S eqaqa so i l s when the h ighes t amo unt of P was added but the P concentrations
in plants grown in th e Batiri and Nadro loulo u soil s were always l es s than
-1
the criti cal concentra t ion (SO mmol kg
, Gonzalez e t al . , 1 9 80 ; Manibool ,
1982) .
Th is was surp rising as the amounts o f P added were consi derably
above tho se commonly used in th e field and the amounts of Ols en-extrac tab l e
P (Fig . 7 . 3) in the s o ils d i d not sugges t P deficiency in the soils
trea ted w i t h medium and high amounts o f P .
As wil l be discussed in
S e c t ion 7 . 3 . 4 , herbage P concen t ra t ion was s trongly inf luenced by l ime
additions and i t is po s s ible tha t , in th ese 2 soils, l ime rates were such tha t
none o f t h e soil pH values ob t a ined in this s tudy were close enough t o the
pH for opt imum growth .
Thus i t is po s s ib l e that if s ligh tly different
l ime rates had b een us ed cons id erab ly higher yiel ds and herbage P
concentrat ions might have b een ob tained .
Somewha t di ff erent resu l t s have previously b een repor ted for th e growth
o f Leucaena leuco cephal a in l imed , a c i d soils (Fox and Whitney , 1 9 81 ;
Fox et a l . , 1 9 8 1 ;
Hu and Wei-Er , 1 9 8 1 ) .
In contras t to the present
ob serva t ions , l iming of Hawaiian acid soils ( Fox and Whi tney , 1 9 8 1 ;
Fox
et al . , 1 9 8 1 ) resul t e d in an increase in yiel d , even at pH Yalues as high
as 7 .
Th is inc r ease in yiel d was at tributed to imp rov�· ea nut rit ion
through l iming·, (Munns et al . , ·1 9 7 7 ;
Fox an-cl \.fui tney , 1 9 8 1 ) .
Howev er , in
the present s tudy, o p t imum l evels o f Ca were added with basal nu t rien t s to
1 10
all t reatme n t s so tha t Ca
was no t l imi t ing plant growth at low pH values .
Al though there was an increase in the concen trat ion o f Ca in plant tops
wi th in creas ing pH through l iming (Appendix 3a- 39) , the concentra t ion o f
Ca in plants grown at low pH was no t deficient ( Gonzal e z e t al . ,
1 9 80 ) ,
sugge s t ing that th e increa s ed growth was no t due to increas ed Ca , � se .
However , pr evious s tudies on L eucaena leucoc epha la have b een conduc ted in
the f ield (Munns and Fox , 1 9 7 6 ) and there are indicat ions that lime
induced yield depress ions in some crops are no t as prono unced under f i el d
cond i t ions as they are in po t experimen ts , particularly b ecause o f the
l imi t ed volume o f s o il in po t experiments .
Al thou gh incubat ion o f soils with added P genera l l y resulted in a
s ign ifican t (P < 0 . 0 5 ) increas e in yiel d wi thin each l ime level (Fig . 7 . 5 ) ,
it did no t elimina te the depress ion in yiel d at h igh pH : ,values .
Sumner
( 1 9 79 ) related a s imilar yield depress ion to P deficiency and the ini t ial
extrac tab le A1 content o f s o il s .
He propo sed that the ini tial l evel o f
exchangeab le Al , at a given pH , r eflected t h e r eac t iv i ty of aluminous
sur faces which, in turn , governed the solub il i ty of P .
From a revi ew o f
lit e ra tu re and f rom h is s tudies , Sumner ( 1 9 7 9 ) concluded that plan t growth
was r educed at high pH only in those soils in wh ich ext rac tab le Al
-l
exceeded 1 . 4 mmo l kg
soil b efo re liming .
He was ab l e to corre ct the
yield depression by the ad d i t ion of high l evels o f P .
Except for the
Bat iri soil , the M KCl-ex t rac tab le Al values in th is s tudy are higher than
the c r i t ical value r epo r t ed b y S umner ( 1 979 ) .
However , the amoun t s of P
used by Sumner at the high es t ra te of appl ication were 2 to 3 t imes high er
than tho s e us ed in the p r es ent s tudy .
I t therefore s eems po ssib l e th at
the d epre s s ions in yield ob s erved in the present s tudy coul d have b een
corre c t e d if very high addit ions of P had b een used .
7 .3. 3
Effec t of l ime and phosphate
addi t ions on roo t growth
L ime and P addi t ions h ad s imilar but l ess pronounced effects on roo t
yield than they had on the yiel d o f tops (Fig . 7 . 6 ) .
Thus, in the low and
med ium P-t reated B a t iri , Nad roloulou , and S eqaqa soil s , l ime add i t ions
had only a small e f f e c t on roo t yield .
In contras t to thes e soils, there
wa s an app reciab l e increa s e in root yield a t L1 in the low and med ium
P-t reated Koronivia s o il s .
Above L1 , l iming caus ed a decrease in roo t
yiel d , as was also ob s erved fo r top yield .
With fur ther addi t ions of P ,
l iming caus ed an inc rease in roo t yield up to the pH a t which maximum plant
111
KORONMA
a
1
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0
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a.
NADROLOULOU
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•
0
Fi g . 7 . 6
•
.1
3
4
�
6
5
pH
=I
7
Effect o f so il pH and added P (Tab l e 7 . 1 ) on
dry ma t ter roo t yield .
Ver t i cal b ars ( I )
represent LSD (pH) a t 5 , 1 , and 0 . 1 % l evel
of signif i canc e .
e
=
P3) .
( •
=
P1 ,
A
=
P 2 , and
112
top growth o ccurred , above wh i ch i t decreased in a l l soils excep t B a t i r i .
In the Batiri so il l iming resul ted in sma l l but cons i s tent increases in
yield with inc r eas ing pH value s .
The mo rphology o f the roo ts also d i f fered among
the l ime trea tments .
Genera l ly , th e p l ant roo t s a t pH values whe re maximum growth was ob tained
had many mo re s econda ry or l a t eral roo ts than tho s e grown in unlimed
soils .
Wh ereas th e unl imed s o ils pro du ced roo ts which were s tun ted and
thickened , overl iming l imi ted the developmen t of both secondary and
lateral roo ts .
7.3.4
Effect of l ime and phosphate addi tions
on the ch emi cal composit ion of Leucaena �co cepha la
I n general , the P concen trat ions in p lant tops followed trends very
s imi l a r to the yiel d respons e curve (Fig . 7 . 7 ) .
Thus , on e i ther s i de o f
the p H a t which maximum grow th occurred , the P concentration in plant tops
in all s o il s , except Koronivia , decreas ed markedly .
Th e Koronivia s o il
showed a s imilar t r end but th e decrea s e was no t as p ronounGed.
S imila r
e f f ec t s have previous ly b een repo rted fo r Leucaena leucocephala
(Fox e t al . ,
1 9 6 4 ) and o ther l egumes and this decreas e in the con c entration o f P was
c it ed as a probabl e caus e of yiel d redu c t ion at high pH values (Fox et al . ,
1964 ;
Lanyon et al . , 1 9 7 7 ;
Sumner , 1 9 7 9 ) .
Al though the concent rat ion
o f P in p l ant roo ts vari ed w i th pH the trend was no t cons is tent b etween
soils (Fig . 7 . 8 ) .
In general , however , the concentration o f P was l owes t
a t h igh pH .
In contrast to th e da ta for P , the Al concentra tion in plant tops was
at a minimum at the pH where maximum growth o c cur re d , on either s ide o f
which i t increas ed (Fi g . 7 . 9 ) .
Th is in c r ea s e was mo s t pronounced in the
tops of p l ants grown in the Nadroloulou and S eqaqa s o ils .
I t is apparent
tha t crop growth was s everely depres sed at Al conc entrat ions exceeding
-1
approxima t e ly 1 . 3 mmo1 kg
DM tops (Fig . 7 . 10a) . and , moreover , the
concentrat ion o f P in tops wh en this Al concentra t ion was exceeded was very
low (Fi g . 7 . 10b ) .
Thes e r esul ts appear to conf irm the findings o f
Fox e t a l . ( 1 96 4 ) and Farina e t al . ( 1 9 80a , b ) i n tha t plants can take u p Al
a t s o il pH values where i t might b e expec ted to b e in the fo rm of an
inso lub l e hydroxide .
Interes tingly , the Al concen tration in the p l ant
t o p s i n the Nadroloulou s oil at the h ighes t p H was high er than that in
the unlimed soils a t pH 4 . 0 (Fig . 7 . 9 ) .
Such an observation was a l so
113
K�ONIVIA
Ill
3
Fi g . 7 . 7
5
pH
4
6
Effect o f soil pH and added P (Tab le 7 . 1 )
on the concentrat ion o f P in plant tops .
Vertical b ars ( I ) represent LSD (pH) at
5 , 1 , and 0 . 1% l evel o f s ignif icance .
( •
=
Pl ,
A
=
P 2 , and
e
=
P3) .
I.
7
1 14
BATIRI
SEQ.AQA
Ill
�
-
-
NADROLOULOU
KORON IVIA
-
Ill
-
.__
4
�
1.8
•
Ill
...
5
6
6
5
Ef f e ct of soil pH and ad ded P ( Tab le 7 . 1) on
the concent rat ion o f P in roo ts .
Verti cal
b ars ( I ) repres ent LSD (pH ) at 5 , 1 , and 0 . 1%
level o f s igni f i c ance .
and e
=
P3) .
( •
=
Pl ,
•
=
P2,
7
1 15
BAT I R I
SEQ AQA
-
p
I
0'1
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Fig .
