technical report on the semi-detailed soil survey of radhi watershed

Transcription

Technical Report on Semi-detailed Soil Survey of Radhi Watershed
Soil Survey Unit (SSU)
National Soil Services Centre, Semtokha
Council for RNR Research of Bhutan (CoRRB)
Ministry of Agriculture
TECHNICAL REPORT ON THE SEMI-DETAILED
SOIL SURVEY OF RADHI WATERSHED
Report No. SS - 6
December 1999
Last updated: 3/19/2013
Page 1 of 73
CONTENTS
SUMMARY................................................................................................................................ 4
ACKNOWLEDGEMENTS ........................................................................................................ 5
ABBREVIATIONS AND GLOSSARY ...................................................................................... 6
1.
2.
INTRODUCTION.............................................................................................................. 9
1.1
Radhi integrated research and extension area ........................................................................9
1.2
Aims of the Radhi semi-detailed soil survey ............................................................................9
SURVEY AREA ............................................................................................................... 10
2.1
Location and extent..................................................................................................................10
2.2
Climate ......................................................................................................................................10
Table 2.1
2.3
Geology and soil parent materials ..........................................................................................13
2.4
Topography and drainage .......................................................................................................14
2.5
Land use and vegetation ..........................................................................................................15
3.
PREVIOUS SOILS INFORMATION.......................................................................... 17
3.1
Sinclair Knight (1983)..............................................................................................................17
Table 3.1
3.2
4.
Climatic summary for Radhi 1985-1997.................................................................................... 12
Sinclair Knight (1983) soil classes at Radi-Phomi .................................................................... 17
Other studies.............................................................................................................................18
METHODS ...................................................................................................................... 19
4.1
Field ...........................................................................................................................................19
4.2
Mapping ....................................................................................................................................20
4.3
Laboratory ................................................................................................................................20
5.
SOIL CLASSIFICATION, CHARACTERISTICS AND CORRELATION ................. 21
5.1
Soil classification ......................................................................................................................21
Table 5.1
5.2
Summary of soil classes in Radhi geog ...................................................................................... 21
Characteristics of soil classes in Radhi geog ..........................................................................22
5.2.1
Grey clay (RGC) ................................................................................................................................. 22
5.2.2
Grey loam (RGL). ............................................................................................................................... 22
5.2.3
Brown loam (RBL). ............................................................................................................................ 23
5.2.4
Brown sand (RBS) .............................................................................................................................. 23
5.2.5
Yellow loam (RYL) ............................................................................................................................ 24
5.2.6
Reddish brown clay (RBC) ................................................................................................................. 24
5.2.7
Humic loam (RHL) ............................................................................................................................. 25
5.2.8
Hillside gleys (RHG) ........................................................................................................................... 25
5.2.9
Analytical characteristics of soil classes ............................................................................................. 25
Table 5.2
Ranges of chemical analyses, by soil classes, Radhi ................................................................. 26
5.3
Soil correlation .........................................................................................................................26
5.3.1
Correlation with Sinclair Knight (1983).............................................................................................. 26
Table 5.3
Correlation with Sinclair Knight (1983) soil classes................................................................. 27
5.3.2
Correlation with soils elsewhere in Bhutan ......................................................................................... 27
Table 5.4
Correlation of Radhi soils with Tethyan phyllite/quartzite Kashi soils ..................................... 27
Table 5.5
Correlation of Radhi soils with Shumar soil profiles at Drametse ............................................ 28
5.3.3
International correlations..................................................................................................................... 28
Table 5.6
International correlations of soil classes at Radhi. ................................................................... 29
5.3.4
Correlation with geotechnical classification of soils ........................................................................... 29
Table 5.7 Geotechnical correlation of Radhi soil classes ............................................................................. 30
Page 2 of 73
6.
SOIL DISTRIBUTION AND MAPPING ........................................................................ 31
6.1
Soil distribution ........................................................................................................................31
6.2
Soil mapping units....................................................................................................................31
Figure 6.1
Table 6.1
Table 6.2
7.
Soil Map of Radhi ...................................................................................................................... 32
Composition of soil mapping units at Radhi .............................................................................. 33
Areas of soil mapping units, Radhi ............................................................................................ 33
LANDSLIPS AT RADHI ................................................................................................. 34
7.1
Overview of Radhi soils ...........................................................................................................34
7.2
Slope instability in Radhi ........................................................................................................34
7.2.1
7.2.2
7.2.3
7.2.4
7.2.5
General ................................................................................................................................................ 34
Natural instability ................................................................................................................................ 34
Anthropogenic factors ......................................................................................................................... 35
Interaction of natural and anthropogenic effects ................................................................................. 36
Implications for intervention ............................................................................................................... 37
REFERENCES ....................................................................................................................... 39
APPENDIX A:
Table A.1
APPENDIX B:
Table B.I.
APPENDIX C:
METHODS OF SOIL ANALYSIS USED AT SPAL, SIMTOKHA .......... 42
Summary of current interpretation of SPAL soil analyses ......................................................... 44
SOIL PROFILE DESCRIPTIONS AND ANALYSES ............................. 45
Summary of Radhi soil profiles .................................................................................................. 45
SOIL CORRELATION ............................................................................. 70
C. I
Soil classification and correlation in Bhutan. ........................................................................70
C.2
General Criteria .......................................................................................................................70
C.2.1
C.2.2
C.2.3
C.2.4
C. 3
C.3.1
C.3.2
C.3.3
C.3.4
C.3.5
C.3.6
Soil moisture regime ........................................................................................................................... 70
Soil temperature regime ...................................................................................................................... 70
Mineralogy class ................................................................................................................................. 71
Particle size class. ................................................................................................................................ 71
Correlation of Radhi soils .......................................................................................................71
Grey clays and loams (RGC & RGL) ................................................................................................. 71
Brown loams and sands (RBL & RBS) ............................................................................................... 71
Brown clay (RBC)............................................................................................................................... 71
Yellow loam (RYL) ............................................................................................................................ 72
Humic loam (RHL) ............................................................................................................................. 72
Hillside gley (RHG) ............................................................................................................................ 72
APPENDIX D:
Table D.1
SSU SOIL SURVEY REPORTS ............................................................... 73
SSU main soil surveys and reports ............................................................................................ 73
Page 3 of 73
SUMMARY
This is the technical report of semi-detailed soil survey of Radhi geog carried out by the Soil
Survey Unit in 1998-1999.
Radhi geog has been selected for intensive research and extension activities by Khangma RNRRC because it is highly productive and also because its physical resource base is seriously
threatened by recent large-scale landslips.
The soil survey was carried out in order to provide colleagues working in the area with a general
account of the soil resources.
Radhi geog covers about 2450 ha in eastern part of Trashigang dzongkhag, and stretches from
about 1200 m to 2700 m asl. The climate is subtropical - warm temperate. Topographically it
has a roughly the shape of part of a cone, and radiates out from a narrow zone at the top to broad
lower slopes. The lower slopes are extensively used for wetland rice and it is this that makes
Radhi an important agricultural production area in Eastern Bhutan. The top of the geog is under
broadleaf forest but this has been severely degraded by excessive timber and firewood
harvesting, and by overgrazing of cattle from Radhi and the higher lands to the east.
The area is mainly underlain by clay-rich marine sedimentary rock, with subordinate quartzite
beds. These have been intruded by granitic and basic rocks. The soils are formed from parent
materials that are derived from these rocks but many of the materials have been moved and
mixed by hillwash and landslips.
The distribution of soils is influenced mainly by altitude, rock type, and topography. There is a
zone of shallow stony soils on the steep lower slopes close of the river. Above that most of the
lower slopes have a mixture of grey clays and loams, which are the main wetland rice soils.
They also appear to be the most prone to landslips. The middle slopes have a complex of brown
loams and sands, which are the main rainfed cropping soils. These soils stretch down to the
lower slopes along the crests of the moderately gentle spurs. The upper slopes have a complex
of brown and yellow loams and clays. At the very top of the geog these soils have deep, dark
humic topsoils. There are small areas of soils with naturally poor drainage mixed in with all of
the main soil types.
The landslips are estimated to cover about 150 hectares (360 acres) at present but are still
expanding, their possible causes are discussed. The area is naturally prone to this kind of
disturbance, and the landscape indicates that similar events have occurred before. The present
cycle of landslipping may be inevitable, but it might have been triggered by recent changes in
land management, such as expansion of wetlands, over-exploitation of forest resources, and road
construction. If the landslips are mainly natural and inevitable, the potential benefits of attempts
at stabilisation and soil conservation may be limited, and need to be carefully investigated.
SSU anticipate doing further, more detailed surveys of the soils of specific areas at Radhi in the
future as the research programme develops.
Page 4 of 73
ACKNOWLEDGEMENTS
The fieldwork for this Soil Survey was done by Tsheten Dorji, Ian Baillie, Kado Tshering, H.B.
Tamang and Tshering Dorji. The report and interim map were produced by Pema Wangmo. The
final soil map was digitised by Sangita Pradhan.
We are particularly grateful to Deputy Forest Ranger Tshering Peljor for his personal hospitality,
great logistical assistance, and guidance in the field.
We are also grateful to the RNR staff of the Trashigang Dzongkhag and the RNR Centre, Radhi
for their assistance on logistics, and to the Gup, Chimi and people of Radhi for their physical
assistance and hospitality in the field. The Director and staff of the Khangma RNR-RC,
especially Mr J N Pradhan, gave valuable assistance and guidance on the initial organisation, and
information on the agricultural and environmental characteristics and problems of the geog.
The analyses of the soil samples were done by the Soil and Plant Analytical Laboratory (SPAL),
MoA, Semtokha.
The supply of meteorological data from the Ministry of Trade and Industry is gratefully
acknowledged.
We are grateful to the staff of Khangma RNR-RC for discussions on the situation at Radhi and
for comments on the draft text and map. Feedback is important, because Radhi is to become a
focus of concentrated multidisciplinary research and extension activities. We would like our
findings and report to be informative and accessible, so that soils characteristics and distribution
receive full consideration in research and extension activities.
Page 5 of 73
ABBREVIATIONS AND GLOSSARY
AAS
AHT
Alluvial fan
AmOAc
AP
AvP, AP
AWC
asl
BHU
BS%
SSU
C
ca
CEC
Chhu
Chhuzing
CL
Colluvium
Complex
Consociation
CoRRB
Creep
Danida
Dzongkhag
EC
Exch
Extr
FAO
FC
fe
FY
FYM
Geog
GIS
Gley
GPS
GSI
Gully wash
ha
HCl
ISRIC
Atomic absorption spectrophotometery
Agrar - und HydrotechniK, GmbH, (Germany)
Poorly stratified and sorted material deposited in side valley
Ammonium acetate (extractant for exchangeable cations and for
measuring CEC)
Arunachal Pradesh
Available Phosphate
Available water capacity (= MC% FC – MC% WP)
Above sea level
Basic health unit
Base saturation percentage
Soil Survey Unit
Clay
Approximately
Cation exchange capacity
Stream or river
Irrigated agricultural land
Clay loam
Local hillwash, moved by surface erosion and slow non-glacial creep
processes.
Soil mapping unit with several co-equal soil classes
Soil mapping unit with one soil class dominant but others as minor
constituents
Council for RNR Research of Bhutan
Slow gravitational mass movement of colluvium downslope.
Danish International Development Assistance
Administrative district
Electrical conductivity
Exchangeable (for cations)
Extractable (for soil nutrients)
Food and Agriculture Organisation of United Nations
Field capacity (MC% at 0.1 atmospheres suction)
fine earth (particle size < 2mm)
Financial year (July – June in RGOB)
Farmyard manure
Block or subdistrict, administrative subdivision of Dzongkhag.
Geographical information system
Soil that is permanently wet, poorly aerated and has predominantly
greyish colours, due to reduction of free iron to ferrous valency state.
May have local oxidising conditions giving rust - coloured mottles,
especially around root channels.
Global positioning system
Geological Survey of India
Rapid movement of coarse, commonly bouldery, unlayered materials
down steep streams.
Hectare
Hydrochloric acid
International Soils Reference & Information Centre, Wageningen,
Netherlands
Page 6 of 73
Kamsing, Kamshing
Krotovina
L
LUPP
LUSS
MC%
me
me%
MLT
MoA
mS/cm
MTI
Nd
NH4OAc
NSSC
OC
OM
Orogeny
P
PA
pH
PD
PM
PPD
ppm
PSC
Puddle
Regolith
REID
RGOB
RNR
RNR-RC
S
Saprolite
Si
Sk
SMU
SoB
SoI
Sokshing
Solum
sp, spp
SPAL
ST
Surface wash
Tr
Rainfed agriculture
Old faunal burrow filled with dark soil from topsoil
Loam
Land Use Planning Project, in PPD
Land Use and Statistics Section, in PPD
Moisture content % (w/w)
milliequivalent (unit of exchangeable cations)
milliequivalents per 100 g fine earth
Miscellaneous Land Type
Ministry of Agriculture
milliSiemens per centimetre (unit of electrical conductivity)
Ministry of Trade and Industry
No data
Ammonium acetate
National Soil Services Centre, REID, Semtokha
Organic carbon
Organic matter
Mountain building by uplift and deformation of the earth’s crust
Phosphate
Project Agreement
Measure of acidity – alkalinity
Programme Director
Parent material
Planning and Policy Division, MoA
Parts per million
Particle size class (Soil Taxonomy)
Intensive cultivation of wet soils before planting wetland rice. Intended to
create fine slurry seedbed and to reduce subsoil permeability and water
losses
Loose material at surface of the Earth. Includes soils and underlying loose
materials such as colluvium, alluvium, and landslip, wind and ice deposits
Research, Extension and Irrigation Division, of MoA
Royal Government of Bhutan
Renewable natural resources (includes agriculture, animal husbandry and
forestry in RGOB sense)
RNR Research Centre
Sand
Soft weathered rock beneath solum, often reddish
Silt
Skeletal (high stone content)
Soil mapping unit
Survey of Bhutan
Survey of India
Forest from which needle or leaf litter is collected for livestock bedding
and FYM.
True soil, in which soil processes have removed many traces of parent
materials structure
Species (singular & plural)
Soils and Plant Analysis Laboratory, NSSC, REID, Semtokha.
Soil Taxonomy (US system of soil classification)
Movement of individual soil particles by running surface water.
Trace
Page 7 of 73
TE
TEB
TLB
TN
TRB
USDA
v/v
WR
WRB
w/w
X
Z,Zi
Trace elements
Total exchangeable bases (= exchangeable Ca + Mg + Na + K)
True left bank (facing downstream)
Total nitrogen
True right bank (facing downstream)
United States Department of Agriculture
% by volume
Weathered rock
World Reference Base for Soil Resources (ISRIC development of FAO
system of soil classification)
% by weight
Exchangeable (for cation)
Silt
Page 8 of 73
1.
INTRODUCTION
1.1
Radhi integrated research and extension area
Khangma RNR-RC has initiated the Radhi research study so as to focus practical research and
research–based extension efforts by different groups of sub-sector professionals on a restricted
area. The idea is that the concentration will enable integration of investigations on crop
production, animal production, and the management of forest, land and water resources. Their
findings can be evaluated in the context of the demographic, cadastral, the availability and
gender partitioning of labour, trading relations, and other socio-economic-cultural features of the
geog. The spatial concentration of research should highlight interactions between the subsectors.
This is particularly important in Bhutan where present agricultural practices and future
developments depend on the judicious and synergistic management of human skills and labour,
water, biomass-energy, nutrients, livestock draught-power, and land between the forest –
livestock – crop subsectors.
The choice of Radhi by Khangma differs from that of Lingmutey Chhu by Bajo RNR-RC. Radhi
is not intended to be representative of large areas in the Eastern Region. It has been selected
partly because it is an exceptionally productive and intensively exploited area. Also its resources
are under considerable stress, and it has serious but locally specific environmental and
production problems. In particular it has lost considerable areas of productive land and access to
water resources due to the recent re-activation of large landslips.
SSU hopes to contribute to these activities by establishing the range and distribution of soil
conditions and resources within the area. This should help sub-sector specialists with some
aspects of the nutrient and water management. In addition, fuller appreciation of the soil pattern
may account for some inter-sub-sector interactions. The survey will also provide data that can
feed into the investigations and on-going discussions of the causes of, and possible responses to,
the landslip problem.
1.2
Aims of the Radhi semi-detailed soil survey
This semi-detailed soil survey was undertaken partly to supply the participants at Radhi with
background soils information but it is also part of the SSU training programme. The objectives
are:
Providing Khangma RNR-RC and other participants in the Radhi integrated research and
intervention activities with information on the soils of the geog.
Contributing to the on-going discussion of the causes of, and possible responses to, the
landslip problem
Further training in semi-detailed soil survey techniques for SSU staff
Adjusting standard soil survey practices to suit conditions in Bhutan.
Providing SSU with data towards the development of a national soil classification, and of
national and regional soil maps.
SSU see this survey as the first stage of their involvement in the work at Radhi. They anticipate
doing more detailed surveys in the future, in response to the needs of other researchers. These
surveys may be undertaken either to clarify soil features of interest, or to refine the soil mapping
of specific areas.
Page 9 of 73
2.
SURVEY AREA
2.1
Location and extent
The survey area covers the whole of the Radhi geog. It stretches from latitude 270 19.25’ to 270
23.30’ N, and from longitude 910 40.35’ to 910 44.50’ E. It is located in the eastern part of
Trashigang Dzongkhag, on the south (true left) bank of Gum Ri, about halfway between its
headwaters near the Arunachal Pradesh border in the east and its confluence with Gong Ri
(Dangme Chhu) near Trashigang in the west. The geog is more or less triangular in shape. The
northern side runs along the south bank of Gum Ri, the western side follows the interfluve spur
between the Buna Ri branch of Nakpo Rong (the Ranjung drainage) and Shong Ri up to near
Shetimi, and the eastern side follows the most easterly (Kardung nunnery) spur west of Yedi Ri,
also up to near Shetimi. The survey covers about 2445 ha (6040 acres).
The geog is about one hour's drive from Trashigang along the recently black-topped Phongme
feeder road. The geog has a recently upgraded junior high school, a BHU, and an RNR Centre
with Forestry, Agriculture, and Animal Husbandry extension staff. It contains about 700
households (REID 1999) and an estimated population of about 4000 people.
2.2
Climate
The geog spans an altitudinal range of over 1500 m, stretching from about 1050 m asl at its
lowest point along Gum Ri to about 2750 m asl near Shetimi. Its climate ranges from
subtropical to cool temperate.
The only climatic data from within the survey area are for Radhi Panthang, at about 1500 m.
They are summarised in Table 2.1. The daily data from the Ministry of Trade and Industry are
complete for the months March – November. However the daily data are missing in many years
for the school holiday period in December – February, although monthly summaries are usually
given. The status of these summaries is uncertain. It is possible that the data in Table 2.1 underestimate the December – February monthly total (and therefore also the annual total) rainfalls.
However this is the driest time of the year and any under-estimates for that period will not
seriously distort the perception of the overall moisture balance. The summaries may also miss
the absolute maximum and minimum temperatures, and also the heaviest daily rainfalls for these
months. As this is the coldest time of the year, the absolute minimum temperatures are therefore
probably lower than indicated in Table 2.1
The climate at Radhi Panthang is warm temperate to subtropical, and the soil temperature regime
is probably thermic (Soil Survey Staff 1999). The data in Table 2.1 indicate that the atmospheric
temperature occasionally drops to just above freezing in January, so that there are likely to be
occasional ground frosts. As the daily records for the winter months are incomplete, there may
be some nights when air temperatures drop below freezing. The temperatures rise steadily until
May when the means level off at about 22-23 0C, and stay at that level until September. The
absence of a sharp summer peak is due to the high cloud cover during the monsoon. This keeps
the mean maximum temperatures down to about 30 0C and the minima up at about 15 0C. The
highest absolute maximum temperatures have been recorded in the spring and autumn when
skies are clear. These are also the months with the highest temperature ranges, as the clear skies
allow for considerable night cooling.
Taking Eguchi’s (1997) generalised lapse rate for Bhutan of about 0.5 0C per 100 m increase in
altitude, the mean temperature in the Shetimi area at the top of the geog is probably about 12 0C,
Page 10 of 73
with mean December – January minima of about 0 - 1 0C, and mean June – September maxima
of about 20-21 0C. The boundary between thermic and mesic soil temperature regimes (mean
annual temperature of 15 0C) is probably at about 2000 – 2200 m a.s.l.
The mean annual rainfall at Radhi Panthang is 1520 mm, of which about 1150 mm (76%) falls in
the monsoon months of May – September. The onset of the monsoon in this part of Bhutan
appears to be erratic, and considerable rainfalls can occur in April and even in March. The end
of the monsoon appears to be more regular, in late September – early October. The autumn and
spring months are not completely dry, and occasional heavy storms can occur. However
December – February is generally dry, with clear skies.
It is not known how rainfall varies spatially in the geog. There is probably an increase with
altitude but the geog is on a north-facing slope in a deeply entrenched East-West valley, so the
increase may be modest. The mean annual rainfall at Shetimi may be no more than about 2000
mm.
The generally warm temperatures and summer rainfall may give winter moisture deficits in the
lower part of the geog. The increase in rainfall and decrease in temperature with altitude
probably means that crops and vegetation on the upper slopes are less stressed for moisture.
Although badly disturbed, the broadleaf forest at the top of the geog appears to be well watered.
The RNR-RC studies on farming systems and cropping pattern will indicate the extent of winter
rainfed cropping in the non – irrigated land on the middle slopes.
Page 11 of 73
Table 2.1
Climatic summary for Radhi 1985-1997
Year
J
F
M
A
M
J
J
A
S
O
N
D
11*
11*
13
13
13
13
13
13
13
13
13
13*
11.8
14.4
15.9
17.4
19.4
22.2
22.8
22.6
22.7
19.8
16.8
14.2
17.2
20.7
22.1
22.7
24.2
26.7
27.2
27.1
26.8
25.9
23.4
20.8
23.6
(Average of monthly maxima)
6.5
8.0
9.7
12.0
14.5
17.7
18.3
18.0
16.6
13.8
10.2
7.5
11.5
(Average of monthly minima)
25.0
29.0
34.0
30.5
33.0
34.0
34.0
33.0
31.5
35.5
29.0
28.0
35.5
(Hottest daily maximum on record)
1.8
2.0
4.0
3.0
6.0
6.0
13.0
14.0
10.0
6.5
5.0
3.0
1.8
(Coldest daily minimum on record)
11*
11*
13
13
13
13
13
13
13
13
13
13*
Mean of monthly totals
33
53
80
104
146
221
295
218
166
73
43
18
Monthly maximum
(Highest total on record for month)
98
114
190
201
248
325
422
285
298
141
96
70
Monthly minimum
(Lowest total on record for month)
0
21
21
8
41
67
126
140
98
11
0
0
Highest daily rainfall
20
40
50
48
96
(Wettest day on record for month)
* Includes records with monthly summaries but incomplete daily data
55
90
50
50
55
80
18
11*
(Years with complete data)
1520
(Mean for years with complete data)
2096
(Highest total for years with complete
data)
1066
(Lowest total for years with complete
data)
96
(Wettest day on record)
Data from MUMTI
Temperature 0C
n (number of complete records)
Mean
(Average of monthly mean
minimum. & mean maximum)
Mean maximum
(Average of daily maxima for
month)
Mean minimum
(Average of daily minima for
month)
Absolute maximum
(Hottest daily maximum on record
for month)
Absolute minimum
(Coldest daily minimum on record
for month)
Rainfall (mm)
n (number of complete records)
Source:
Page 12 of 73
11*
(Years with complete data)
18.3
(Average of monthly means)
2.3
Geology and soil parent materials
The geog is underlain by rocks of the Merak-Sakten Tethyan block. These overlie the high grade
metamorphic rocks of the Himalayan Central Crystalline Complex. The Tethyan rocks were laid
down as marine sediments in the former Tethys Ocean, which used to lie between the Indian and
Eurasian continental plates. As the Indian Plate moved northwards against Laurasia (Eurasia),
the ocean was squeezed and eventually disappeared. Its floor was forced up and much of the
Tibetan plateau is formed from marine sedimentary rocks. They were subject to pressure, heat
and deformation during their compression and uplift.
The Merak-Sakten block, like the Black Mountain block in Central Bhutan, is a separate outcrop
of Tethyan rocks, and not a southwards extension of the Tibetan Tethyan basin, (unlike the
Lunana and Lingshi Tethyan blocks in the north and northwest of Bhutan). The Merak-Sakten
and Black Mountains Tethyan rocks are thought to have been rafted into their present position
when the underlying sheets of gneiss of the Himalayan Central Crystalline Complex were thrust
southwards and outwards during the Himalayan orogeny. The Merak-Sakten block are the oldest
Tethyan rocks in Bhutan and are of Paleozoic age. So far no fossils have been recorded, whereas
Paleozoic and Mesozoic fossils have been found in the younger Tethyan rocks of the Black
Mountain and Lingshi blocks (Singh 1978 a, b & c; Chaturvedi et al. 1983; Ganesan 1982;
Ganesan & Bose 1982).
The Merak-Sakten block was included in the Tirkhola Formation in some geological summaries
(ESCAP 1991), but has been amalgamated into the revised and expanded Chekha Formation in
the recent synthesis of the geology of Bhutan (Tangri & Pande 1995). This designation accords
with the original mapping in the area, in which the main formation mapped was Chekha (Verma
1972). The outcrops in the Radhi area are mainly those of the basal members of the formation.
The predominant rock type is greenish grey, highly lustrous, and argillitic. It varies between
phyllite, mica schist, and schistose phyllite. The original mapping noted that the phyllite/schist
in the Radhi area contains staurolite and has a 'greasy' lustre. The rocks further east, in the
Phongme area, are garnetiferous rather than staurolitic (Verma 1972). Both types are
characteristic of intermediate metamorphism of sedimentary rocks with high clay contents. The
predominant phyllite/schists are interbedded with quartzite and transitional phyllitic quartzite. In
Radhi the general dip of the Chekha beds is northwards, down to the axis of the Sakten syncline,
which is mapped as running more or less East-West, to the north of Gum Ri (Bhargava 1995).