NADROLOULOU
KORONIVI A
""""'
Ill
\
4
7.9
I
5
7
6
pH
5
4
6
Effect o f soil pH and added P ( Tab l e 7 . 1 ) on
the concentrat ion o f Al in plant tops .
Verti cal b ars ( I ) represent LSD (pH) at 5 , 1 ,
and 0 . 1 % l evel o f s ignificance .
( •
=
Pl ,
•
=
P 2 , and
e
=
P 3) .
7
1 16
(a)
•
•
•
•
•
•
t
•
•
\
0
0
P treatments
soil
..
I
�
�
'E
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KORONIVI A
11
•
•
NADROLOULOU
o
0
•
BATIRI
0
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SEO.AO.A
0
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•
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a..
b 30
11
•
•
c:
0
•
� 15
...
c:
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5 o
l.J
1
3
2
4
Concentration of Al in plant tops (mmol kg- 1 )
Fi g . 7 . 1 0
0
Relat ionsh ip b etween the concentration o f Al
in plant tops and
(a) dry mat ter top y i e l d , and
(b ) concentrat ion o f P in the tops .
117
recorded by Farina e t al . ( 1 9 80a) in l e af t issue o f corn plants grown in
a ci d s o i l s t reated wi th l ime .
In the p resent s tudy , an increas e in the
amount o f a d de d P resul ted in a small d ecrease in Al concentra tions in
pl ants grown in all s o il s .
In cont ra s t to the trends in Al concentra t ion described above for
p lan t tops , th e Al concentrat ion in roo ts d ecreas ed wi th l ime add i t ions
unt il the pH at whi ch maximum growth o c c urred and then remained cons tant
( Fi g . 7 . 1 1 ) .
In addi tion, the Al concentration in th e roo ts was always
10 to 2 0 t im es h igher than in the tops .
This is no t surpri s ing b ecause
Al i s known to accumulate in the roo ts o f Al -s ens i t ive plants (Fay et al . ,
19 7 8 ;
Helya r , 1 9 7 8 ) .
Al though the Al c oncentration of th e plant
roo t s may h ave b een influenced by s ome contamina tion from adhering soil
par t i cl es , this woul d al ter only the abs o lute values measured and no t the
trends obs e rve d .
Fox e t a l . ( 1 9 64) attribut ed high p l ant A1 concentrations at high
soil pH values to the loca l i zed e f f e c t o f Ca (OH)
high p H s uch that aluminat e ions formed .
resul ting in zones o f
2
If this mechanism was
operat ive then it would b e expec ted to influence the Al status o f plants
grown in s o i l , regardless of exchangeab l e A1 con t ent .
The f indings o f
S umner ( 1 9 7 9 ) , tha t yield depress ion was ob served only in soils in whi ch
-1
exchangeab l e A1 exceeded 1 . 4 mmo l kg , casts some doubt on the reason
propo s e d by Fox et al . ( 1 9 64 ) .
Furth ermo re , al l the soils in the pres ent
s tudy were thoroughly mixed during the l ime incub a tion perio d .
S tudies
by Kwong e t al . ( 19 78) and Y oung and B a ch e ( 1 98 5 ) suggest that a ctive A1
copl d be p r e s ent in soils at a pH > 5 . 5 in the t o rm o f soluble organic
compl exe s .
Perhaps it is these solub l e Al -o rganic complexes which are
taken up b y p l an t s at high pH values .
Of the s o il proper ties cons idered
in p revious chap t ers and in the present s tudy , only pH 4 . 8 NH�OAc
extrac tab l e Al increased at h igh pH values (Appendix 1 ) .
However , thes e
p H va lues were much higher than thos e a t which growth depression f irst
o c curre d .
Al though the conc entrat ion o f Al i n the plant tops increased markedly
at h igh pH, i t is interes ting to no te that there was no cl ear trend wi th pH
in th e to tal amount o f Al taken up into the plan t tops (Fig . 7 . 1 2 ) .
Ind e e d , the uptake by the roo t s ( and the who l e plan ts ) declined s tea dily
wi th inc reas ing pH (Fig . 7 . 1 3 ) .
This sugges ts tha t the incr eased Al
concentration in plant tops a t high pH values may b e due , a t l ea st in part ,
to the concentrating effe c t o f reduced growth wh ich has resul ted f rom fa ctors
118
80
....
I
en
�
0 6()
E
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(/)
�
0
0
L..
BATIRI
0
NADROLOll.OU
0
KffiONI VIA
ll.
SEQAQ A
o
c
I
Ill
I II
%11
'+0
4
Fig .
7 . 11
5
pH
6
7
E f f e c t o f s o i l pH on the concentrat ion o f Al
in roo t s o f plants grown in s o ils trea ted
with 4 . 84 mmol P kg- 1 •
Vertical bars ( I)
represent LSD (pH) at 5 , 1 , and 0 . 1 % l evel
of s igni f i c anc e .
119
SEQAQ A
10
ttl
05
-
I
�
0
�
�
BATIRI
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Fig. 7 . 12
5
6
7
7
pH
E f f ect o f soil pH and added P (Table 7 . 1 ) on
Vert ical
the uptake of Al by plant tops .
b ars (I) represent LSD (pH) at 5 , 1 , and 0 . 1%
l evel o f s ignif i c ance .
and
•
=
P3) .
( •
=
Pl ,
.A
=
P2 ,
1 20
SEQAQA
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KORONIVIA
NADROUlJLOU
Ill
4
Fig . 7 . 13
I
5
111
6
pH
I
4
I
5
:
I
6
Ef f ect o f so il pH and added P (Table 7 . 1 ) on the
up t ake o f Al by p lant roots .
Vert i cal b ars ( I )
represent LSD (pH) at 5 , 1 , and 0 . 1 % l evel o f
s i gni f icance .
( •
=
Pl ,
&
=
P 2 , and
•
=
P 3) .
�
I
7
121
other than increas ed Al up take .
The resu l ts for o th er major and minor nu trients are presented in
App endix 3a-3d .
Al tho ugh comp l e t e maj o r ( except P ) and minor nutrients
were added r egularly th ere was some var ia tion in the uptake o f nutrients
with pH .
The plants g rown at bo th low and a t h igh pH va lues showed a
range o f symptoms , al though nu trient concentrations generally did no t
fall b elow the c rit ical concentr ations (Gon zalez e t al . , 1980) and none
of the symp toms coul d b e associated wi th a specific el ement .
7 .3. 5
L ime-aluminium-phosphate
interact i ons in soils and plan ts
The ini t ial , pos i tive r esponse to l ime by plants a t med ium and high
l evels o f added P was p robably assoc iated with a decrease in M KCl
extractab l e Al and to enhanced availability of added P .
In a l l soils ,
l ime add i t i ons cau s ed a d ecreas e in M KCl -ext ractabl e Al (Fi g . 7 . 2 ) and
a co rresponding increas e in r es in-extra c tab l e P up to pH values of 4 . 5
to 5 . 5 (Fig . 7 . 4 ) .
This is con s i s tent with the general finding that
the initial response o f plants to l iming is due to enhanced availability
o f added P ( Fox et a l . , 1 964 ;
Wo od ruff and Kamp rath , 1 9 65 ) and a
decrease in M KCl -ext rac tab le Al .
The r easons for the depress ion in plant growth at high pH values are
l ess c l ear and many conf l ic t ing mechanisms have been p ropo sed (Kamprath ,
1971 ;
Amaras iri and Olsen , 1 9 7 3 ;
Farina et al . , 1 9 80a , b ) .
Sumner , 1 9 7 9 ;
Fries en et al . , 19 80b ;
In this s tudy, the obs erva tions that the
concentration o f P at extreme pH va lues was be low the crit ical
concentrat i on requi red for normal growth coupled with the e l evated ratio
o f P in roo t s to P in roo ts + tops (Fig . 7 . 1 4 ) sugge s t s P d e f i c i ency at
these pH values .
However , the pattern o f Al uptake by plant roo t s
(Fig . 7 . 1 3 ) � toge th er wi th the r e sul ts for the concentra tion o f P in roo ts
( 7 . 8) , sugges ts that different mechanisms were respons ible fo r th e caus e
o f P def i c i ency in p lan ts at low and high pH values .
Thus, a t low pH
values , P d e f i ci ency app ears to r esul t l argely from the blocking o f P
movement f rom the roots to the tops by the presence o f high concentration�
of Al in the root s (Fi g . 7 . 1 1 ) .
Phos pha te deficiency at h igh pH values
has b een r el ated to a decrease in solution P concentration by the formation
of inso lub l e Ca-P c ompoun ds (Kamprath , 1 9 7 1 ;
Fox e t al . , 1 9 64 ) .
Support
fo r this me chanism in the p res ent s tudy comes f rom the resin P resul ts
wh ich decreased at high pH val ues .
Further evidence to s uppo rt this
1 22
KCRlNIVIA
I
+
�
0
'-
NADROLOULOU
O·
ul
0·1
Q..
ul
BATIRI
-
�
Q..
0
19
a::
0
0·5
0·1
Fig . 7 . 1 4
SEQAQA
3
.111
5
pH
4
7
6
Effect o f so il pH and added P ( Table 7 . 1 ) on
the rat io o f P roo t s to P ( roots + tops )
in plant s .
Vertical b ars ( I ) represent
LSD (pH) at 5 , 1 , and 0 . 1 % l evel of
s ignificance .
e
=
P3) .
(
•
=
Pl ,
A.