The Chekha phyllite and quartzite beds are intruded by large bodies of the Chomolhari
leucogranites, which are of Miocene age. The largest outcrops are to the north of Gum Ri and
there are no large outcrops in the Radhi area. However beds of granitic pegmatites and many
small anatectic bodies of granite have intruded the Chheka rocks throughout the survey area, and
especially on the slopes in the east of the geog. The country rock quartzites are often locally
mylonised (crushed fine) in the vicinity of these intrusions (Verma 1972). The Tethyan beds are
also intruded by later quartz veins and by dykes of greenish mafic/ultramafic rocks, mostly
amphibolite. Boulders of the latter are particularly common in the bed of Yedi Ri to the east of
the survey area, and have been used as decorative elements in retaining walls and revetments
along Yedi Ri section of the Phongme feeder road.
Many of the soils are formed in slope drift rather than residual parent materials. The
geotechnical section of the Sinclair Knight study (1983) designated the slope deposits as
colluvium, and differentiated three types:
Reddish brown silty clays with some sand and stone/boulders on the upper slopes
Page 73 of 73
Yellowish, low plasticity clay and silt, with subordinate sand, on the ridge lines
Brown and yellow micaceous clays and silts on the middle and lower slopes.
The term 'colluvium' has been used so far by SSU for hill slope drift that has been deposited
gradually by surface wash and slow gravitational creep. Many of the slope deposits at Radhi
appear to have been moved more rapidly than this, by the action of periodic landslips. Many
sites and deposits have been disturbed more than once, so that the drift material has been
frequently re-mixed and is now highly variable. The main types of colluvial/landslip drift on the
slopes identified in this survey are:
Phyllitic (schist) material, which weathers to grey and brown silty clay, with soft soapy and
silvery fragments of phyllite, which are usually platy. This material has variable contents of
sand and hard quartz stones, depending on the proportion of quartzite in the source beds.
Grey and brown sand - sandy clay loam with quartz stones, derived from beds where quartzite or
quartz veins predominate.
Yellow – brown sand - sandy clay, derived from granites and pegmatites.
Reddish brown clay, derived from the mafic/ultramafic intrusive rocks.
Many slope deposits are mixtures of these types.
The only alluvial deposits in the area are small patches of bouldery river alluvium on low
terraces along Gum Ri. There are more extensive and higher terraces on the north bank of Gum
Ri, in northern section of Phongme geog. The main side streams in the Radhi area are Shong Ri
and Yedi Ri. They are steeply graded, and transport unsorted gully wash, mostly derived from
landslips. The deposits have not splayed out in their lower courses nor deposited as significant
alluvial fans in the survey area. However there are some well developed fans to the north of the
survey area, along the tributary streams in Phongme geog on the north bank of Gum Ri.
There are no obvious wind-blown deposits in the survey area. However the soils on the upper
slopes have high contents of silt and fine sand. These may be aeolian, but could also be derived
from the weathering of the underlying rocks.
2.4
Topography and drainage
The geog occupies the mainly north and northwest facing slopes of a section of the main valley
of Gum Ri. This part of the valley is strongly asymmetrical, with moderate - gentle slopes on the
south bank, in the survey area, but steeper slopes in Phongme geog to the north of the river. This
valley form is common in eastern and southern Bhutan. It has been attributed to the progressive
sideways movement of the riverbed in the direction of dip where a river runs along the strike of
moderately dipping and landslip-prone beds (Takada 1991). Gum Ri is currently down-cutting
and presumably still migrating northwards down the dip towards the axis of the syncline, and
there are cutoff bluffs along the lower ends of terraces, fans and colluvial slopes on the north
bank. The river is clearly effective in rapidly removing large volumes of debris coming down
from the landslip area, and Shong Ri does not have a large alluvial fan or debris cone in its lower
stretches or at its confluence with Gum Ri.
The basic shape of Radhi geog is part of a large cone, sloping down towards the North and
Northwest. It is drained and dissected by a number of minor and intermediate streams that
radiate outwards from the Shetimi area, such as (from West to East) Shong Ri, Khochi Ri,
Samtharong Ri, and Yedi Ri. The slopes are moderate and more or less rectilinear in the upper
part of the geog. The ruling slopes are also rectilinear for most of the lower sections, except for
the convexities where the toe has been steepened by the recent downcutting of Gum Ri. The
Page 14of 73
lower slopes above the convex section are undulating, with broad gentle spurs and intervening
wide re-entrants. As well as clearly defined drainage lines, there are a number of seepage zones
and springs where the throughflow of water in the subsoil comes up the surface. These seepages
are most commonly seen on the lower slopes, and are often associated with landslips and the
slumping of agricultural terraces.
The generally conical configuration has resource implications, which make the geog unusual. It
has relatively extensive gentle lower slopes and relatively small areas of upper slopes. This
distribution means that the geog is relatively well endowed with irrigable and arable lands, but
has relatively small areas of pasture and forest.
The most notable topographic feature of Radhi is the recently re-activated landslip complex in
the catchment of Shong Ri, near the western boundary of the geog. The area affected covers
about 150 hectares, but appears to be still expanding. The main area is in the middle part of the
catchment, with a scar several hundreds of metres wide and about a kilometre long. The back
and side walls are over 10 m high in places. The floor of the scarred area is mostly a jumble of
wet soils and fragments of weathered rock, mainly phyllite and but also with quartzite and
crystalline rocks. The Shong Ri catchment was noted as unstable by Sinclair Knight (1983) in
the early 1980’s, and the rumpled land surface in the general area suggests a previous history of
mass movements. The recent bout of slipping began in the middle-late 1980’s. The short history
and large size of the recent slips indicate massive and efficient removal of debris by Shong Ri,
possibly of the order of 100 000 to 500 000 cubic metres per year. There are also landslips on
the lower slopes along Yedi Ri, and also where road cuttings and wetland terrace walls have
collapsed elsewhere in the geog (Sharma et al. 1998). However none of these other slips are
anything like the same size as those in the Shong Ri catchment.
Figure 2.1 Major Slip on Road to Radhi
Figure 2.2 Regenerated Areas of the Radhi Slip
Fuller accounts of the recent slips and the attempts to control them are available from RNR-RC
Khangma. The origins and implications of the slipping are discussed in Section 7.
2.5
Land use and vegetation
As the geog is to be the focus of concentrated research and extension activities, its land cover,
farming systems and agronomy are described in detail elsewhere. Those needing more
information on these aspects of the geog should refer to the relevant reports by Khangma RNRRC.
Page 15of 73
The general land use pattern shows a clear altitudinal and topographic sequence. At the very
bottom of the geog, along the banks of Gum Ri, cultivation is patchy. This is because the slopes
have been steepened by the currently active downcutting of the river. Above that there is a broad
belt of irrigated rice lands (chhushing). Above that there is a belt of rainfed cultivation
(kamshing). This appears to be cropped on a rotational basis, and there are considerable areas of
fallow. The rainfed cropping may have formerly extended further upslope than it does now, and
some of the fallow may actually be early regrowth after abandonment. Most of the top end of the
geog is under broadleaf forest. There are some clearings in the forest along the higher spur
crests. These are under a short sward of unimproved pasture species.
The geog is one of the main rice-producing areas in the Eastern region. Its rice surpluses are
widely traded, particularly with the pastoralist people in the mountains to the east. Terraced
irrigated arable lands are reported to total about 300 ha, which is about 13 % of the total area of
the geog (Kuensel, 5.12.1998), but this may be an underestimate. The wetlands are used for rice
in the summer. Nurseries are established in May, transplanting is done in June-July, and the crop
is harvested in October-November. In winter the wetlands are used for some wheat and mustard
cultivation. An important aspect of the irrigated rice cultivation is the effect it has on regolith
and slope stability. Many of the rice soils are medium or coarse textured. They are ploughed
and leveled after the first flooding but do not appear to be intensively puddled. It seems that they
are irrigated liberally throughout the cropping season and there appears to be considerable deep
percolation. This significantly increases the weight and probably decreases the strength of the
subsoil and weathered rock, predisposing them to failure and slippage. The possible interactions
of cultivation practices with other factors affecting slope stability are discussed in chapter 7.
The main crops grown in the belt of rainfed cultivation above the rice wetlands are maize and
vegetables. Rainfed cultivation is thought to have previously extended further upslope, but some
of the higher areas have been abandoned or are used only occasionally as tseri. The contraction
in rainfed cropping appears to have happened fairly recently and there is little colonisation so far
by trees and shrubs. The dominant vegetation is dense stands of tall herbaceous weeds,
particularly Artemisia myriantha. The combined area of the currently active dryland and
abandoned/tseri areas totals about 450 ha (Kuensel, 5.12.1998).
The main vegetation in the upper part of the geog is broadleaf forest. This contains high
proportions of oak and rhododendrons. The forest is intensively used for grazing and for
firewood cutting. In places all visible trees that have been severely lopped. The intensity of
cutting is such that the trees are not always able to recover, and patches of stag-headed, dying
and dead trees are common. These eventually develop into open glades. In moist sites near
drainage lines they have moderately thick stands of bamboo. Elsewhere the ground cover is a
mixture of grasses and herbs. The opening up of the forest may increase its grazing productivity,
but it reduces the wood resources available. Fuelwood is becoming scarce in the geog, and
householders now need to go considerable distances to collect it.
Page 16of 73
3.
PREVIOUS SOILS INFORMATION
3.1
Sinclair Knight (1983)
Radhi was one of the areas covered by the Sinclair Knight study (1983). This is the first soil
survey in the country for which we have documentation. They covered four potential irrigation
areas in the east of Bhutan, totaling over 3000 ha. As well as Radi-Phomi (their spelling),
covering 1400 ha, they surveyed:
Khamdang, Trashiyantse, ca 1200 ha command, altitude 1200 – 2000 m.
Chali, Mongar, ca 350 ha command, altitude 750 – 1700 m.
Chaskar, Mongar, ca 300 ha command, altitude 1150 – 2000 m.
At the time of the study Trashiyangtse was not a separate dzongkhag; hence the apparent
contradiction between their study title and the location of the Khandang survey area.
Their survey in the Radi-Phomi area covers part of our area, but they stopped at 2200 m altitude,
as that was the limit of command of the contour canal. They surveyed about 2000 ha, of which
1400 was in Radhi geog. The intensity was about detailed reconnaissance. They examined the
soils at 49 sites, of which 30 are in Radhi. A positive feature of the study is that they produced a
soil map of the area, at a scale of 1:20 000. It does not map continuous tracts but only areas of
potentially irrigable land. Steep rocky and poorly drained areas along streams are not included.
The study used simple soil classes, which are correlated to the classes of the 1974 FAO Soil Map
of the World, but are given relatively straightforward names, the meanings of which are clearly
explained. The classes used at Radi-Phomi are listed in Table 3.1. It should be noted that their
FAO soil class of Lithosol has since been renamed as Leptosol (FAO 1998). The absence of
poorly drained soils and Gleysols is probably due to the exclusion of areas near drainage lines
from their mapping.
Table 3.1
Sinclair Knight (1983) soil classes at Radi-Phomi
No.
1
Sinclair Knight
Name
Shallow stony soils
Lithosols & Regosols
2
Alpine clay loams
Phaeozems
3
Reddish or yellowish clays
Orthic Acrisols
4
Brown clay loams
Humic Cambisols
5
Reddish sandy clay loams
Dystric Cambisols
6
Grey micaceous
clay loams
7
Silty alluvial terrace soils
Equivalent in 1974 FAO system
sandy Humic Cambisols
Dystric Cambisols
Page 17of 73
The only soil that they did not map at Radi-Phomi is No. 7, the black sandy clay loam (Humic
Cambisol), which occurs in only one of their four survey areas, at Khamdang. The river terrace
alluvial soils are mapped at Radi-Phomi but only as small discontinuous pockets on the south
bank of Gum Ri.
Because they actually mapped the soils, rather than just describing them, they were able to
identify broad patterns and relationships in soil spatial patterns. For Radi-Phomi, they identified
a topo - lithological sequence of soils running downslope of:
Dark 'alpine' clay loam on the ridge line
Reddish clay derived from pegmatites, with subordinate reddish sandy loam on the upper slopes
Brown and dark grey darker soils on micaceous schists downslope.
Steep non-arable land just above Gum Ri
Their study includes a geotechnical assessment, with particular emphasis on methods of terrace
construction and slope stability. The geotechnical emphasis is on the subsoil and underlying
regolith, rather than the upper one metre or so of agricultural soils, and distinguished three main
groups of colluvial deposits:
Reddish brown silty clays with some sand and stone/boulders on the upper slopes
Yellowish low plasticity clay and silt, with sand, on the ridge lines
Brown and yellow micaceous clays and silts on the middle and lower slopes.
This study was done before the start of the current phase of massive landslipping in the Shong Ri
catchment, but they did identify the area as susceptible. Their observations on the causes and
nature of landscape instability in the area are incorporated into the discussion in Section 7.2.
3.2
Other studies
The only other soil study known to include soils developed on Tethyan parent materials is that of
the Kashi geog in Wangdue-phrodrang dzongkhag (LUPP 1997). This is over 100 km to the west
and is mostly at higher altitudes than Radhi, so it is of limited relevance. Nonetheless the
phyllitic soil profiles at Kashi are correlated with the Radhi soil classes in Section 5.3
The Drometse study (LUPP 1996) covers an area about 25 km west of Radhi. About half is
underlain by phyllites and quartzites. However these belong to the Lesser Himalayan Shumar
Formation, and are not Tethyan. In Drametse they appear to the granites, pegmatites and
ultramafic intrusions of the Chheka formation at Radhi. They also differ from the Radhi rocks in
that they include subordinate limestone bands. The tectonics of Drometse are less active and the
landscape appears to be more stable than at Radhi. The main phyllitic soils at Drometse are
yellowish brown sandy clay loams - sandy clays. The quartzite soils are of similar colours but
coarser textures. Despite the geological and topographic differences, there are some similarities
between the soils of Drometse and Radhi. Some Drometse profiles are correlated with the Radhi
soil classes in Section 5.3.