=
P2, and
123
hypo the s is is presented b y the work o f Murrmann and P eech ( 1 9 6 9 ) and by
obs e rvat ions on iso topic exchange and P sorption presented earlier in
this s tudy ( Chap te r 5 and 6 , respec tively) .
Reduced ava il ab i l ity of P
at h igh pH values has also b een related to an incr eas ed s orpt ion o f added
P by f reshly p recip i ta ted Fe and Al hydroxides in limed soils (Amarasi r i
a n d Olsen , 19 7 3) .
However , Farina e t al ( 1 9 80a) reported tqat P defic iency , per s e ,
may no t explain the depress ion in yield at high pH values .
In subs equent
s tudies , Farina et al ( 1 9 80b ) r epor ted that as much as 7 0 % of the varia t ion
in the yield of co rn at high pH va lues was exp l i cab le in terms of l eaf Al
con tent a n d that inclus ion o f P did no t improve the
variance accounted fo r .
A s imi lar t re a tment o f the data f rom this study revealed tha t when yie l d
and P con c en tr a t ion data f rom plan t tops in which Al concentra tion
-l
exceeded approxima tely 1 . 3 mmol kg
( S ec tion 7 . 3 . 4 ) were regres sed together ,
P concent rat ion accounted for 69 % o f the variab i lity in yiel d .
Inc lus ion
o f Al in the mul tiple regress ion analysis improved the variance accounted
f o r to 74 % .
Howeve r , when the r egres sion was conduc ted by taking resul ts
of the p l an t s grown a t high pH values only , where growth was depres s e d ,
P conc entration alone a c counted for 8 2% o f the variab i l i ty in yield .
Incl us ion o f Al improved the rel a t ionsh ip such tha t ove r 9 2 % of the
variab i l i ty in yiel d was ac counted fo r .
The concentra t ion o f Al
a ccounted for 55% o f the variab il i ty in yield .
alone
Thu s , the p res ent r esul ts
sugges t that the var iab il i ty in yield at high pH values can largely be
explained by the concentration o f P and to a smaller extent by the
concen tr at ion of Al in the plant t is sue .
7.4
Con c l us ions
Liming had a variab le e f f e ct on the growth of Leucaena l euco cephala
in all s o il s .
In general , p l an t growth was s everely a f f ec ted at both low
and high pH values and th is e f f e c t persisted even wi th addi tions o f P .
The pH at which maximum yiel d o ccur red varied b etween the soils but was
generally in the range 4 . 4 - 5 . 2 .
Incubat ion o f s o i l s with P resul t e d in
a s i gni f i can t (P < 0 . 0 5 ) improvement in yield within each l ime l evel in
all s o il s .
The response o f p lants to l ime varied wi th both the amount o f added P
and soil typ e .
Thus , al though yield increase d with a dde d P in the
1 24
Nad ro lo ul ou soil , th e re was no obvious intera c t ion b etween l ime and P .
Th is e ff e c t o f l ime and P in th e Nad ro loulou soil coul d have arisen from
the fac t that the l ime rates were such that the pH value s a chieved were
e i th er too h igh or too low for opt imum growth .
In cont ras t to the
Nadroloulou soil , a pos i tive l ime X P intera c t ion was ob tained in medium
and h igh P - t reated B a t iri , Koronivia , and S eqaqa soils .
This highly
s i gn i f i c an t l ime X P interact ion is typical o f P -d e f i c i ent , h ighly
weath e re d , ac id soils (Munns , 1 9 6 5 a , b ;
Fri e s en et al . , 1 9 80b ) .
From the P concen tration data and yiel d respons e curves , i t is
apparent that near maximum growth was a t tained only in the Koronivia
and S eqaqa soils t r ea ted wi th the highes t l evel of added P .
It is also
apparent f rom the P concentrations o f plants growing in Batiri and
Nadro loulou s o i l s , that poor growth in thes e soils is la rgely related to
a d e f i c i ency of P .
The init ial increase in yield with increasing pH was probably due to
enhanc e d availab il i ty of added P and also to a decreas e in a c t ive Al .
The depress ion in p l ant growth at high pH va lues appears to b e largely
related to P deficiency and to a smaller extent to the presenc e of excess
Al .
Al though the ratio o f P in roo ts to P in roo ts + tops indicates
defic iency o f P at b o th low and h igh pH values , the data for the A1 and P
concentrat ions in the roo ts al s o indicate Al -induced P def iciency in plants
g rown a t low pH value s .
The o b s e rve d increase in Al concentra t ion in plants at high pH values
is due , at l eas t in par t , to the concentration effec t of reduced growth on
Al concentration .
This contras ts wi th the s ituation at low pH where ,
d es pi t e lower growth , the uptake o f Al by p l ant root s was higher .
CHAPTER 8
125
CHAPTER 8
AS SES SMENT OF PLANT-AVAILABLE PHOSPHATE
US ING S EVERAL S O IL-TESTING PROCEDURES
8. 1
Introduc t ion
The r e i s a n incr easing awar eness of the need t o l im it appl ica tion o f
f er t il i z er P t o rates wh ich are ad equate f o r plant growth .
However , to
determine o p t imum appl ication rates , soil-t es t ing pro cedures are required
to asses s s o i l P s t atus .
Two o f the basic c r i t eria of a s o il - tes t ing
p roc edure are that i t requires a minimum amo unt o f cal ibration and ,
once c a l ib ra t ed , it shou ld successfully pred ict the r es pons e o f plants
to added P ( Thomas and P easlee, 1 9 7 3 ) .
The soil-tes ting pro cedure
s ho ul d a l s o be convenient for the routine analys is of large numbers o f
samples .
In F ij i , a s ingl e soil - tes t ing pro cedur e involving extra ction with
0 . 0 2 5M H 2 S 04 is currently used to assess th e P s tatus o f so ils (Fij i SCEP ,
. 1 9 84 ) .
However , the extent to which this a cid extractant is success ful
over a range of soil typ es , which vary widely in chemical and mineralogical
p roper t i es , is no t known .
Becaus e the rela t ionships b etween 0 . 025M
HzS04 -extractabl e P and plant r esponse to P has no t b een evalua ted there
is a need to s tanda r d is e soil- t e s t ing pro c edures for the range of soils
in Fij i ( Fij i SCEP , 1 9 84 ) .
Al though only four Fij ian soils were s tudied in Chapter 7 , the
p erformance o f a numb er of soil-testing pro c edures can be a s s essed us ing
the r es ul t s obtained for the e f f ec t of l iming on the g rowth of Leucaena
l euco cephala . However , in assessing th e usefulnes s o f soil-tes t ing
p ro c e dures , it is n ec essary to ensure tha t plant growth is no t l imited
by o ther f ac tors .
As d iscus s ed in Chapter 7 , there was a cons iderab l e
growth l imi tation in Batir i , Nadroloulou , and S eqaqa so il s b ecause of Al
toxici ty , particularly at low pH values , such tha t some doubt exis ted a s
to whe ther availab l e P was compl etely exhaus ted .
To inves tiga t e this further , subsampl es o f the s o ils which had
init ially b een planted with Leucaena l eucocephala , were grown with
p er ennial ryegras s (Lol ium perenne L) us ing the pro c edure of S tanford and
126
Dement ( 1 9 5 7 ) .
This t e s t is d e signed to b e h ighly explo itive o f the
nutr ient under study .
Ryegrass was cho s en a s the crop becaus e it is
known to b e l es s sens i t iv e to Al toxicity than Leucaena l euco c ephala .
Thus , in this chap t er , a range o f so il-t es ting proc edures was used
to a sses s the plant-avail ab l e P s ta tus o f s o i l s which had b een trea t ed
with l ime and added P .
The soil-tes ting procedures cho sen were O l s en
(Ol s en et a l . , 1 9 54 ) , Colwell ( Co lwel l , 1 9 6 3 ) , Bray (I and I I )
(Bray
and Kur t z , 1 9 4 5 ) , Mehl ich (Nelson et al . , 1 9 5 3 ) , Cl-exchange r es in
( Cooke and H islop , 1 9 6 3 ) , and iso topic exchange (Chapter 5) .
The
Mehl ich so il-tes t ing proc edure was us ed in pref erenc e to the proc edure
used in Fij i becaus e it ha s b e en shown to s uc c es sfully predict th e P
requirement s of plants in highly�eathered s o i l s (Thomas and Peasl ee ,
1973) .
8.2
Ma terials and Me thods
8. 2. 1
Soils
The s o ils used in this s tudy and th eir pr epara tion have b e en describ ed
in detail in Chapter 7 ( S ec t ion 7 . 2 . 2 ) .
Each of the 4 highly�eathered ,
acid so il s w ere l imed a t either 4 o r 5 rates to obtain a range o f soils
Subsampl e s of the l imed soils were incubated
with pH values up to 7 .
s ub s equen t l y with 3 ra t es o f P .
The rates o f P were such tha t all s o i l s
r ec e ived 1 common P t r ea tment a n d the o ther 2 rates were low o r high ,
depending o n the P-sorption capa c i ty of the s o ils .
a re l is t ed in Table 7 . 1 .
at a tempe ra ture of 20
±
°
These treatments
Fol lowing incuba tion , the soils were a ir dried
2 C and s to red in po lythene bags for chemical
analysis and plant growth s tudies .
At th e complet ion o f the Leucaena �uco c ephala growth s tudy ( Chap ter 7 ) ,
all s o il s ampl es were a ir d r ied at a temperature o f 20
±
0
2 C , pas sed
through a 2-mm s ieve , and again s tored in poly thene b ags for the ry egrass
growth s tudy .
8.2.3
Extrac tab l e Phospha t e
Mehl ich- and Ols en-extrac t ab l e P in the t rea ted so ils were determined
by the methods d es crib ed in Chap t er 5 ( Sec t ion 5 . 2 . 2 ) , wh il e r es in
extrac tab le P was determined u s ing the method describ ed in Chapter 7
( S ec t ion 7 . 2 . 3 . 1 ) .
127
Colwel l-extrac t ab l e P (Colwell , 1 9 6 3 ) ; 0 . 2 g o f so il was shaken fo r
1 6 h wi th 20 mL of 0 . 5M NaHco 3 (pH 8 . 5 ) so lu t ion in polypropylene tub es ,
c entrifuged , and f il t ered through Wha tman No . 5 f il t er paper .
Inorganic
P was d e t e rmined in the extrac t s by the method of Murphy and Ril ey ( 1 962) .
Bray ( I ) -extrac tab le P (Bray and Kur t z , 1 94 5 ) : 1 g of s o il was shaken
by hand fo r 60 s with 7 mL of the Bray r eagent (0 . 0 2 5M HCl + 0 . 0 3M NH4 F)
in polypropyl ene tub es and f il t ered through Wha tman No . 5 f il t er paper .
Inorganic P was determined in the extra cts , as descr ib ed ab ove for
Colwell -ex t rac t ab l e P .
B ray ( ! I ) -extrac tab le P (Bray and Kurt z , 194 5)
: 1 g of s o il was shaken
by hand fo r 40 s with 7 mL of the Bray reagent ( O . lM HCl + 0 . 0 3M NH4 F )
i n pol ypropylene tub es , and f i l t ered through Whatman No . 5 f il ter paper .
Inorganic P was det ermined in the extra cts a s describ ed above for
Colwel l - extrac tab l e P .
I s o topically-exchangeable P : 1 g o f so il was shaken with 30 mL of
d eionized wa ter fo r 1 6 h , following which , 1 mL of carrier- free P solution
(2 to 4 � C i ) wa s added to the s o il suspension .
The tub es conta ining the
s o il suspension were shaken for a fur ther 2 h , c entrifuged , th e supernatant
solut ion passed thro ugh a mil l ipor e memb rane (< 0 . 4 5 �m) , and 3 1 p and 32 p
determined in the f il tra t e .
The con tac t t ime was d e c r eased from the 4 h
us ed in Chapter S , to 2 h to ensure exchange with th e frac t ion o f P wh ich
woul d b e mo s t easily exchangeabl� .
The pro cedure u s e d . for the determina t ion
of 3 2 p is des c r ib ed in Chapt er 5 ( S ec t ion 5 . 2 . 2) .
8.2. 3
Determina t ion o f buffer capa c i ty
To d e t ermine the P-buf f er capacity o f s o ils , s o rp t ion s tudies were
conduct ed by shaking so il samp l es ( 1 g) wi th 0 . 0 2M KCl ( 30 mL ) solution
contain ing varying amo unts of P (Holfo rd , 1 9 79 ) .
The buffer capa c i ty o f
each o f the so ils was then det e rmined b y 3 d i ff erent methods , which
included
(i)
the maximum s lope o f the Langmuir isoth erm , as d es cribed by
Hol fo rd ( 1 97 9 ) ,
(ii)
and
t h e s lop e of th e sorpt ion iso therm (Peasl ee and Phil l ips , 1 9 8 1 ) ,
( i i i ) the amount o f P r equired to increase the so lution P conc entration
to 0 . 00 8 mmol L - 1 ( 0 . 25 vg mL- 1 ; Jones and Fox , 1 9 7 7 ) .
In addi t ion , the P r e t ent ion of unl imed soils was determined by the
method o f Saunders (19 6 5 ) .
1 28
8.3
P l ant Growth S tud i es
A mod if ied S t anfo rd and Dement ( 1 9 5 7 ) p ro cedure was used to ass ess
the amo unt o f ava il abl e P remaining a f t e r the growth o f Leucaena l eunocepha la
( Chapt er 7 ) .
A plas tic po t , wi th the bot tom removed , was p laced ins ide a second
po t and sub s equen t ly f i l l ed with 300 g o f wa shed river sand .
A solut ion
containing compl e t e maj o r and mino r nu trients was added to each po t .
Perennial ryegras s (Lol ium perenne �) was used as the test plant and
approximat ely 50 seeds were s own per po t .
The po ts were wa tered
r egul a rly to the r equired mo isture content until th e seeds germina ted .
Af ter 6 weeks of plant growth , wh en th e roo ts o f the plants had fo rmed
a dense mat at the bo t tom of the po t , th e plants were cut to a level o f
200-300
mm
above th e sand surface .
Th e inner po ts, containing th e sand
and the roo ts , were then trans f erred into ano ther inta ct po t conta ining
20 g of the t r ea ted so ils which had earl i er b een used to grow Leucaena
l eucocephala .
S ix po ts conta ining ryegrass growing in sand were kep t
as co n trol pl an ts .
mo i s t ure con tent .
These pots were wa tered regul arly to the required
All maj o r and mino r nutrients , except P , were added
twic e each week in addition to the r egular watering .
At the end of 28 d , the tops were harves ted , the roo ts washed free o f
adhering sand and s o il partic l es , and the herbage dried at 6 5 ° C for 4 8 h .
Dried herbage ( roo t s and tops ) was weighed t o ob tain the yie l d , ground ,
and P concentrat ion determined by the me thod o f Twine and Wil liams ( 19 7 1 ) .
Aluminium in the tops o f pl ants grown in Seqaqa soil was determined a f t er
d iges tion in perchloric a cid us ing the mod i f i ed aluminon method (Rsu ,
1963) .
8.4
Resul t s and Discus s ion
8 .4 . 1
Plant growth s tudies
For al l but the Koronivia soil, there was an increase in g rowth wi th
increasing l ime rate up to either 11 o r L 2 and this was followed by a
depr e s s ion in growth a t h igher pH values .
With th e Koronivia s o i� :yield
was almo s t constant up to L l , above which it declined .
Only the resul t s
f o r the commo n P trea tments are presented b ecaus e l ime addi t ions had
s imilar effects a t a l l P l evels .
Th es e trends are s imilar to those
1 29
repo r ted earl i er for the growth o f Leucaena � co c ephala on these soils .
Interes t ingly , the maj ority o f the plants grown in B a t iri and S eqaqa so ils
a t e i ther low o r h igh pH graduall y died .
Th is o ccurred after about
3 weeks , du r ing which t ime the ini t ially heal thy roo ts of ryegras s ( in
s and cul ture) gradually b e came spo t ty in appearance .
This contra s t ed with
the plan t s in the contro l t r eatment which cont inued to grow s lowly even
without any addit ion o f P .
Thus , the yiel d o f plant tops grown at low
and h igh pH va lues was generally l e s s than tha t in the contro l po ts
(Fig . 8 . 1 ) .
In Chap ter 7 i t was sugges ted that the maj o r fac tor l imi t ing plant
growth in s o i l s at low pH va lues wa s A1 toxicity and / or Al- induced P
d e f i c i ency and at h igh pH values was soil P supply and to a smal l er ext ent
the in crea s ed concentrat ion of A1 in the plant t is sue .
In the present
s tudy, the concentra t ion o f P in plant tops was low a t b o th low and a t
h i gh pH values ( l ine b , Fig . 8 . 2) .
Only the resul ts f o r th e medium
P - treated Se qaqa so il s are pres ented b ecaus e the plants growing in th is
so il were the mo s t s everely af f ec t ed .
In contras t , the concentra t ion
of A1 was at a minimum a t the pH of maximum growth , on either s ide o f
which i t increased ( l ine a , Fig . 8 . 2 ) .
Al though reduced p lant growth
in s o ils a t high pH values coul d have been due to a decreas e in plant
availab l e P , comparison w ith the control plants woul d sugge s t tha t this
alone could no t have caus ed the d eath of plants .
Th is prob ab ly aros e
f r om the inc r eased conc en t r a tion o f A1 in the p lants and provides comp e l l ing
evidence for some A1 toxi c i ty e f f ec ts at high pH .
8.4 . 2
Relat ionsh ip b e tween plant uptake
of pho spha t e and so il- t e s t resul ts
The rela tionship b e tw een uptake of P by p lants (Leucaena + ryegrass )
and P extrac t ed by the s ev en so il-tes ting pro c edures is illustra t ed in
Fig . 8 . 3 a-g .
Wi th the exception o f the resul t s for iso to p ic exchange ,
a l l p roc edures gave resul t s wh ich were correlated with plant P uptake
data .
However , only f o r r es in P was the rela t ionship a c l o s e one .
The s lope coe f ficient ( 0 . 9 7) for the regres s ion equa tion , together with
a very low intercept , s ugges t ed an a lmost , 1 : 1 relationship b etween
r e s in-extrac tab le P and P uptake .
This high co rrela t ion b e tween resin
P and plant P suppor t s the f ind ings o f a number of workers who have
ob s erved that the resin-ex t ra c t ion pro cedure is
a
good indicator o f P
availab i l i ty in a wid e range o f s o i l s (Bache and Ro gers , 1 9 70 ;
and Co rnfo r th , 1 9 7 3 ;
Le Mare , 1 9 8 1 ) .
Walms l ey
1 30
BATIRI
SEQAQA
control
KORONIVIA
atI
�
NADROLOULOU
111
�
control
4
Fi g . 8 . 1
5
6
I
4
5
6
Effect o f soil pH and added P ( 4 . 84 mmol kg- 1 ) on the
dry weight of ryegrass tops .