Page 18of 73
4.
METHODS
4.1
Field
This was the second semi-detailed soil survey undertaken by SSU, and it was partly done as a
field training exercise. As most of the soil surveyors had about one year of field experience
when they did the fieldwork, they did much of the routine observation independently. The
fieldwork was done in two spells, in April and October 1998.
The soils were examined on a routine basis at 96 sites, mainly with a 1.2 m Edelman auger, fitted
with a 7 cm combination head where possible, but switching to a 7cm stony soil head where
necessary. At 13 sites, where the first attempt to auger was stopped by stones at less than 50 cm,
a second attempt was made within a few metres. A few of the routine examinations were done in
road cuttings, cut back at least 15 cm to expose fresh soil. Most of the observations were located
at regular altitude intervals (20 or 50 m) along footpaths that run up-and-down slope and are
marked or easily located on the base map. The density of observations is lower in the upper
parts of the survey area.
For routine soil observations the following site data were collected:
Location, GPS; general topography and site position; the angle (in %), aspect, length and
form of the slope; soil parent material; general land use and current crops/vegetation;
presence and type of irrigation; artificial land shaping features; fertiliser use, if known;
site drainage; and surface stones.
The soil profiles were described according to their natural layering (horizons) in the upper one
metre, and not at fixed depths. The following data were collected for each horizon:
Munsell colour of matrix (in field moisture condition); number, size, contrast and colour
of mottles; field texture; number, size and type of stones; moisture condition; and
consistence on the auger.
Particular attention was paid to the presence or absence of puddled layers and plough pans In this
survey, as puddling practices affect water movement and slope stability.
The soils were described in more detail at 12 sites. Five of the detailed descriptions were done in
freshly cleaned-back cuttings, and the remaining seven in purpose – dug profile pits. The site
data were the same as for the routine sites, with the addition of a detailed description of surface
features, including:
Microrelief, rock outcrops, litter, cracks, faunal activity, sand wash, and capping.
The soils were described by horizons. The data collected for each horizon were the same as in
the routine descriptions, with the addition of:
Strength, size and type of soil structure; number and size of pores; presence, strength and
continuity of cutans (shiny coatings on surfaces of soil structural units); moisture state
and consistence, in situ and in hand; number size and type of roots; reaction to HCl (to
test for presence of free carbonate minerals); concretions of iron, manganese or other
secondary formations; presence and effects of animals (wormcasts etc.); any other
features (e.g. charcoal); clarity and shape of lower boundary.
Page 19of 73
4.2
Mapping
The Survey of Bhutan topographic mapping of the area is sheet 78 M/11 at scale 1:50 000. The
current edition was originally produced by the Survey of India in 1963, and is based on 1:40 000
aerial photography from 1956-8, and ground control from 1959-61. Despite its age and scale,
the map gives a reasonably accurate depiction of the landscape. The interim soil map is based on
a 125 000 photo-magnification of this map. The Survey of Bhutan is currently re-mapping the
area at a scale of 1:25 000, and the soil map will be re-drafted using GIS when the new SoB
mapping is available as an improved topo base.
The interim map shows only the soil boundaries. However the infrastructure and the locations of
the soil augerings and the profile pits have also been plotted on photo – magnified hard copies of
the base map. Eventually all of these data will be digitised into the GIS as three separate covers:
infrastructure; soil inspection sites; and soil boundaries.
4.3
Laboratory
The 39 soil samples collected from the main horizons of the 12 detailed profiles were analysed
by the Soil and Plant Analytical Laboratory (SPAL) of the National Soil Service Centre, (NSSC)
of the Research, Extension and Irrigation Division (REID) of the Ministry of Agriculture at
Semtokha. The methods of analysis used by SPAL are summarised in Appendix A.
The only chemical methodological points that need to be mentioned here concern the
measurement of cation exchange capacity (CEC) and calculation of base saturation (BS%). CEC
can be measured by saturating the soil with ammonium cations, and then displacing and
measuring the quantity required. This is referred to as CEC (NH4OAC). An alternative is to
estimate CEC by summing the exchangeable bases (Ca + Mg + K + Na = total exchangeable
bases (TEB)), and the extractable aluminium and hydrogen. This is known as the 'effective
cation exchange capacity (ECEC). SPAL does not measure extractable Al or H in soils with pH
(water) greater than 5.5. Few of the Radhi samples from this survey are that acid. There were no
determinations for extractable Al or H for the less acid soils. In such cases the ECEC is identical
with the TEB, and is not informative, so it not been given for these soils. Base saturation is the
quotient TEB/CEC. If the TEB and the ECEC are identical (as is the case where there is no
extractable aluminium or hydrogen), the 'effective base saturation' (EBS % = TEB/ECEC) is
automatically 100 %. Such values add no information, and have not been given for these soils
report. Unless specified otherwise, the base saturations in the soil class descriptions in Section
5.2 and in the soil profile data in Appendix B refer to TEB/CEC (NH4OAC). ECEC, EBS% and
extractable Al and H are given only for the more acid soils.
Page 20of 73
5.
SOIL CLASSIFICATION, CHARACTERISTICS AND CORRELATION
5.1
Soil classification
As SSU is still in its early stages, soil classification is being done in an interim ad hoc way.
Until we have formulated and tested a national soil classification, we are treating each survey as
a separate task, and setting up local soil classes. Nonetheless, this is the seventh soil survey done
by the Project and we are beginning to build up a picture of the soils of Bhutan. We are
therefore able to draw parallels between the soils of Radhi and some other areas in Bhutan in
Section 5.3.
The soils of Radhi are grouped into eight local classes, which are defined on field characteristics,
such as colour, texture, depth, drainage and stone content. Topographic position and parent
material are also taken into account, as this aid field differentiation and affect important soil
properties that are not directly apparent in the soil profile, such as soil water movement and
organic matter dynamics. The classes are summarized in Table 5.1. The ‘R’ prefix in the class
code signifies Radhi.
Table 5.1
Summary of soil classes in Radhi geog
Soil class
Code
RGC
Main features
Profiles
and
analyses
Appendix B
Name
Grey clay Dark grey loam; over grey silty clay loam - clay; over PC007
stony clay or loam. Lower slopes
RGL
Grey
loam
Dark grey loam; over brownish grey sandy loam - silty Pd023
loam; over stony loam. Lower slopes.
PH036
RBL
Brown
loam
Greyish brown sandy loam; over brown - reddish brown PC009
sandy clay loam; over weathered quartzite or pegmatite. PC010
Midslope & spurs
Pd022
RBS
Brown
sand
RYL
Yellow
loam
Deep, dark brown - grey sandy loam; over brown-dark PC008
brown sandy loam – loamy sand. Midslope & spurs
PC026
PC027
Brown sandy clay loam; over bright brownish yellow Pd021
sandy or silty clay loam – silty clay; over weathered
phyllite. Mid & upper slopes
RBC
Reddish
brown
clay
Dark olive-yellowish brown silty clay loam; over strong Pd016
or reddish brown clay loam – clay; over weathered
amphibolite/phyllite. Mid & upper slopes
RHL
Humic
loam
RHG
Hillside
gley
Deep dark silty topsoil; over dull yellowish sandy loam- Pd020
sandy clay loam; over weathered phyllite & quartzite.
Upper slopes.
Grey, rust mottled, wet soils of variable textures. Drainage lines, springs & seepage zones
Page 21of 73
5.2
Characteristics of soil classes in Radhi geog
5.2.1 Grey clay (RGC)
These are moderately important soils for rice cultivation in the geog. They occur on the side
slopes of the spurs and in the re-entrants between spurs in the middle and lower parts of the area.
They do not occur on the crests or upper slopes of the spurs, or in the upper section of the geog.
Where they are not cultivated, these soils have an irregular, jumbled looking meso-topography
due to repeated landslipping. However this is not apparent in most areas of these soils, as their
surfaces have been greatly modified by the construction of agricultural terraces. There is one
fully described and sampled profile in these soils (see PC007 in Appendix B).
The main morphological features are the dark colours and the medium-fine textures. The
topsoils are mostly greyish brown with reddish brown mottles. Some of them are quite light
coloured (light brownish grey) due to prolonged use for irrigated rice. Textures vary from fine
sandy loam to silty clay. Topsoil structures are medium subangular blocky when moist but these
soils dry to hard clods. The subsoils are mainly dark grey or dark greyish brown with faint
brown mottling. Textures vary according to the mixing history by landslipping, but fine textures
predominate with many silty clay loams and silty clays. The structures are suprisingly fine,
mainly fine subangular blocky and crumb when moist, but these subsoils were not seen when
dry. Subsoil consistence is mostly slightly firm when moist. There are variable contents of
phyllite and quartzite stones, depending on mixing. The subsoil overlies colluvial or landslip
mixtures of weathered phyllite and quartzite stones at depths of 50 -150 cm. These soils are not
very deep, with weathered rock or dense stones preventing augering to100 cm in more than half
of the routine inspections. The weathered rock is predominantly dark grey, but the generally high
contents of muscovite flakes give it a shiny and silvery appearance.
The topsoils are acid, with pH in water about 5.5. The pH increases to become only slightly acid
or neutral at depth. The exchangeable base status also increases with depth, with base saturation
rising from about 50% in the topsoils to about 65-80% in the subsoils. Organic matter levels are
low, probably due to the prolonged cultivation for irrigated rice. The artificial nature of some of
these soils shows up in the organic carbon profiles, with rises in the subsoils attributed to former
topsoils that have been buried during terrace construction. Total N is also low but C:N ratios are
mostly satisfactory, in the range 10-15. Available P contents are high in Profile PC007, probably
due to recent FYM applications. The contents of exchangeable and available K are both
surprisingly low in view of the abundance of K-bearing micaceous minerals.
5.2.2 Grey loam (RGL).
These are the more extensive medium and coarse textured equivalents of the grey clays. They
are found in similar locations, in the shallow lower areas (re-entrants) in between spurs on the
middle and lower slopes. They are widely used for rice cultivation. There are two fully
described and sampled profiles (see Pd023 & PH036 in Appendix B).
The profiles are similar to the grey clays but the colours are slightly more brownish (brownish
grey rather than dark grey) in the subsoils, and the textures are coarser, mainly heavy sandy
loams and sandy clay loams. The two groups of soils appear to be of similar modes of
formation, formed in seepage areas on landslip deposits. The parent materials of these soils
contain more quartzite and pegmatite and less phyllite than those of the grey clays.
Page 22of 73
The topsoil is dark grey fine sandy loam with district reddish brown mottles. It has a coarse
subangular blocky structure, which dries to hard clods. However the moist consistence is friable.
The subsoil is dark greyish or yellowish brown heavy sandy loam or sandy clay loam, with
reddish brown mottles and scattered multi-coloured patches derived from fragments of
weathered pegmatite and phyllite. It has a moderate – strong subangular blocky structure with
strong but discontinuous clayskins. Like the grey clays, these are not particularly deep soils, with
more than half of the routine augerings unable to reach 100 cm. The underlying material is a
jumbled mixture of soil and weathered pieces of phyllite and quartzite.
These soils have neutral topsoils, possibly due to prolonged use for irrigated rice. The subsoils
are acid, with pH (water) in the range 5 – 5.5. In Profile Pd023 the base status does not accord
with the pH values as the base saturation is lowest (<40%) in the neutral topsoil, and rises to
>50% in the acid subsoil samples. Organic carbon, total nitrogen and available P are all low very low. As in the grey clays, the contents of exchangeable and available K are both suprisingly
low.
5.2.3 Brown loam (RBL).
There are extensive areas of reddish and brownish soils on the middle and lower slopes of the
geog. They predominate on the crests and side slopes of the spurs that run down the lower
slopes. However they are also found as minor components in the areas of mainly grey soils in
the intervening re-entrants. There is a considerable textural range and they are divided into two
classes on texture – these medium textured loams and the coarser textured reddish brown loamy
sands (RBS, see 5.2.4).
The reddish brown loams are used for irrigated rice, dryland crops, but there also still a few
small areas of remnant broadleaf woodland used for fuel and sokshing (leaf litter for livestock
bedding and organic fertilizer). There are three fully described and sampled profiles (see PC009,
PC010 and Pd022 in Appendix B).
The topsoil is dark greyish brown very fine sandy loam with a medium subangular blocky
structure. The subsoil is brown sandy clay loam but grades to reddish brown at depth. It has
yellow, black and orange but no grey mottles and appears to be well drained. Subsoil structures
are coarse subangular blocky with weak discontinuous clayskins.
These soils are neutral or slightly acid. In one profile the pH is highest at the surface and
decreases in the subsoil, but the trend is reversed in the other two. However the differences are
not very marked in any of them. Base saturations are high (>50%) throughout, except in the one
topsoil in which the pH has been raised by rice cultivation. The high base status is mostly due to
moderate contents of exchangeable Ca. Organic carbon, total N and available P are very low in
all profiles. The contents of exchangeable K are low - moderate but available K is low.