Vertical bars ( I )
represent LSD (pH) at 5 , 1 , and 0 . 1 % l evel o f
s ignif icance .
7
131
SEQAQA
-
I
Cl
�
-
E
E
0
a
-
6
2
�
-
E
E
0
-
V)
CL
V)
CL
0
.E
�
c
-
<
�
0
c
.2
"13
�
CS
'-
'�
c
QJ
u
c
0
LJ
, &I
4
Fig . 8 . 2
5
pH
6
0·002 �
�
control
b
7
0·0
E f f e c t o f soil pH and added P (4 . 84 mmo l k g- 1 )
on the concent rat ion o f
rye grass tops .
( a ) Al and (b) P in
Vertical b ars ( I ) represent
L S D (pH) a t 5 , 1 , and 0 . 1 % l evel o f significance .
u
c
0
u
1 32
0·4
a.
•
-10'1
X
C5
E
E
-
•
K or on i v i a
• •
"
"
•
• •
0
b.
"
•
.
- -.,-- -i...
--· ·
� ·· " . .
0
c.
Resn
"
•
•
--
•
•
•
-
•
4
•
---
r = - 0·1 2
•
N cid rolou lou
B at ir i
S e q aq o
t1tU:h
••
.
()-4
•
.
. . ;
� ....... . ' .
I
(}2
0
•
•
•
0· 4
"
•
0·2
0
Ex chargeable
•
- -- - - -,
.
r = 0·43
•
--- -
-
-- --
--- -
-
•
•
***
-
••
2
1
"
•
(}2
0
Fig . 8 . 3
(con t d .
p . 1 3 3)
0
0·2
Ex t rac t a bl e P (rrmd
k;J-1)
0 ·5
Relat ionship between upt ake o f P by plan t s
(Leucaena l eucocephala and ryegra s s ) and
amounts of iso topical ly-exchangeable P ( a )
Mehl ich- ( b ) , resin- ( c ) , Bray ( I ) - ( d ) ,
B r ay ( I I ) - ( e ) , O l s en- ( f ) , and Colwell
extractab l e P (g) .
1 33
d.
Bray (I)
0· 4
-
I
0'1
.X
•
0
�
•
. ..,..
_ _
E
E
0· 4
•
•
.,
,...
.. ... .....
r
Clsen
•
••
:
•
.
0
__
. .. -
•
I
0-5
.
•
-
3
=
•
•
\
1�
-
.:-
•
•
. .
'
.
.
:·
1�
0
__
,....
E x t rac t able P ( m mol
.
1
kg-1 )
-
•
•
-
--
2
Fig . 8 . 3
Relat ionship between up take o f P by plants
( cont d .
f rom
p . 1 32)
(Leucaena l euco cepha l a and ryegras s ) and
amounts o f iso top ically-exchangeable P (a) ,
Mehl i ch- (b) ,
Bray ( ! ! ) - ( e ) ,
r e s in- ( c ) , Bray ( ! ) - ( d ) ,
Ol s en- ( f ) , and Co lwell
extractab l e P (g) .
-
- --
- --
. . .
•
--
•
••
2
---- ·
--.;;;; . •
.
•
.
-
• •
.
-
- _ _ ... _ _ .... ...-
Colwell
•
***
r = 0.46
.
--
1
0
***
6
0·4 •
•
- - -&
•
•
•
..
•
..,... . ...-
-- - -- ·
· -- .
·- - - - .
.
.
... -:.- -_.. .,.
I
__
_ _
2
•
•
• •
• •• J.
--
-
1
0
f.
•
•
Bray (ID
•
-·
•
-
Ql
.X
IV
�
:::>
•
•
0·2
e.
•
•-
•
•
-
-
·
•
-
-
-
..
.
3
---
.
- · · ··
•
1 34
The s uper io r i ty o f the r e s in s o il - tes t ing proc edure over the o ther
method s in predic t ing p l ant P up take is p robab ly due to two reasons .
Firstly ,
a
weak anion-exchange r esin , such as Cl , does not al ter the
surface properties o f the so il as do the o ther reagents and , mo r eover ,
i t ac t s p r inc ipally by r emoving solu t ion P and by deso rb ing loos ely
h eld P (Amer et al . , 1 9 5 5 ) .
S econdly , but mo re impor t antly , the
cont inuing s ink provided by th e Cl r es in makes it func t ion rather l ike a
p l ant roo t wi th a v ery h igh capab il i ty for P uptake .
The r el a t ionship b e twe en iso top ically - exchangeabl e P a nd plant P
uptake was v ery poo r .
The s e r esul ts a gre e wi th the f indings o f a numb er
o f inves t igators (Mc Conaghy et al . , 1 9 66 ;
Amer et al . , 1 9 6 9 ;
Dalal
and Hall swo r th , 1 9 7 7) who r epo r ted tha t iso topic-exchange measurements
g enerally over e s t ima ted plant - availab le P in soils o f high P-sorption
capaci t y .
This is b ecaus e the concentrat ion o f P in the s o il so lution
d ep ends no t o nly on the amount of P on the surfac e , which is measur ed by
i so topic exchange , but also on the numb er o f vacant s i tes .
Dalal and
Hall swo r th ( 1 97 7) a l so ob served that in soils with h igh P- sorpt ion
capacity , the r el a t ionsh ip b e tween iso top ic-exchange and P uptake by
plants d epended to a large extent on the reliab il i ty of measuring solution
P.
A s imilar d i f f icul ty in measuring solut ion P in the s o ils us ed in
th is s tudy was d i s cus sed in Chapter 5 .
The resul ts in Fig . 8 . 3 s how that for all the so il- tes ting procedures ,
except the r e s in procedure , the P up take values rel a t ive to extractab l e P
fo r the B a t ir i , Nadroloulou , and S eqaqa soils were generally c lus tered
together , whe r eas tho s e for the Koronivia soil were segrega ted for all
but res in P .
Th e Ko ronivia s o il d i f f ers from the o ther soils in having a
r el a t ively lower P - so rption c apacity .
However , even exclusion o f this
soil d i d no t imp rove the rel a t ionship b e tween soil - tes t resul t s and P
upt ak e .
To account f or the var iat ion in soil proper ti es , several inves t iga tors
( e . g . Ho l fo r d and Mat t ingly , 1 9 7 6 ) inc luded a buffer capaci ty term in th e
mul t ipl e regress ion analys i s b e tween P up take and extrac tab l e P .
B ecause
th ere i s no agreed method o f determining buffer capacity , 4 dif ferent
ind i ces of buffer capac ity were used in the mul tipl e regres s ion s tudies
in th e present s t udy .
These indices included the maximum slope of the
Langmuir s o rp t io n iso therm (HBC) , measured over an equil ib rium concentration
-1
range o f 0 to 0 . 16 mmo l L
(0 - 5 �g mL -1 ) in 0 . 0 2M KCl ru
\ uo lfo rd , 1 9 79 ) ;
1 35
the s lo pe o f the P -s o rp t ion iso the� (SBC) ;
the quant ity of P needed
to increase the solu t ion P concentra t ion to 0 . 008 mmo l 1-1 (0 . 25 � g mL-1
FBC) , which is cons idered by Jones and Fox , (197 7 } to provide o p t imum
plant growth ;
and P - re t ent ion (PR) values o f the unl imed soils
( S aunders , 1 96 5) .
When the buffer capa c i ty indices were included in mult iple regress ion
analyses b e tween P uptake and extrac tab le P th ere was only a small
improvement in R 2 values , par t icul arly wi th the HBC , FB C , and SBC indices
(Tab l e 8 . 1} .
In contras t , th ere was an apprec iab l e improvemen t in R 2
values when PR was included in the co rrela t ion s tudies .
Nevertheles s ,
even when the PR te rm was incl uded , none o f the s o il - t es ting pro cedures ,
exc ep t the r es in a c coun ted for mo re than 50% of the variat ion in P up take .
The r e s in p ro c edure accounted for over 8 0 % o f the var iation in P upt ake ,
even wi thout the inc lus ion o f a buffer capa c i ty term .
A numb er o f inves t igators have sugge s t ed that the relationship
b etween plant P uptake and soil - t e s t resul ts may b e influenced by the
d ens i ty of soil (Mengel and Kirby , 1 9 8 2) and also by soil texture
(Novai s and Kamprath , 1 9 7 8 ) .
For exampl e , Mengel and Kirby ( 19 82)
h ave sugges ted that organic s o ils have a low bulk density and the
exp r es s ion of resul ts on a we ight basis can be misl eading .
Furthermo r e ,
such f a c t o r s wil l no t b e acco unted for by b uf f er capacity t erms .
Of
t h e 4 so ils u s e d in the pres ent s tudy , the Ko ronivia and the S eqaqa soils
d i f f ered from the o ther 2 in having a low c lay and a h igh o rganic ma t t er
content , respec t iv ely .
When either of these so il s was excluded from
the c o rre l a t ion s t udies ther e was only a small improvement in the R 2
value .
The se r esul ts con tra s t with tho s e presented by Ho lford ( 1980 )
and Dalal an d Hallsworth ( 1 9 7 6 ) who reco rded a s ignificant improvement
in R 2 values when buffer capacity terms were included .
However , unl ike
the s o il s used by these inves t igato rs , the soils us ed in the present
s tudy wer e l imed .
Quo t ing the work o f F i t t s ( 1 956) , Kampra th and Wa tson
( 1 980) sugges ted that l iming may influence the amount of P extrac ted
by chemical extractants , th ereby a f f e c t ing the rela t ionship b e tween
plant P uptake data and extractab le P .