5.2.4 Brown sand (RBS)
These are the coarse textured equivalents of the brown loams. They occur on the crests and
upper side slopes of the spurs on the middle and lower slopes of the geog. They are used for
irrigated rice but the textures are too coarse for them to be wholly satisfactory, and they tend to
be left for woodland. There are two fully described and sampled profiles in these soils (see
PC008 and PC027 in Appendix B).
The profiles of some of these soils are very simple. Profile PC027 consists of very dark greyish
brown fine sandy loam over deep very dark brown loamy sand-sandy loam. The horizonation is
Page 23of 73
more complex in other profiles, depending on the local history of colluvial and landslip
deposition. Most profiles have dark brown – very dark greyish brown sandy loam topsoils with
crumb or moderate fine compound subangular structures. The subsoils are brown, strong brown
or reddish brown. Textures vary from sand to sandy loam, with loamy sands predominant in
lower subsoils.
These soils are highly leached and quite acid, with all pH values (in water) below 5.5 in all
samples, and less than 5 in some subsoil horizons. The base status is extremely low, with base
saturations below 10% in all samples from two of the profiles. Exchangeable Ca is particularly
low, reading as a trace or zero in several horizons. Exchangeable K however is high in one
profile and low, but not extremely, in the other two. Topsoil organic carbon contents are
moderate, and C:N ratios are good. The reasonable levels of organic matter in all three profiles
are attributed to the woodland vegetation, even though it is probably depleted by sokshing.
Available P levels are mostly high, possibly another result of the woodland vegetation. As in the
brown loam, K status is low-moderate.
5.2.5 Yellow loam (RYL)
These are brightly coloured medium and fine textured soils that are found on the upper and
middle slopes of the geog. Small areas are used for irrigated rice but they are mostly under
rainfed cropping or herbaceous fallow, with under some disturbed broadleaf woodland. There is
one fully described and sampled profile in these soils (see Pd021 in Appendix B).
The topsoil is dark brown – brown fine sandy clay loam with a friable consistence and coarse
crumb structure. The subsoils are brownish yellow and range in texture from sandy clay loam to
silty clay. Horizon 38-78 cm in Profile Pd021 is darker coloured, and this looks like a former
surface soil that has been buried by later colluviation or minor slumping. The subsoils have
moderate subangular blocky structures, some of which are compound and break to crumbs and
some of which have discontinuous clayskins. The underlying weathered rock at about 80 cm in
Profile Pd021 is phyllite. However it is thought that quartzite and pegmatite also contribute to
the parent materials of the more textured of these soils.
The single analysed profile has a slightly acid topsoil, with a pH (water) of 5.5. The pH
increases to about neutral (pH 6 - 7) in the subsoil. However the lab data show the whole soil to
extremely base-depleted, with TEB values < 1 and base saturation values of <5% throughout.
Organic carbon contents are high and C:N ratios are good, but available P contents are all very
low, as are both exchangeable and available K.
5.2.6 Reddish brown clay (RBC)
These soils occur on the upper slopes of the geog, particularly along the eastern spur up to
Shetimi. It is thought that they are derived from amphibolitic parent material. Most of them are
under intensively exploited and highly disturbed broadleaf forest. Some were cleared for
dryland cropping but many of these areas appear to have been recently abandoned. There is one
described and analysed profile of these soils (see Pd016 in Appendix B).
The topsoil is olive brown-dark yellowish brown silty clay loam. The yellowish colours and
high silt contents may due to aeolian deposition and make the surfaces of these soils look similar
to the humic yellowish loams further upslope. The topsoils of the brown clays have blocky
structures and friable consistence. The subsoil is distinctly reddish, ranging from reddish yellow
to reddish or strong brown. The texture is clay-clay loam. The structures are moderately
developed blocky and have clayskins, although these are weak and discontinuous. The
Page 24of 73
consistence is firm and the subsoil appears to be fairly densely packed and is not profusely
rooted. There are common boulders and stones of fine-grained dark crystalline rock, thought to
be amphibolite.
The topsoil is slightly acid, but the subsoil is almost neutral. Base saturation is much higher than
in this profile in any of the other soils at high altitude (compare with low values in RYL and
RHL). The moderate content of bases is attributed to amphibolite in the parent material.
Exchangeable Mg:Ca ratios are moderate, in the range 0.5 – 1 , and also reflect the amphibolitic
influence. Contents of organic carbon, Total N, and available P and K are low to very low.
5.2.7 Humic loam (RHL)
These soils were seen only on the upper slopes and higher ridge crests in the Shetimi area at the
top of the geog. They are under highly disturbed broadleaf forest or short sward unimproved
pastures. None were seen under cultivation. These soils are gullying rapidly and extensively.
There is one fully described and analysed profile in these soils (see Pd020 in Appendix B).
The striking feature of these soils is the deep, dark coloured topsoil. This may be up to 60 cm
thick. It is dark grey-dark brown in colour and has silty loam - silty clay loam texture. It is
friable and porous and several earthworms were seen in the topsoil of the described profile.
However, the structure is subangular blocky rather than crumb. The subsoil is dull yellowish
coloured fine sandy loam-fine sandy clay. The consistence is still friable and the subsoil appears
to be loosely packed, with many medium earthworm burrows. The structure is subangular
blocky, with discontinuous organic cutans washed down from the humic topsoil. The described
profile is of moderate depth with weathering rock at less than 1 m. The underlying bedrock is
mostly quartzitic phyllite. This is highly weathered and crumbly and in places it is deeply
gullied. The gullies are active, and are still cutting down and backwards at rates of several
metres per year.
The single analysed profile is acid and has very low exchangeable base status, with base
saturations of less than 10% and very low exchangeable Ca contents. The deep dark topsoil is
reflected in the moderately high organic carbon contents down to about 50 cm. Total N levels
are moderate and C:N ratios are good, but available P and K are very low throughout.
5.2.8 Hillside gleys (RHG)
These are soils that are permanently wet. They are found scattered as small patches along
drainage lines and around springs and seepage zones. There are no described or analysed profiles
in these soils.
They are predominantly grey in colour, but some of them have distinct reddish brown 'rust'
mottles. Some also have black humic topsoils. This is particularly noticeable in bamboo patches
in the forest. Textures are variable. Structures are weak, and consistence varies from fairly
loose in sandy gleys to sticky and plastic heavy loams and clays. There are no analytical data for
these soils
5.2.9 Analytical characteristics of soil classes
Table 5.2 summarizes the ranges of chemical characteristics of the soil classes, derived from the
analyses of the profile samples collected during the survey. There are no data for the hillside
gleys (RHG). The most noteworthy feature is the generally low contents of exchangeable and
Page 25of 73
available K in the soils. The high proportions of micaceous phyllites in the parent materials
should be a good source of K.
Table 5.2
Ranges of chemical analyses, by soil classes, Radhi
Soil class
(number of
profiles
analysed)
Topsoil only
Topsoil and subsoil (T/S)
Org. C
(%)
Total
N
(%)
C:N
AvP
(pp
m)
RGC (1)
1.5
0.2
8
RGL (2)
1.0 - 1.8
0.1
RBL (3)
0.9 - 1.3
0.1
13
14
13
18
RBS (3)
2.2 - 3.6
Tr - 0.2
13
16
RYL (1)
5.0
0.8
RBC (1)
1.1
RHL (1)
5.0
pH
TEB
me %
BS (%)
Exch K
(me %)
AvK
(ppm)
35
5.3/
6.4
3.1 /
12.9
56 /
79
0.1- 0.4
/ 0.1- 0.5
57/68
-
1- 6
1-6
4.1 - 7.3
/ 6.0 - 8.4
3.6 - 13.5/
6.8 - 17.5
37 -64
/ 65-70
34 - 84/
52 - 97
0.3/ 0.3
-
5.3 -7.0
/ 5.4- 7.0
5.4 - 7.0/
6.3 - 6.7
0.2 - 0.5/
0.1 - 0.5
19-39/
19-20
106-109
/20-66
-
7 35
4.7 - 5.3/
4.7 - 5.3
0.6 - 1.4/
0.4 - 0.7
4 - 19/
5 - 13/
0.2 - 1.1/
0 1 - 0.2
75-206
/16-56
7
2
5.5/ 6.9
0.7/ 0.4
3/ 2
0.2/ 0.1
32/9
0.1
16
3
5.3/ 6.5
9.5/ 16.4
92/ 87
0.1/ 0.2
0.3
15
2
5.4/ 5.3
1.8/ 0.5
6/ 3
0.8/ 0.2
164/
24
18/16
See Table A.1 in Appendix A for interpretation of these values
5.3
Soil correlation
The local soil classes are given simple descriptive names that are only applicable in the Radhi
area. In order to see how the Radhi soils fit into the regional and national context, it is necessary
to correlate them with others soils in Bhutan and elsewhere.
5.3.1 Correlation with Sinclair Knight (1983)
The approximate correlation between our soil classes and those used in the Sinclair Knight
(1983) reconnaissance study of Radhi are summarised in Table 5.3. The correlations are not
very firm because the Sinclair Knight soil classes are not described in detail nor are there any
analytical data. Table 5.3 shows that the main arable soils of Sinclair Knight relate fairly well to
our soil classes. They had separate classes for shallow and stony soils. As is clear from the soil
class descriptions (see section 5.2), most of the main soil classes are stony at depth, and we
found few deep, stone-free profiles. Separation of a single class for shallow and stony soil is
therefore not appropriate, although we do map a band of shallow soils on the lower slopes (see
Section 6). They also had a separate class for silty alluvial terrace soils. They indicated that the
river terrace soils are insignificant in Radhi. Because they mapped discontinuous sections of the
slopes and avoided drainage lines, they have no real equivalent of the hillside gleys
Page 26of 73
Table 5.3
Correlation with Sinclair Knight (1983) soil classes
Approximate equivalent
Sinclair – Knight (1983) soil class
SSU soil class
Grey clay (RGC)
Grey loam (RGL)
Brown loam (RBL)
Brown sand (RBS)
Yellow loam (RYL)
Reddish brown clay (RBC)
Humic loam (RHL)
Hillside gley (RHG)
# 6 Grey micaceous sandy clay loam
# 5 Reddish sandy clay loam
# 3 Reddish or yellowish clay
# 4 Brown clay loam
# 2 Alpine clay loam
-
5.3.2 Correlation with soils elsewhere in Bhutan
SSU have now completed the fieldwork for seven soil surveys and are able to start drawing
tentative parallels between soils they have seen in different parts of Bhutan. In addition
correlations are possible with soils data in some of the 1995-7 land resource evaluations by
LUPP.
None of the previous SSU soil survey areas have been on Tethyan bedrock or in parent materials
derived predominantly from phyllites or related argillites. However the LUPP (1997) study area
at Kashi is partially underlain by Tethyan sedimentary rocks, some of which are argillaceous.
These appear to be more competent than the Chheka (Tirkhola) Formation at Radhi, and have
not given rise rapid and large scale mass movements. More importantly they are intricately
intermixed with limestones. This has affected soil texture, pH, base saturations and the contents
of exchangeable Ca. Also the Kashi study appears to have few phyllitic soils at the higher
altitudes. However, there are some similarities between some phyllitic profiles at Kashi and the
Radhi soil classes, as summarised in Table 5.4. It seems that no naturally wet gleys were
examined at Kashi, and there are no profiles equivalent to the Radhi hillside gleys (RHG).
Table 5.4
Correlation of Radhi soils with Tethyan phyllite/quartzite Kashi soils
Radhi soil class
Similar Kashi profiles (LUPP 1997)
Grey clay (RGC)
KP10
Grey loam (RGL)
Brown loam (RBL)
Brown sand (RBS)
Yellow loam (RYL)
KP16, KP17
-
KP11, KP12
Humic loam (RHL)
KP18
Hillside gley (RHG)
-
Page 27of 73
The LUPP study area that is closest in distance, altitude and climatic conditions is at Drametse,
about 20 km to the west of Radhi (LUPP 1995). About half of Drametse overlies argillites and
phyllites. Although these are of the Lesser Himalayan Shumar formation and are not Tethyan,
the soils are somewhat similar to those of Radhi, as summarised in Table 5.5
Table 5.5
Correlation of Radhi soils with Shumar soil profiles at Drametse
Radhi soil class
Drametse profiles (LUPP 1995)
(phyllite/quartzite)
Grey clay (RGC)
-
Grey loam (RGL)
DP11
Brown loam (RBL)
DP10, DP13, DP14, DP16, DP37, DP39
Brown sand (RBS)
DP8, DP12
Yellow loam (RYL)
-
DP15, DP17
Humic loam (RHL)
-
Hillside gley (RHG)
-
There is also a LUPP study area at Bongo-Gengu (LUPP 1997) which has soils that are derived
from phyllites and other argillaceous rocks. The rocks are of the Shumar and other Lesser
Himalayan formations (Bhargava 1995) and are thus similar to the argillites at Drametse. The
Bongo-Gengu area is very wet, with mean annual rainfalls of three metres or more, so that the
soils are much more leached than those at Radhi.
5.3.3 International correlations
The local names given to classes and soil mapping units are too vague for soil scientists outside
Bhutan. For them it is necessary to give the equivalents in the international systems of soil
classification. Unfortunately there are several of these, with little agreement or standardisation
between them. For the present SSU is correlating its soil classes with the systems of FAO and
the US Department of Agriculture. In the first batch of reports (Nos 1, 2, SS3, SS4 and SS5), the
SSU soil classes were correlated with the 1988 Revision of the FAO Legend of the Soil Map of
the World and the 5th Edition (1992) of the Keys to USDA Soil Taxonomy. Since then SSU has
received updated versions of both systems. Table 5.5 correlates the Radhi soil classes with the
1998 version of the World Reference Base for Soil Resources (FAO, with ISSS and ISRIC) and
with the 8th edition (1998) of the Keys to the USDA Soil Taxonomy.
Because full and definite correlations require laboratory and environmental data that are not
available for the Radhi soils, the correlations in Table 5.6 are approximate. The basis of the
correlations is discussed further in Appendix C.
Page 28of 73
Table 5.6
International correlations of soil classes at Radhi.
Radhi Soil Class
Subunit in World
Reference Base for Soil
Resources (FAO 1998)
Great group in USDA Soil Taxonomy
(Soil Survey Staff 1998)
[Family in italics]
Code
RGC
Name
Grey clay
Hydragric Anthrosol
(Aquic) Haplanthrept, Anthraquic & Typic
Haplustept
[thermic, fine loamy, mixed]
RGL
Grey loam
Hydragric Anthrosol
(Aquic) Haplanthrept, Anthraquic & Typic
Haplustept
[thermic, loamy, mixed]
RBL
Brown
loam
Dystric or Haplic
Cambisol, Hydragric
Anthrosol
Typic & Anthraquic Haplustept,
(Aquic) Haplanthrept,
[thermic, loamy, mixed]
RBS
Brown
sand
Dystric or Haplic
Arenosol or Cambisol
Dystric or Typic Haplustept
[thermic, coarse loamy & sandy mixed]
RYL
Yellow
loam
Dystric or Haplic
Cambisol
Dystric or Typic Haplustept
[mesic, fine loamy & mixed]
RBC
Reddish
brown
clay
Chromic or Haplic
Luvisol or Cambisol
Typic Haplustalf, Dystric or Typic
Haplustept
[mesic, clay mixed]
RHL
Humic
loam
Siltic or Haplic
Phaeozem
Humic Dystrustept, Dystric or Typic
Haplustept
[mesic, loamy, mixed]
RHG
Hillside
gley
Dystric or Haplic
Gleysol
Endo- (& Epi)aquept (& -aquent)
(thermic & mesic, variable PSC, mixed)
5.3.4 Correlation with geotechnical classification of soils
Stability of soil terraces and water conveyance systems are important for irrigated agriculture in
Bhutan. They are particularly important at Radhi, because it is an important rice producing area
in eastern Bhutan, and because areas of its terraced land are under apparent threat of terrace
failure and large-scale landslides. Table 5.7 correlates the soil classes of Radhi with the
geotechnical classification of the Irrigation Section of REID of MoA.
As discussed in section 7.2, it is not the agricultural soils of the top metre or so, but rather the
regolith and saprolite beneath them, that that are thought to be unstable. Because the soil classes
are defined on features in the top metre, the classification does not account for some aspects of
the distribution of erosion features in Radhi. The soil classes most prone to landslips are RGC
and RGL, derived from the phyllites and other argillaceous rocks. Gullying appears to be more
Page 29of 73
widespread in the finer textured soils in the upper parts of the area, i.e. classes RYL, RBC, and
RHL.
Table 5.7
Geotechnical correlation of Radhi soil classes
Radhi soil class
REID Irrigation Section Geotechnical Soil Classification
Land unit
RGC
RGL
RBL
RBS
RYL
RBC
RHL
RHG
Soil class
CL
3A/B & 4B/C/D
(low plasticity clay)
(upper & mid hillslopes & valley SC
side slopes, including slips and (mixed sand & clay)
debris deposits)
SM
(mixed sand & silt)
ML
3B & 4B/C/D
(low plasticity loam)
(upper & mid hillslopes & valley CL/ML
side slopes, including slips and (low plasticity clay & loam)
debris deposits)
ML/OL
(low plasticity & organic loam)
3 A/B and 4A/C/D
CL/OL
(most slope positions but (low plasticity clay & organic loam)
especially in valley heads &
lower slopes)
Source for class criteria: CIP (1993)
Page 30of 73
6.
SOIL DISTRIBUTION AND MAPPING
6.1
Soil distribution
The distribution of the soils is complex, due to varied parent materials, climatic and vegetation
differences with altitude, and soil mixing by landslips, and the construction of wetland
agricultural terraces. Nonetheless it is possible to see a general pattern.
The grey soils on the lower slopes just above the river tend to be shallow and stony. The soils of
the re-entrants on the lower slopes tend to grey colours and fine textures, with a high proportion
of phyllite in their parent materials. Their grey colours are accentuated by the majority of them
being used for padi rice. The soils of the midslopes and of the spurs running down to Gum Ri
are mostly brown. Those on the eastern side of the area are medium textured, with loams
predominant and sands and clays of lesser extent. On the western side, the soils are sandier. The
most extensive soils on the upper slopes are the yellow loams. These are replaced by the reddish
brown clays, where amphibolites are important components of the parent material, and by the
dark-surfaced humic loams in the grasslands around Shetimi at the top of the area. Small areas
of gley soils (RHG) are associated with all of the other soil types, especially the grey clays and
loam (RGC and RGL)
The pattern we found corresponds more or less with that reported by Sinclair and Knight (1983).
Our mapping units are based rather more on topography and less on assumptions about the
underlying geology. Also we map the whole geog rather than discontinuous patches, and our
survey area extends higher upslope.
6.2
Soil mapping units
Because of the complex pattern and the limited field data, it is not possible to map any simple
units in which one soil class predominates. Instead the soil mapping units are mostly complexes.
In some of these there are two main soil classes, with other minor constituents. In others one soil
class is most extensive but accounts for less than two thirds of the area, and the minor constituent
classes are significant.
There are two miscellaneous land types, for the bouldery areas on the terraces and for the
landslips and gullies. These units are defined in terms of surface features rather than soil profile
characteristics. The compositions of the SMU are summarised in Table 6.1, and their areas in
Table 6.2.
Table 6.2 shows the grey clays and loams are the most extensive soils. This is a consequence the
conical shape of the area, with a high proportion of lower slopes. The brown loams are the most
extensive mid-slope soils, and the yellow loam the most extensive at the top of the geog. The
landslips are estimated to cover about 145 hectares (ca 360 acres). However they are very
dynamic and may have extended since the time of our fieldwork.
Page 31of 73
Figure 6.1
Soil Map of Radhi
Page 32of 73
Table 6.1
Composition of soil mapping units at Radhi
Mapping unit
Type
TX
Miscellaneous
Main soil classes
Bouldery terrace soils
Minor soil classes
GSX
Complex
Shallow RGL, RGC
RGC, RCL,RHG
GX
Complex
RGC, RGL
RBL, RBS, RHG
BLX
Complex
RBL
RBS, RBC, RGL, RGC, RHG
BSX
Complex
RBS
RBL, RGL, RHG
YX
Complex
RYL
RBC, RHL, RHG
HX
Complex
RHL
RYL, RBC,RHG
LG
Miscellaneous
Landslip & gullywash
Table 6.2
Areas of soil mapping units, Radhi
Area
acres % of survey area
Soil mapping Unit
Ha
TX
20
49
0.8
GSX
140
346
5.8
GX
880
2174
36.0
BLX
600
1482
24.5
BSX
230
568
9.4
YX
390
962
16.0
HX
40
100
1.6
LG
145
358
5.9
TOTAL
2445
6039
100.0
Page 33of 73
7.
LANDSLIPS AT RADHI
7.1
Overview of Radhi soils
The landscape of Radhi is unusual for eastern Bhutan, in its extensive tracts of gently sloping
land, with potential for wetland rice production, on the lower slopes of the valley. This
topography is partly due to the distinctive geology, with large areas of gently dipping and
erodible phyllites. The predominant erosion process is mass movement, with wide and frequent
shallow landslips down the bedding planes. This combination of geology and topography partly
explains the soil pattern. The soils of the lower part of the geog are largely formed in landslip
and hillwash deposits derived from phyllite and subordinate quartzite. This accounts for the
generally fine and medium textures on the lower slopes, and the moderate and high frequencies
of stones. The mainly grey colours are partly due to the predominance of wetland rice
cultivation and also to the relative youth and limited weathering of the frequently disturbed
parent materials. There are also some browner or redder and coarser textured soils on the lower
slopes, but these are mainly on the spurs. These are due to the presence of granite components in
hillwash and landslip slope deposits.
Further upslope the soils are generally more brightly coloured, often yellow, reddish yellow and
reddish brown. The brighter colours are partly because there are no significant areas of wetland
on the higher slopes. It is probably also partly due to higher proportions of non-phyllites in the
parent materials. The soils have moderate-high silt and clay contents. It appears that mafic
intrusive rocks are as important as quartzites and granites. The presence of deep, dark humic
soils at the very top of the geog is attributed to the cooler climate and the activities of vigorous
earthworm populations.
7.2
Slope instability in Radhi
7.2.1 General
The landslips are the most serious issue in the management of natural resources in the geog, and
are the main reason why the area has been selected by RNR-RC Khangma for focused multidisciplinary research and extension. The vulnerability of the slopes of the Shong Ri catchment
was explicitly identified in the Sinclair Knight study (1983). The catastrophic landslips that have
occurred since then have been well publicised, and now feature in the general press (e.g. Kuensel
5.12.1998 and 16.1.1999).
The current burst of landslipping can be viewed as:
A set of natural and inevitable processes that as going through a particularly active phase
at present
Or the result of recent increases in human pressure on the land resources and
deterioration of land management practices.
Or the interaction of both sets of effects.
7.2.2 Natural instability
Features that tend to support the 'inevitable natural catastrophe' interpretation include:
The lower slopes have an undulating and jumbled meso- and micro-topography, consisting of
what looks like the overlaying of several phases of previous landslipping. Newer headwalls and
Page 34of 73
sidewalls appear cut into older features. This suggests that there have been several earlier phases
of extensive landslipping. Much of the land affected by earlier slips has since been terraced and
is now irrigated for wetland rice.
Gum Ri, like most of the main trunk rivers of Eastern Bhutan, is currently in a phase of active
downcutting. At Radhi this results in the steeper lower slopes just above the river and in the
cutoff bluffs where the river has sliced the lower ends off terraces and fans. As well as actual
erosion, the main stream of Gum Ri is also effective in removing the debris resulting from
erosion in tributary valleys. The large volumes of material generated by the landslips in Shong
Ri have not been deposited as massive debris fans in its lower valley nor in the main valley of
Gum Ri. The whole lot has been rapidly and efficiently swept away by the main river.
Moreover the material has been transported for long distances, and it has not been dumped as
large sandbanks and bars just downstream.
The Phongme-Ranjung stretch of the Gum Ri valley, including Radhi, has an asymmetric profile,
with a steep northern slope and a relatively gentle southern slope. This valley form has been
interpreted as characteristic of areas in which mass movement down dip planes is the dominant
erosion process (Takada 1991).
The underlying rock is not very hard and weathers readily to give deep saprolite. ‘Saprolite’
(derived from the Latin word for soap) is a really appropriate term for the weathered micaceous
phyllite in Shong Ri, as it can be very slippery when wet.
Because phyllites and other argillaceous rocks are important, many of the soils have moderate or
high clay contents. This makes many of the subsoils only slowly permeable, which favours
subsoil throughflow. Areas where this emerges at the surface as springs or seepage zones are
particularly susceptible to slippage.
The moderate angles of dip favour landslipping. Steep or near–vertical dips would facilitate easy
percolation of water and deep weathering but would not give rise to very active slip planes. Zero
or low angles of dip (i.e. near horizontal) reduce the potential for slipping along bedding planes.
They also tend to give lower permeabilities and more limited weathering.
Although the main landslips are moderately shallow relative to their size, many of the failures
occur at depths of several metres. This is below the zone of influence of most effects of
agricultural land management.
7.2.3 Anthropogenic factors
If land mismanagement is seen as the main cause, the kinds of abuse should be identifiable.
There appear to be a number of anthropogenic processes that could have caused, or at least
triggered, the current land slipping:
Clearing or severe cutting of forest reduces its use of soil water because the leaf area is lower.
This increases the water available for surface runoff and deep percolation. The runoff can lead
to increased gully formation. Several substantial gullies were seen in the forest and grassland
upslope from the area of the current landslips. The gullies tend to peter out downslope, and few,
if any, seem to run down and cut into the landslip back wall. However the situation is very
dynamic and it is possible that the present isolated gullies may grow downwards and link up with
landslip complex in the future.
Page 35of 73
Reduction of water use by forests because of over-cutting and also the irrigation of increased
areas of wetland both have the effect of increasing the percolation of water into the subsoils and
below. This has the effects of both increasing the weight of the soils and the underlying soft
weathered rock, and at the same time reducing their strength and making them more likely to
slide.
The effects of wetland irrigation on the loading and weakening of slopes are increased if the soils
are not effectively puddled. Irrigation then tends to over-liberal, with high percolation losses
being made good by application of additional water. We found no or only faint traces of pans at
plough depth in the wetland soils at Radhi. This ties in with farmer reports that they level the
plots, often with a heavy wooden board, after pre-irrigation and cultivation but do not puddle the
soils intensively. Percolation rates are therefore high, predisposing the soils to slippage.
Small increases in irrigated areas may cause disproportionate slipping, if the expansion is into
marginal land near springs, seepage zones, or existing free faces. Once started, the fresh
landslipping may eat back into areas that were previously stable
The construction of irrigation bench terraces involves the creation of many new free faces in the
landscape. On the steeper terraced slopes the free faces may be high enough to be footed in
colluvium or saprolite that is wetted by subsoil lateral throughflow and prone to slip.