Thus, i t is l ikely that the
d i f ficul ty in relating plant P uptake data to extractab le P aris es f rom
the p rob l ems a s so c iated with the ex trac t ion o f P f rom limed s o il s .
It has b een shown that the amount o f P extrac ted by the O l s en
(Sorn-s r ivichai e t al . , 1 9 8 4 ) and Mehl i ch ( Chap ter 5 ) reagents is a f f e c t ed
136
Tab l e 8 . 1
Propor t ion o f variat ion in plant P uptake (R � a c co unted f o r b y
ext ractab l e P a lone and in comb inat ion with various ind ices o f
buf f er c apa c i ty , as determined b y mul t iple regr es s ion analyse s .
Phospha t e
Extractab le P
Extrac tab l e P + Buf f er Capac ity
PR 11
parameter
a lone
BBC
SBC
FBC
Ols en
0.21
0 . 21
0 . 26
0 . 28
0 . 44
Colwell
0. 12
0 . 17
0 . 21
0 . 25
0.43
Mehl ich
0 . 18
0 . 12
0. 15
0 . 20
0 . 30
Bray ( I )
0 . 24
0 . 20
0 . 24
0 . 25
0 . 34
Bray ( I I )
0.21
0 . 18
0. 23
0 . 24
0 . 33
Res in
3 2p
0 . 81
0 . 80
0 . 83
0 . 84
0 . 90
0 . 01
0. 17
0 . 19
0 . 27
0 . 29
maximum s lope o f the Langmuir iso therm as describ ed by Ho lford
HBC
(Ho l ford , 1 9 79 ) .
SBC
FB C
PR
slope of sorpt ion iso therm .
=
amount o f P required to increase the solution P conc entra tion
t o 0 . 008 mmo l L- 1 .
phosphat e r e t ention values o f the unl imed so ils .
1 37
by t he concentration o f Ca in t�e ex tract a nd b y the pH o f th e s o il s ,
res p e c t ively .
The curves in Fig . 8 . 4 show that the amount o f P extrac ted
by the Bray ( I ) reagen t al s o decr ea sed with an increase in s o il pH ,
contrary t o the trends ob s e rved f o r plant growth .
The decreas e in
B ray- ext r a ctab le P was probab ly due to a decrease in
F- activity in the
suspens ions o f h igher pH sampl es due to compl ex forma tion b e tween F
Ca 2+ ( Sy e r s et al . , 1 9 7 2) .
8.5
·
and
Conclus ions
Th e p l ant availab il ity of P in 4 soils l imed to varying pH values
and varying widely in P-so rp t ion capac i ty and P s tatus , was ass essed by
the growth o f ryegrass subs equent to the growth of Leucaena �cocephala ,
in a cont r o l l ed-clima t e labora to ry , using a modif ied S t andford and Dement
( 1 9 5 7 ) p ro ce dure .
General ly , l iming caused an increas e in p lant growth
at l ow r a t es o f appl i c a t ion and this was fol lowed by a marked decrea se in
growth at h igh pH values .
A comparison o f p lants grown on s o ils wi th
tho s e in co ntrol po ts ( s and cul ture only) showed tha t poor growth at
bo th low and high pH values was due in part to a toxici ty effect rather
than s impl e P defic iency .
I t was propo sed , that toxicity could have
resul te d f rom increas e d Al concentra tion in th e h erbage .
Amon g the soil - tescing procedures examined , res in-extrac table P
mo s t closely approximated the p l ant - availab l e P s tatus of the so ils .
General ly , the amount o f P extrac ted by th is test wa s small and almo s t
equivalent ( slope
=
0 . 97 ;
r
=
0 . 9 0) to the amount of P taken up by
plant s .
In contras t to r esin P , iso topically-exchangeable P showed no
co rrel a t i on with plant P uptake data .
This was a t t r ibuted to the difficul ty
in measuring solution P in s o i l s with a very h igh P-sorption capacity and
also tha t a l though iso top ical ly-exchangeab l e P measures th e amo unt o f P on
surface i t do es no t reflect the concent ra t ion o f P in soil solution .
All Chemical extrac tants were only weakly correlated wi th plant P
up t ake .
Inclus ion o f 4 diff erent ind ices o f P-buffer capac i ty did no t
markedly improve the correl a t ion .
Th is was a t tributed to the effect o f
l ime o n t h e effectiveness of so il-tes ting procedures .
1 38
Fig . 8 . 4
Effect o f soil pH and added P (4 . 84 mmo l kg
-1
)
on the amount o f P extrac t ed by the Bray ( ! ) reagent .
SUMMARY AND CONCLUS IONS
1 39
SUMMARY AND CONCLUS IONS
The work p r e s ented in this thesis may be summa r i s ed as follows :
(1)
A review o f the l i terature ind ica tes tha t , although l iming ac id
so i l s generally incr eas es plant growth , there is cons iderable
controversy r egarding the e f f e c t s of l iming on the extrac tability
and / o r retention o f P by s o il s .
I t i s a l so apparent that i f the
amount o f added l ime exc eeds that need ed to neutral ize exchangeabl e
Al , a depression i n plant growth i s l ikel y .
The conf l i c t ing
views r egarding the effec t s o f l iming on ex trac table P , P sorpt ion ,
and p l ant growth appear to arise becaus e invest iga t ions were
con£ ined either t o labo rato ry st ud ies or to plant growth studies
only .
(2)
In Fij ian soils a c id ity , Al toxic ity , and / o r P deficiency prob lems
l imit agr icul tural produc t ion .
Lack o f suitab l e s o il - t est ing
proc edures and a poor unders t and ing o f l ime-Al-P int eract ions
limi t improved ut ili zat ion o f these a c id soil s .
Fo llowing
prel iminary inves t igat ions , 4 contra s t ing Fij ian so ils were cho s en
for a l ime-Al -P int erac t ion s tudy .
(3)
An inv e s t iga t ion was conduc t ed t o compar e exchangeabl e Al ext ract ion
and d e t erminat ion methods .
The resul t s o f this study showed large
d if f er ences in the amount s of Al removed from any soil depending
on the method of extract ion and det ermina t ion .
Th is ob serva tion
has impor tant impl icat ions f o r l iming of soils b ecause in recen t
yea r s there has b e en a n incr eas ing t endency t o b a s e appl ication
r a t e s on the l evel of exchangeable Al in so il s .
From this
inv e s t igat ion , it appears tha t the mos t suitab l e method of
ext r a c t ing Al from Fij ian s o ils would involve 2 X 1 -h shaking o f
so il followed by the determination o f Al i n M KCl ex tracts _ us ing
t he oxine t echnique .
(4)
Resul t s o f a preliminary l ime incub a t io n s tudy showed that there
was considerab l e variat ion in the pH buffering capac ity of the s o il s .
140
The M KCl-ex trac table Al content o f al l soils was reduced to near
z ero at so il pH values exc eeding 5 . 2 .
( 5)
The ion- r e t ent ion method is commonly used to a s s ess the surface
cha rge chara c t e r i s t ics o f soils .
The method involves an in it ial
was h ing of so i l s with an electroly t e ( prewash el ectrolyte ) of
high concentra t ion to r emove exchangeable ions , equil ibrat ion o f
the washed s o i l s with an elec trolyte o f appropriate concentra tion ,
and s ub sequent ext rac t ion o f the sa turated soils .
In an init ial
s t udy , the e f f ec t of the concent rat ion of the pr ewash elec trolytes
on the magni tude o f surface charge of unlimed and s el ected l imed
s o i l s was inves t iga ted .
Resul ts showed that the amo unt o f Al
removed dur ing the prewa sh and the equil ibrat ion s t ep had a marked
e f fe c t on surface nega tiv e charge .
However , when the Al released
in the extract ing solut ion was inc luded in the c alcul a t ion o f
char g e , the d i ff er ences i n the measur ed negative charge ob tained
wi t h varying concentrat ions of prewash el ectroly te were reduced .
As wi th the concen tra t ion o f the prewash electro lytes , the
s o i l : solut ion ratio also had a marked ef f ec t on the nega t ive
cha rge but the d i f ferenc es were largely related to the amo unt of
Al r emoved during the equil ibra t ion s t ep .
Thes e resul ts sugges t
tha t it is impor t ant to include the Al con tent o f the extract ing
solut ions in all calcula t ions o f charge .
( 6)
A s t udy of the e f f ec t o f different index cations on the surface
charge o f l imed soils showed that surface charge mea sured in
O . O lM CaC1
i s always higher than tha t determined in 0 . 0 3M NaCl .
2
Interestingly , the diff erences in sur face negat ive charge increa sed
w i th increas ing s o il pH values .
Sub s equent s tudies showed that
the d i f f er ences b e tween the 2 metho d s aro se largely f rom the
inabi l i ty of Na to effec t ively exchange with a dsorb e d Ca at high
pH values .
(7)
Following the invest igat ions reported in ( 5 ) and ( 6 ) it was
concluded tha t a suitabl e method for measuring the surface charge
o f l imed so il s woul d use O . O lM CaC 1 2 as the equilibrat ion el ectro ly t e
and include i n the calcul a t ion o f charge the amount o f Al released
in the O . SM KN0 3 extrac t ing solution .
This method cons isted of 6
equ i l ib rations o f soils with O . OlM CaCl z a t a s o il : solut ion ratio o f
1 : 2 0 , and ei ther 4 o r 5 ext ra ctions o f the equi l ibrated soils with
O . SM KN0 3 s ol u tion .
14 1
(8)
L iming had a marked effect o n the surface nega t ive and po s i t ive
char g e of the s o il s .
With increas ing pH there was a marked
increase in the surface nega tive charge and a smaller decrea s e in
sur f a c e po s i t ive charge of the s o il s .
However , the magnitude o f
the increa se i n surface nega t ive charge varied cons iderably
b e tween s oil s .
I n all s o il s , surfa c e po s i t ive charge was
det e c tab l e at pH values as h igh as 7 and this was thought to b e
due t o th e presence of permanent po s i t ive charge aris ing from
i sormorphous sub s t itut ion o f Ti 4 + and,O:r Mn4+ in the Fe oxide la t t ice .
(9)
In contras t to l iming , incub a t ion o f s o i l s with added P resul ted
in o nly small changes in bo th surface nega t ive and pos i t ive charge .
(10)
The resul ts o f an isotopic-exchange s t udy sugges t ed tha t th ere was
an increase in the amount o f surface P up to a pH of approxima tely
7.
At higher pH values exchangeable P decrea s ed , pos s ibly
b ecause of the precipita t ion of Ca-P compounds .
In contras t ,
Ol sen-extrac tab le P was at a minimum b etween pH values 5 . 5
6.0,
-
on e i ther sid e of which it incr eased , part icularly in soils t r ea t ed
w ith high amo unts of P .
The init ial decreas e in Olsen P was
a t t r ibuted to an artefact in the O l s en procedur e .
The increase
at h igh pH values was thought to b e due to the slow rel eas e of P
from precipitated Ca-P compounds .
There was a consi s t ent decrease
in Mehl ich-extrac t ab l e P with increas ing soil pH .
In a series o f
extr ac t ions th e pH o f the Mehl ich extrac tant was kept cons tant
us ing an
autoburet t e and the decrea s e in extrac tab le P was s hown
to b e largely due to the n eutral izing effect o f lime on the Mehl ich
ext ractant .
( 1 1)
Resul ts of a sorp t ion s tudy conducted in 0 . 0 1M CaCl 2 demons trated
that P sorp t ion decreas ed with increas ing pH up to pH 5 . 5
-
6.0
and th en increa s ed again .
At low initial solution P concentrat ions
( 1 . 6 1 mmo l L- 1 ) the effects were sma l l in the Batir i , Nadroloulou ,
and S eqaqa soils al though they were suff ic iently large to cau s e
pronounced changes i n the equilibrat ion solut ion P concentra t ion .
The effects were always larger in the Koronivia so il , and a l so in
the o ther so il s if h igher init ial solution P concentrat ions were
us e d .
The ini t ia l d ec r ease in P s o rp t ion with pH appears to resul t
f rom an in teract ion between added P , nega t ive charg e , and the
e l ec t ros tatic po tential in the plane of sorp t io n .
Resul ts o f a
142
comparative sorpt ion s t udy , involving KCl and CaC1 2 o f varying
concentrations as the b ackground electrolyte and us ing Nadroloulou
so il incub ated with KOH o r Ca (OH ) 2 , sugges ted that the increase in
s o rpt ion at high pH values was due to the f o rmat ion o f insoluble
Ca -P compounds .
This is cons is tent with the resul ts o f the
iso topic - exchange s tudy .
Sorp t ion s t udies conduc ted us ing soils
which had been incub ated with P a fter l iming showed a marked
redu c t ion in the amo unt of P sorbed .
( 1 2)
Resul t s o f a contro l l ed - cl imate , plant-growth s tudy us ing the
trop i cal legume L eucaena leu co c ephala showed marked d i f f erences in
plant growth between the 4 soils .
For all 4 soil s , l iming and P
addi t ion caused an ini t ial increas e in plant growth . which was
followed by a marked r educ t ion i n growth at h igh pH val ues .
Th is trend was mo s t p ronounc ed in the S eqaqa soil s .
The pH at
wh ich maximum growth o ccurred varied b etween the soil s but was
determined by the l evel of l ime needed to r educe M KCl-extractab l e
Al t o low values .
Add i t ions o f P caused a signif icant (P < 0 . 0 1 )
incre as e in plant growth within each l ime l evel in a l l s o il s and
in the B a tir i , Ko ronivia , and S eqaqa soil there was a posi tive
interact ion b e tween l ime and added P .
Al tho ugh Nadroloulou s o i l
was P def icient , a s i gnif icant interac t ion b etween l ime and
added P was no t ob s erved .
( 13)
For all 4 so ils the conc entra t ion of P in the Leucaena tops followed
trends s imilar to the yield r esponse curve .
In contra s t , the
con c entration of Al was lowes t at the pH corresponding to maximum
growth , and on eithe r s ide o f t his , Al concentration increas ed .
The increased Al in tops at low pH values was a t tributed to the
increase d availab il i ty of Al under acid conditions .
At h igh pH
values the increased A1 conc en trat ion appears to be due , at l eas t
in par t , to the concentrat ing e f fect of r educed growth wh ich has
r esul ted from factors o ther than increa s ed A1 uptak e .
eleva t ed ratio of P in roo ts to P in roo t s
+
From the
tops and low P
concent rations in plant tops i t appeared that poor growth a t
extr eme pH values was due to reduced avail ab il ity o f P .
However ,
cons ideration o f the pattern o f A1 uptake by the roo ts to gether
with the concentra t io n of P in roo ts ind ica ted that . ·Ai ·. Within the
pl ant induced P defic iency a t low pH values .
At h igh pH values
143
P d e f i c i ency was p robably r el a t e d t o a d e c r ease in s o il P supply .
This s eemed consi s t ent with resul ts o f the iso topic - exchange s tudy .
( 14 )
A glass hous e s tudy was conduc ted us ing s o i l s wh ich had been
init ially planted wi th Leucaena kucocephala .
The s tudy involved
an i n i t i al growth of ryegras s in sand cul ture and sub sequent
t r ansfer of thes e plants onto the soils .
When the ini t ially
heal thy ryegrass plant s were trans ferred onto the soils mo s t of the
p l ants grown at low and high pH values gradual ly died .
A
c omparison with control plants g rown in a s imil ar manner but no t
t rans f erred onto the soil showed that the reduced growth at ext reme
pH values was due , in part , to a toxic i ty effect rather than s impl e
P d e f ic i ency .
( 1 5)
I t was proposed that A1 was respons ib l e .
Comparison o f the da ta ob tained by res in extract ion and pl ant P
up t ake gave a cl o s e 1 : 1 relat ionship .
Colwel l- , Bray ( I ) - , B ray ( I I ) - ,
I n contras t , O lsen- ,
and Mehl i ch-extrac tab le P were
only weakly correla t e d with P uptake .
Inclus ion o f 4 different
ind ice s of P-buf fer capacity did no t improve the rel a t ionship
b e tween P uptake and extractab le P .
The diff icul ty in rela ting
plant P uptake data to extractable P l evels was a t t r ibuted to the
prob l ems as sociated with extracting P from l imed soil s .
There
was no us eful rel at ionship b e tween P up take and isotopi cal ly
exchangeab le P .
APPENDI CES
144
60
�
I
Cl
�
-
0
E
E
SEQAO.A
•
NAOROLOULOU
•
K OOO N IVIA
�:.
BAT I RI
o
-
<(
QJ
-
..0
d
�
u
d
L..
�
X
QJ
I
u
<(
0
'lit
I
z
a:>
�
I
'K)
�
0
4
5
6
pH
8
APPENDIX 1 .
Effec t of s o il pH on the amounts o f
Al extracted f rom l imed s o i l s by
pH 4 . 8 NH 4 0Ac solut ion .
9
145
APP ENDIX 2
pH of l ime- and pho spha te-treated so il s
b e fore and a f t e r growth o f Leucaena leucocepha1a .
pH
before
S o il
S eqaqa
B a t ir i
Nadro lo ulou
Ko ronivia
11
a fter .
P i ll
P2
P3
Pl
P2
P3
L 1 11
4.5
4.5
4.5
4.5
4.5
4.5
L2
4.7
4.7
4.6
4.7
4.7
4.8
L3
5.2
5.1
5.0
4.8
4 .9
4.9
L4
5.8
5.9
5.9
5.8
5.8
5.8
L5
6.8
6.8
6.8
6.5
6.6
6.6
L1
4.9
4.9
4.9
4.9
4.9
4.9
L2
5.0
5.0
5.0
5. 1
5.0
5.0
L3
5.2
5.2
5.2
5.1
5.1
5. 1
L4
5.5
5.5
5.4
5.4
5.4
5.6
L5
6.0
6.0
6.0
5.9
5.9
6.1
L1
4.0
3.9
3.9
4 .0
3.9
3.9
L2
4.1
4.1
4.1
4.1
4.1
4. 1
L3
4 .4
4.4
4.4
4.5
4.5
4 .4
L4
5.5
5.5
5.5
5.3
5 .4
5.4
L5
6.6
6.6
6.6
6.5
6.5
6.5
L1
4.2
4.2
4.2
4.2
4.2
4.2
L2
4 .4
4 .4
4.4
4.4
4.5
4.4
L3
5.4
5.4
5.4
5.3
5. 3
5.3
L4
6.1
6.1
6. 1
6. 1
6. 3
6.2
rates o f a dded l ime and P are d i s c ussed in Chapter 7 (Sec tion 7 . 2 . 1 ) .
"'
...;:r
�
APPENDIX 3a
E f f ec t o f so il pH and phosphate add i t ion on the concentra t ion o f nutrients in
Leucaena 1 e ucocepha 1 a tops grown in Koronivia so il
Ammo unt o f
added P
mmol kg - 1
pH
(M KCl )
Ca
Fe
K
•
.