Construction activity, such as of the Phongme feeder road also creates new free faces, some of
which are higher (up to 10m) than those created by agricultural terracing, and thus more liable to
failure. There are many slope and bank failures along the Phongme road. These have been
investigated in detail by PWD engineers (Sharma et al .1998). It appears to take very small
events or factors to trigger slippage in unstable road cuttings. For instance, leakage from a single
domestic tap was blamed for one failure just east of Shong Ri on the Radhi-Ranjung section of
the road. Although serious, these failures are at present much smaller than the Shong Ri
landslips. However it is possible to envisage fairly small landslips starting from such failures
and enlarging as they work back upslope by progressive undercutting.
7.2.4 Interaction of natural and anthropogenic effects
The present situation probably results from a combination of natural instability and recent
changes in land management. The topographic features noted in Chapter 2 and Section 7.2.2
suggests the area is currently going through a phase of intense tectonic uplift, river down-cutting
and resultant geomorphological adjustment. These are natural and inevitable consequences of
being located in an area of tectonic activity, such as the Merak-Sakten block of Tethyan rocks.
These processes do not always proceed smoothly and may be naturally jerky, with phases of
relative calm interspersed with intense bursts of disturbance.
In such a situation of spasmodic instability, it may take only minor changes to trigger fresh
activity. This may have happened in Radhi. It is not known which, if any, of the recent changes
in land management - increased wetlands, forest degradation, or road construction – may have
contributed most to the triggering of the current burst of landslipping.
Intensive observation during the wet season is required to determine the details of the
landslipping process. For instance we need to know more about:
Main routes followed by the main volumes of runoff water,
Locations of initial failure sites,
Relationships between the initial sites and the sequence of subsidiary slipping upslope,
Page 36of 73
Roles of wetland terracing and road cutting free faces in initiating slips,
Routing and progress of landslip debris once it is detached.
Interactions between upslope gullying and downslope landslips, and especially the routes
and effects of runoff from the gullies
We also need some medium term observations to determine how often landslipping recurs at any
site, and the likelihood of substantial natural regeneration of vegetation.
7.2.5 Implications for intervention
The interpretation of Radhi as a naturally unstable area, currently in an active phase, suggests
that major landslips are inevitable. Mass movements are positive feedback processes. The free
face created by one landslip makes it easier for another to follow by failure of the new head- or
sidewalls. Once started, such sequences of landslips run their course, and fade only when the
land surface has adjusted to a new, river-controlled base level. This scenario implies that there
is little can be done to stop the process once it has started, and that appropriate management
responses are mainly restricted to:
identification and evacuation of the most vulnerable sites,
identification of warning signs of imminent major failures, and continual vigilance to
prevent loss of life and movable property,
a flexible agricultural system to exploit landslip sites once they appear to have become
moderately stable.
In contrast to this rather fatalistic approach to response and remediation, the implication of the
'anthropogenic trigger' scenario is that landslipping may be reduced if the exacerbating land
management practices are modified. The present cycle is now clearly in an active phase.
Because of the self-reinforcing nature of landslip sequences, reduction of mismanagement
practices may not even stabilise the status quo immediately, let alone reverse the damage already
done. However remedial action and improved land management may halt the current phase
earlier, limit the area affected, and allow for earlier rehabilitation. For example, the reduction of
wetland cultivation near springs and seepage zones or close to the edges of slips and gullies,
more attention to the drainage and strengthening of road cuttings, and measures to stop forest
degradation may act to slow down the expansion of the landslips.
These measures are going to involve disturbance for the people of Radhi and surrounding areas.
They will also mean some engineering costs for the PWD. It is not justifiable to advocate such
disruptive or expensive measures unless we have some indication that they will be effective.
This requires some idea of what is actually happening and some details of the physical processes
involved. Remedial intervention measures need to be based on a correct and comprehensive
understanding of the processes and factors involved if they are to be of lasting benefit.
The weathered phyllite is moderately fertile, relative to most saprolites. Once an area is stable
the rate of natural re-vegetation appears to be rapid. There are substantial saplings already
established in stabilised spots on the current landslip debris. However the trees seem to follow
the stability rather than cause it. Once established, their uptake and transpiration will increase dewatering of the immediate area around them and their roots may bind together the soils and
underlying weathered rock. These benefits are likely to accelerate stabilisation after the main
slipping has stopped but are probably too small scale to actually prevent large slips.
It is not easy to see identify those areas where the slipping is complete or where the quiet is only
temporary. It is therefore difficult to predict where tree planting will be beneficial. Blanket
Page 37of 73
afforestation of the landslip scars will lead to much wasted effort and high losses of planting
material, because renewed slipping is likely to remove seedlings over large areas.
However one measure that can be recommended immediately is the complete protection of any
natural regeneration on landslip areas. No livestock access or any form of cutting should take
place in these areas. The same level of protection should be extended to existing forest areas
within 50 or 100m m of the edge of the landslip sites.
Once the current phase of landslipping is complete, the landslip sites can be seen as an
opportunity rather than a threat. In Radhi itself, in many other places in Bhutan, and elsewhere
in the Eastern Himalayas, there are many areas where farmers have created productive irrigated
and rainfed arable lands on the sites of old landslips. Deposits of landslip debris have some
advantages as parent materials for the development of agricultural soils. They tend to be deep,
and therefore have moderate or better capacities for storing available water. They contain fine
earth mixed up with pieces of fresh or only moderately weathered rock, which can act as new
sources of some plant nutrients, especially K, Ca and Mg. The deposits tend to be loosely
packed, which facilitates deep rooting. On the debit side the new surface layers have low
contents of organic matter and of available N and P. The build-up of organic matter in the
topsoils is essential in order to bring these sites into full production.
For Shong Ri, as in any other landslip area, a difficult problem is deciding when the landslipping
has ceased and when it is safe and worthwhile to begin rehabilitation. Premature landshaping,
cultivation and especially irrigation might easily reactivate the slips. This could delay recovery,
as well as being a waste of resources and effort. It will not be possible to be absolutely certain
when rehabilitation can begin, but a programme of continued detailed observation should prevent
premature attempts.
Page 38of 73
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Dasgupta S. (1995). Shumar Formation. Pp 64-78 in ‘The Bhutan Himalaya: a geological
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- Riju feeder road. DGM, MTI, Thimphu.
Eguchi T (1997). Regional and temporal variations in precipitation in the Eastern Himalayas.
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Eguchi T (1987). Topographic features in the central part of the Bhutan Himalayas. Pp 185 208 in ‘Life Zone ecology of the Bhutan Himalaya’. (Ed. M. Ohsawa). Laboratory of Ecology,
Chiba University (Japan).
Eguchi T (1997). Regional and temporal variations in precipitation in the Eastern Himalayas.
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178-193.
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Page 41of 73
APPENDIX A:
METHODS OF SOIL ANALYSIS USED AT SPAL, SIMTOKHA
The full details of the methods used at SPAL are given in ‘Soil Analysis’ (SPAL 1993).
The SPAL methods vary slightly according to soil pH. The methods summarized below are
those for soils of pH (water) both > 7 and < 7, as samples of both types were collected during
this survey. However there are no samples of pH (water) < 4.5, so that there are no
determinations of extractable acidity or Al, nor computations of ECEC or EBS%.
Sample preparation
Samples are air - dried, aggregates are hand crushed, and the soil is sieved to 2 mm.
pH
Soil pH is measured in suspensions of the soil in distilled water and 1M KCl (both 1:2.5) using a
PHM 83 automatic pH meter.
Soil extracts
The fine earth fraction is subject to a number of extraction procedures:
Total N is extracted and converted into ammonium form by micro-Kjeldahl digestion
with H2SO4 and a Se-based catalyst.
Ammonium – N and nitrate – N are extracted by shaking with 0.01 M CaCl2 for two
hours.
For soils with pH (water) < 7, available P is extracted by shaking 5 g of fine earth with 35
ml of the Bray and Kurtz extractant of 0.5 M HCl and 1 M NH4F for 1 minute. For soils
with pH > 7, available P is extracted by shaking 5 g of fine earth with 35 ml of the Olsen
extractant of 0.5 M Na HCO3 and 1 M NaOH for 1 minute.
Available K is extracted by shaking 5 g of fine earth with 50 ml of 0.01 M CaCl2 for 2
hours.
Exchangeable Ca, Mg, K and Na are extracted by leaching 5 g of fine earth with 100 ml
of 1M ammonium acetate (NH4OAc).
For the soils with pH (water) < 7.5, the ammonium is extracted by leaching the soil with
excess 1 M KCl, and measured to give the Cation Exchange Capacity. For the soil with
pH (water) > 7.5, the ammonium is extracted by leaching with excess 1 M sodium
acetate.
Assays of extracts
The NH4 in the extracts from the Total N digestion, the KCl leaching for CEC determination,
and from NH4 – N; NO3 – N; available P; available K; and exchangeable K and Na in the
different extracts are measured with the Skalar Segmented Flow Analyser system, which
includes colorimeters for NH4, NO3 and P, and a flame spectrophotometer for K and Na.
Page 42of 73
Exchangeable Ca and Mg in the NH4OAc leachate are measured with a Unicam Atomic
Adsorption Spectrophotometer.
Organic carbon
OC is measured by the Walkley – Black method of low temperature oxidation with acidified
K2Cr2O7 and titration of the excess dichromate.
Particle size analysis
Particle size fractions are measured by the pipette method after pre-treatment of the fine earth
with H2O2 to remove organic binding effects, and with HCl to remove aggregation effects of
carbonates, Fe and Al oxides, and other mineral cementing agents. They are dispersed with
sodium hexametaphosphate.
TEB, ECEC, BS and C:N
Total exchangeable bases, effective cation exchange capacity, base saturation, and C:N ratios are
derived by computation, i.e.;
TEB
BS (NH4OAc)
C:N
= Exchangeable Ca + Mg + K + Na.
= TEB / CEC (NH4OAc).
= Organic C / Total N. [NB High ratios indicate low availability of N]
Note that the BS% values for the Radhi samples refer to TEB/CEC (NH4OAc)
The analytical results from SPAL are interpreted according to the criteria summarised in Table
A.1.
Page 43of 73
Table A.1
Summary of current interpretation of SPAL soil analyses
pH
EC mS/cm
CEC
(NH4OAc)
me%
XCa me%
XMg me%
XK me%
Xna me%
TEB me%
XAl me%
ECEC me%
BS % (NH4OAc)
EBS %
AvK ppm
Very High
High
Moderate
Low
Very Low
> 7.6
(alkaline)
6.6 - 7.5
(neutral)
5.6 - 6.5
(s. acid)
4.6 - 5.5
(v. acid)
> 2.00
0.8 - 1.99
0.4 - 0.79
0.15 - 0.39
< 4.5
(ext. acid)
< 0.15
> 40
25 - 39.9
15 - 24.9
5 - 14.9
<5
> 20
10 - 19.9
5 - 9.9
2 - 4.9
<2
>8
3 - 7.9
1.5 - 2.9
0.5 - 1.4
> 1.2
0.6 - 1.19
0.3 - 0.59
0.1 - 0.29
>2
0.7 - 1.99
0.3 - 0.69
0.1 - 0.29
> 30
15 - 29.9
7.5 - 14.9
3 - 7.4
> 10
5 - 9.9
2 - 4.9
0.5 - 1.9
> 30
20 - 29.9
12 - 19.9
4 - 11.9
> 80
65 - 79
50 - 64
35 - 49
> 80
50 - 79
35 - 49
20 - 34
> 300
200 - 299
100 - 199
40 - 99
15 - 29
5 - 14
1.2 - 3
0.6 - 1.1
0.5 - 0.99
0.2 - 0.49
0.1 - 0.19
20 - 49
(poor)
15 - 19
(moderate)
10 - 14
(good)
AvP ppm
> 30
Org. C %
>5
Total N %
C:N
Source:
>1
50
(very poor)
AHT 1995.
3.1 - 4.9
< 0.5
< 0.1
< 0.1
<3
< 0.5
<4
< 35
< 20
< 40
<5
< 0.6
< 0.1
< 10
(very good)
Page 44of 73
APPENDIX B:
SOIL PROFILE DESCRIPTIONS AND ANALYSES
This appendix includes the detailed descriptions and analyses of the 12 soil profiles. The
profiles are in the sequence in Table B.I.
Table B.I.