Mg
Mn
Na
s
Zn
mmol kg- 1 DM tops
0 . 40
0 . 40
0 . 40
0 . 40
4.2
4.4
5.3
6. 1
96 . 2
1 26 . 0
2 19 . 2
244 . 1
1 .0
1 .0
1. 1
0.9
298 . 9
2 78 . 9
2 76 . 5
265 . 8
65 . 2
48 . 8
41 . 7
38 . 2
1.6
1.1
0.8
0.6
20 . 4
19 . 9
17. 5
20 . 1
20 . 1
26 . 7
35 . 6
36 . 1
0.3
0.3
0.2
0.2
2.42
2.42
2.42
2 .4 2
4.2
4 .4
5.3
6. 1
95 . 4
127 . 1
217 . 8
2 38 . 3
1.0
0.9
1.0
0.9
389 . 9
3 74 . 4
3 74 . 6
365 . 0
63 . 4
56 . 8
44 . 9
4.7 . 2
1.6
1.1
0.7
0.5
20 . 1
20 . 4
19 . 2
17 . 7
34 . 7
36 . 5
46 . 0
47 .9
0.4
0.3
0.2
0.2
4 . 84
4 . 84
4 . 84
4 . 84
4.2
4.4
5.3
6. 1
94 . 5
1 26 . 7
1 84 . 6
2 20 . 1
1.0
1.0
1 .0
1.0
40 1 . 5
390 . 0
39 2 . 9
37 3 . 3
63 . 8
54 . 9
50 . 4
34 . 3
1 .4
1. 1
0.7
0.5
15.6
13. 2
17 . 3
12 . 6
36 . 0
38 . 1
48 . 9
51 . 7
0.4
0.3
0.3
0.2
7.2
0.1
9.3
2.4
0.1
NS
1.6
0. 1
6.2
0. 1
8.0
2. 1
0.1
1.7
1.4
NS
LSD ( 5 % , pH )
( 5% , P)
APPENDIX 3b
Effect of so il pH and phospha t e addition on the concentrat ion o f nutrien ts
in Leucaena 1 eucocepha 1 a tops grown i n Nadroloulou soil .
Amo unt o f
added P
mmol kg- 1
pH
ea
Fe
K
Mg
Mn
Na
s
(M KCl )
Zn
mmol kg - 1 DM tops
0 . 80
0 . 80
0 . 80
0 . 80
0 . 80
4.0
4.1
4.4
5.5
6.6
44 . 0
48 . 0
1 15 . 1
1 40 . 6
2 33 . 2
1 .4
0.5
0.8
1.2
1 .0
1 16 . 9
1 59 . 7
1 76 . 8
20 7 . 7
172.0
69 . 0
59 . 9
56 . 6
43. 8
38 . 2
0.9
0. 7
0.5
0.4
0.6
30 . 4
14 . 6
8. 1
10 . 4
14 . 0
13.9
16.0
23. 5
35 . 1
29 . 0
0.5
0.4
0.3
0.2
0.2
4 . 84
4 . 84
4 . 84
4 . 84
4 . 84
4.0
4.1
4 .4
5.5
6.6
32 . 6
46 . 2
95 . 1
121 . 3
218 . 7
1.4
1.2
0.8
1.1
0.9
1 35 . 3
164 . 9
181 . 2
2 15 . 2
206 . 8
68. 1
51.2
49 . 9
46 . 0
38 . 2
0.8
0.7
0.5
0.4
0.5
19 . 8
3.6
7. 7
8.0
14 . 9
17.9
20 . 1
25. 7
34 . 0
13 . 7
0.6
0.4
0.3
0.2
0.2
9 . 68
9 . 68
9 . 68
9 . 68
9 . 68
4 .0
4.1
4 .4
5.5
6.6
37.4
46 . 6
92.9
1 24 . 4
2 10 . 1
1.1
1 .0
0.7
1.1
0.8
147 . 8
1 50 . 9
1 84 . 0
259 . 8
2 15 . 4
67 . 8
52 . 3
so . 7
45 . 9
35 . 8
0.7
0. 7
0.5
0.5
0.6
14 . 2
1.8
4.7
7.7
16.4
17.8
21.0
25 . 3
32 . 6
25 . 6
0 .4
0.3
0.2
0.2
0.2
5.9
0.2
7.8
1.9
0.1
1.3
1.5
0. 1
4.6
NS
6.1
1.4
0.1
1 .0
1.2
NS
LSD ( 5% pH )
( 5 % P)
�
.c
-...J
APPENDIX 3c
Effect o f s o il pH and phos phate addit ion on the concentration of
nut rients in · Leucaena 1 euc o ceph a 1 a tops grown in Batiri soil .
Amount o f
added P
mmo l kg- 1
pH
ea
Fe
K
Mg
Mn
(M KCl )
Na
s
Zn
mmol kg- 1 DM tops
1.61
1 . 61
1 . 61
1.61
1.61
4.9
5.0
5.2
5. 5
6.0
49 . 5
57. 7
84 . 4
1 30 . 2
1 34 . 8
1.8
1.7
1.3
1. 1
1.3
37 2 . 3
324 . 3
289 . 3
277 . 8
262 . 6
50 . 0
40 . 3
36 . 7
37 . 1
31 . 5
1.4
1.1
0.7
0. 7
0.5
10 . 7
10 . 3
10.4
10 . 0
6.6
31 . 0
30 . 2
31 . 7
32 . 7
38 . 6
0.3
0.3
0.3
0.2
0.2
4 . 84
4 . 84
4 . 84
4 . 84
4 . 84
4.9
5.0
5.2
5.5
6.0
38 . 7
52 . 5
66 . 2
122 . 4
1 29 . 8
1.6
1.5
1.1
0.9
1.0
418.0
34 7 . 6
343 . 5
332 . 8
30 1 . 8
47.6
38 . 4
37 . 0
28 . 5
30 . 0
1.3
1.0
0.7
0.6
0.5
40 . 6
8.4
10 . 4
6.9
6.4
32 . 6
37 . 8
40 . 3
42.4
40 . 8
0.3
0.3
0.2
0.2
0.2
14 . 5 2
14 . 5 2
14 . 5 2
14 . 5 2
4.9
5.0
5.2
5. 5
6.0
27. 2
41.0
63. 2
1 14 . 0
1 20 . 0
1.7
1.3
1.3
1.2
0.9
667 . 6
5 76 . 2
50 8 . 2
39 1 . 8
355 . 4
47.3
37 . 2
37 . 7
37 . 8
35 . 7
0.8
0.8
0.6
0.6
0.4
27 . 3
9.9
5.2
7.0
4.1
36 . 3
38 . 7
40 . 2
43 . 7
17.0
0. 2
0.2
0. 1
0.1
0. 1
LSD ( 5% , pH )
5. 1
0.1
12. 8
1.4
0. 1
1.3
1.6
0. 1
LSD (5% , P)
4.0
0.1
9.9
1.1
0.1
1 .0
1.2
0.1
1 4 . 52
.....
.t:00
:APPENDIX 3d
Effect o f soil pH and phosphate addition on the concentration o f
nutrients in Leucaena leucocepha 1a tops grown in Seqaqa so il
Amount o f
·
added P
mmol k g_ 1
pH
(M KC! )
ea
Fe
K
Mg
mmol kg- 1
�
,.
Mn
DM
Na
s
Zn
tops
1.61
1.61
1.61
1 . 61
1 . 61
4.5
4. 7
5.2
5.8
6.8
38 . 9
145 . 6
162 . 5
2 70 . 0
291 . 5
2.0
1.6
1.2
0.9
1.4
79 . 4
220 . 5
250 . 7
280 . 4
269 . 3
69 . 8
33 . 0
30 . 0
14 . 5
22 . 2
13. 3
2.1
1.9
1.5
1.2
58 . 8
22 . 9
21.0
27.0
18 . 9
9.6
23 . 2
29 . 0
31 . 2
41.0
0.2
0.2
0.2
0.1
0. 1
4 . 84
4 . 84
4 . 84
. 4 . 84
4 . 84
4.5
4.7
5.2
5.8
6.8
36 . 7
14 2 . 1
168 . 2
245 . 6
2 70 . 6
1.4
1.2
1.0
1.4
3.4
128 . 1
272 . 3
277 . 3
2 74 . 1
289 . 8
52 . 5
34 . 7
33 . 8
27 . 0
20 . 4
12.8
1.8
1.5
1.2
0.8
60 . 0
19 . 9
17. 5
28 . 3
17.9
11 . 8
23. 6
29 . 8
32 . 9
43.4
0.2
0.2
0.2
0.1
0.1
14 . 5 2
14 . 52
14 . 52
14 . 52
14 . 5 2
4.5
4.7
5.2
31.4
142 . 0
155 . 1
246 . 8
262 . 0
1.6
1.2
0.8
2.4
1.8
197 .0
352 . 2
326 . 7
243 . 9
3 16 . 7
37 . 9
37 . 5
39 . 8
27 . 5
23 . 6
10 . 7
1.5
1.2
1.1
0.9
60 . 0
18 . 2
15 . 8
27 . 9
17.2
12 . 8
24 . 5
30 . 6
36 . 9
48. 5
0.2
0.2
0.2
0. 1
0.1
4. 7
0.3
20 . 1
1.5
0.6
2. 1
1 .0
0 . 04
3.6
0.2
15 . 5
NS
0.5
1.6
0. 7
0 . 03
LSD (5% , pH)
(5% , P)
5.8
6.8
f(
8
.....
�
\D
B IBLIOGRAPHY
1 50
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Lime-aluminium-phosphate interactions in selected acid soils from Fiji

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