Summary of Radhi soil profiles
Profile number
Radhi soil class
Number of horizons sampled
PC007
RGC
3
PC008
RBS
4
PC009
RBL
3
PC010
RBL - RBS
4
PC026
RBS
3
PC027
RBL
3
Pd016
RBC
3
Pd020
RHL
3
Pd021
RYL
3
Pd022
RBL
3
Pd023
RGL
3
PH036
RGL
3
12
39
Page 45of 73
Profile:
PC007
Map unit:
GX
Soil Classification:
Survey area:
Location:
GPS:
Altitude:
Radhi soil class: Grey clay (RGC)
Soil Taxonomy: Typic Haplanthrept (thermic, fine loamy, mixed)
WRB:
Hydragric Anthrosol
Radhi
Ca 500 m E of Radhi Panthang
270 21.19’N, 910 41.85’ E
1550 m a.s.l
Described & sampled:
15.4.1998, IC Baillie
Climate:
General:
Warm temperate, P = ca 1500 mm p.a
Recent weather: Showery
Regional topography:
Site position:
Low mountain
Mid slope
Slope:
Site drainage:
24%, ca 1km long, terraced slightly convex, aspect WNW (3000)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Chhuzing fallow
Low grass & forb weeds
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Chekha Formation phyllite
Colluvium
None
None
Few common quartz & pegmatite
None
None
Terraced
None
None
Profile description: (Colours are moist unless indicated)
cm
0 -39
2.5YR 3/2 (very dark greyish brown) with common medium prominent reddish brown mottles;
silty loam; moderate medium angular blocky breaking moderate fine crumb; few fine pores;
common fine roots; moist-slightly friable; few slightly hard – soft grey weathered phyllite; HCl
negative; clear regular boundary to:
[Sample PC007/1 @ 0-10cm]
39-57
10YR 4/2 (dark greyish brown) with common few fine faint reddish brown mottles; silty loam;
moderate medium subangular blocky; few fine pores; few fine roots; moist-slightly firm; common
few slightly hard grey weathered phyllite – soft; few medium charcoal; HCl negative; gradual
regular boundary to:
[SamplePC007/2 @ 45-55 cm]
57-80
10YR 3/2 (very dark greyish brown ) with no mottles; very fine sandy loam; weak medium
angular blocky breaking to weak fine crumb; few fine pores; rare fine roots; moist – firm; few fine
hard quartz & few medium slightly hard grey weathered phyllite stones; few medium charcoal;
HCl negative; gradual regular boundary to:
[Not sampled]
80-107
10YR 3/1 (very dark grey) with few fine faint brown mottles; silty clay loam; moderate medium
angular blocky breaking to weak fine crumb; common medium & stones fine pores; rare fine
roots; moist –firm; few fine hard quartz slightly hard weathered phyllite; fine medium charcoal;
HCl negative; gradual regular boundary to:
[Sample PC007/3 @ 90-100]
Page 46of 73
107-135
10YR 5/3 (brown) with common medium fine yellow brown & few medium prominent orange
mottles; silty clay loam; moderate medium subangular blocky breaking to weak fine crumb;
common medium & fine pores; moist – firm; few fine hard quartz & slightly hard weathered
phyllite; fine medium charcoal; HCl negative; clear slightly boundary to:
[Not sampled]
135-150+
Grey & slivery slightly hard weathered phyllite mixed with few hard quartz stones; few patches of
brown silty clay; no roots:
[Not sampled]
Comment:
Buried topsoil (57 - 107 cm). Silty clay textured weathered phyllite at base is very like Iandslip
debris in Shong Ri, and is probably colluvial
SPAL analytical results for SSU Profile PC007
Reaction, P & organic matter
SSU
Depth
SPAL
pH
No.
cm
Lab
H2O
No
PC00
7
0-10
5054
5.3
/1
Survey area: Radhi
Diff
EC
mS/cm
Avail.
P ppm
Organi
c
C%
Total
N%
C:N
KCl
3.9
1.4
0.02
35
1.5
0.2
7.5
/2
45-55
5055
6.3
4.5
1.8
0.01
10
0.9
0.1
9.0
/3
90-100
5056
6.4
4.6
1.8
0.01
19
1.9
0.1
19.0
Exchangeable base status
SSU
Exchangeable
No.
Ca
Mg
TEB
K
Na
Extr
Al
CEC
AmOA
c
ECEC
BS%
AmOA
c
EBS%
PC007
/1
2.0
0.5
0.4
0.2
3.1
nd
5.5
nd
56
nd
/2
2.8
0.4
0.4
0.1
3.7
nd
6.6
nd
57
nd
/3
10.0
2.2
0.5
0.1
12.8
nd
16.4
nd
79
nd
Fine earth granulometric
SSU
Sand
No.
>1000
425micron
1000
212425
106212
50106
Total
sand
20-50
micro
n
2-20
Total
silt
PC007
/1
nd
nd
nd
nd
nd
54.4
23.4
3.3
/2
nd
nd
nd
nd
nd
53.6
13.2
/3
nd
nd
nd
nd
nd
47.6
24.8
Silt
Clay
Texture
26.7
18.9
SL
16.8
31.0
16.4
SL
9.5
34.3
18.1
L
Page 47of 73
Profile:
PC008
Map unit:
BLX
Radhi soil class: Brown sand (RBS)
Soil Taxonomy: Dystric Haplustept (thermic, coarse loamy over sandy, mixed)
FAO:
Dystric Arenosol
Survey area:
Location:
GPS:
Altitude:
Radhi
On eastern (Kardung) geog boundary spur, above Phongme feeder road.
Not available
1975 m a.s.l
Climate:
General:
Recent weather:
Site position:
Slope:
Low mountain
Upper flank of broad spur
37%, ca 0.5 km long, convex, aspect ENE (700)
Site drainage:
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Small copse, heavily cut for fuel & browse
Coppiced & pollarded oak, moss & bare ground surface
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Sunny after showers
Chekha quartzite & pegmatite
Colluvium
Discontinuous 1 cm oak leaves
None
Few medium platy phyllite stones
None
None
None
None
None
cm
0- 6
10YR 4/4 (dark yellowish brown) with common medium distinct reddish yellow & yellowish
brown mottles; fine sandy loam; strong fine crumb; many medium & fine pores; slightly moist &
soft - friable; common fine & medium & few coarse roots; many medium slightly hard & hard
grey, white & black phyllite & few quartz stones; some ant frass; HCl negative; clear regular
boundary to:
[Sample PC008/1 @ 0-6]
6 - 40
9YR 5/6 (strong brown) with no mottles; very fine sandy loam; weak medium subangular blocky
breaking to moderate fine crumb; many medium & fine pores; moist & very friable; common
coarse, medium & fine roots; common medium slightly phyllite stones; HCl negative; clear
[Sample PC008/2 @ 30-40]
40 – 112
10YR 5/3 (brown) with common medium faint reddish yellow mottles; very stony loamy fine
sand; stony with interstitial single grain; abundant fine pores; stony with interstitial moist &
extremely friable; common coarse medium & fine roots; abundant coarse platy phyllite & fine
quartzitic sandstone stones; HCl negative; clear regular boundary to:
[Sample PC008/3 @ 60-70]
112 – 130+
10YR 7/4 (very pale brown) with common medium faint yellow & pale brown mottles; loamy fine
sand (weathered fine sandstone); single grain; abundant fine pores; moist & extremely friable; few
coarse medium & fine roots; few slightly hard weathered fine quartzitic sandstone stones; HCl
negative:
[Sample PC008/4 @ 120-130]
Page 48of 73
Comment:
Typical of spur soils at Radhi, with quartzite more stable. Lab textures seem improbable.
SSU
Depth
SPAL
No.
cm
Lab No
PC00
8
0-6
5040
/1
Survey area: Radhi
pH
H2O
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
5.3
3.8
1.5
0.01
7
2.2
0.01
220
/2
20-30
5041
5.0
3.8
1.2
0.01
3
1.2
0.1
12.0
/3
60-70
5042
5.1
3.7
1.4
nd
35
0.3
0.01
30.0
/4
120130
5043
4.9
3.9
1.0
nd
6
0.1
0.01
10.0
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
SSU
Exchangeable
No.
Ca
Mg
PC008
/1
0.2
0.2
K
Na
1.1
0.01
1.51
nd
7.4
nd
20
nd
/2
0.2
0.2
0.4
0.1
0.9
nd
1.9
nd
47
nd
/3
0.2
0.1
0.2
0.01
0.51
nd
1.3
nd
39
nd
/4
0.2
0.1
0.2
0.01
0.51
nd
1.2
nd
43
nd
TEB
SSU
Sand
No.
>1000
425micron
1000
PC008
/1
nd
nd
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
nd
nd
nd
/2
nd
nd
nd
nd
nd
48.8
21.1
/3
nd
nd
nd
nd
nd
66.2
/4
nd
nd
nd
nd
nd
40.2
Silt
Clay
Texture
nd
nd
nd
17.0
38.1
13.1
L
17.1
1.9
19.6
14.2
SL
7.7
24.0
31.7
28.1
SCL
Page 49of 73
Profile:
PC009
Map unit:
BLX
Radhi soil class: Brown loam (RBL)
Soil Taxonomy: Typic Haplustept (thermic, loamy, mixed)
FAO:
Haplic Cambisol
Survey area:
Location:
GPS:
Altitude:
Radhi
Below chorten, down hill from RNR centre, on Radhi – Ranjung road.
Not available
1350 m a.s.l
Climate:
General:
Recent weather:
Heavy rain yesterday
Site position:
Slope:
Site drainage:
Parent material:
Low mountain
Lower slope
37%, ca 0.5 km long, convex, aspect ENE (700)
Good
Solid: Pegmatite intrusion in Chekha Formation
Drift:
Shallow colluvium & residual
Dryland arable, formerly in maize
Bare recently cultivated
Land use:
Vegetation:
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Microrelief:
Faunal activity:
Other features:
Rare maize stovers
None
Few hard quartz & dark grey fine grained stones
None
None
Cultivation furrows
None
None
cm
0- 16
5YR 4/3 (reddish brown) with no mottles; very fine sandy clay loam; moderate medium clods
breaking to moderate medium crumb; abundant medium & fine pores; slightly moist & slightly
hard - friable; few fine & medium roots; rare medium subangular quartz stones; HCl negative;
clear regular boundary to:
[Sample PC009/1 @ 0-10]
16 - 36
7.5YR 5/3 (brown) with no mottles; clay loam; moderate medium subangular blocky breaking to
moderate fine crumb; moderate discontinuous clayskins; common coarse & many medium & fine
pores; moist & friable; few medium & fine roots; few medium slightly hard olive & greyish green
weathered rock; HCl negative; clear regular boundary to:
[Sample PC009/2 @ 20 - 30]
36– 54
7.5YR 5/6 (strong brown) with many medium faint brown, yellow, orange & grey mottles;
medium sandy clay loam; moderate fine subangular blocky - crumb; many medium & fine pores;
moist & friable; rare fine roots; many soft yellow, orange black, bluish-grey weathered rock; HCl
very weakly positive; clear slightly wavy boundary to:
[Sample PC009/3 @ 40-50]
54 – 91
Very soft yellow, brown, & grey, white & black weathered rock (hand textures as loamy fine
sand); extremely friable; no roots; HCl very weakly positive: [Not sampled]
91 – 110+
Very soft white, brown, & olive weathered rock (hand textures as loamy fine sand); extremely
friable; no roots; HCl very weakly positive:
[Not sampled]
Comment:
Parent material weathers to very sandy hand textures, but colour & high base status look mafic.
Page 50of 73
SSU
Depth
SPAL
pH
No.
cm
Lab No H2O
PC00
9
0-10
5078
5.7
/1
Survey area: Radhi
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
3.9
1.8
0.01
1
0.9
0.1
9.0
/2
20-30
5079
5.7
3.9
1.8
0.01
1
0.7
0.1
7.0
/3
40-50
5080
6.3
3.6
2.7
0.01
1
0.1
0.01
10.0
SSU
Exchangeable
No.
Ca
Mg
PC009
/1
10.3
2.2
K
Na
0.5
0.1
/2
8.0
1.8
0.4
/3
12.9
4.4
0.1
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
13.1
nd
16.0
nd
82
nd
0.1
10.3
nd
15.8
nd
65
nd
0.1
17.1
nd
14.3
nd
120
nd
Fine earth granulometric.
SSU
Sand
No.
>1000
425micron
1000
PC009
/1
nd
nd
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
52.9
15.7
16.2
/2
nd
nd
nd
nd
nd
48.2
20.4
/3
nd
nd
nd
nd
nd
56.7
23.7
Silt
Clay
Texture
31.9
15.2
SL
1.4
21.8
30.2
SCL
6.8
30.5
12.7
SL
Page 51of 73
Profile:
PC010
Map unit:
BLX
Soil Taxonomy: Typic Haplustept (thermic, loamy over sandy, mixed)
FAO:
Haplic Cambisol
Survey area:
Location:
GPS:
Altitude:
Radhi
Below Kado chorten on Radhi – Ranjung road, 30 m uphill from PC009.
Not available
1360 m a.s.l
Climate:
General:
Recent weather:
Site position:
Slope:
Site drainage:
Low mountain
Lower slope
30%, ca 0.5 km long, concave, aspect NW (3200)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Wetland arable, formerly in rice
Rice stubble & forb weeds
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Profile description:
cm
Cool temperate, P = ca 1500 mm p.a
Heavy rain yesterday
Pegmatite intrusion in Chekha phyllite
Probable deep pegmatitic colluvium
Common rice straw
None
Few hard quartz & dark grey fine grained boulders
None
None
Slight poaching
None
None
(Colours are moist unless indicated)
0- 11
10YR 5/3 (brown) with many medium distinct orange & reddish brown mottles; silty loam;
moderate fine subangular blocky; many fine pores; moist & slightly friable; many fine roots; few
fine hard subangular quartz gravel; common wormcasts; HCl negative; clear regular boundary to:
[Sample PC010/1 @ 0-10]
11-21
10YR 4/2 (brown) with many medium distinct orange & reddish brown mottles; silty clay loam;
moderate medium subangular blocky; moderate discontinuous clayskins; many fine pores; moist
& slightly firm; common fine roots; few fine hard subangular quartz gravel; common wormcasts;
HCl negative; clear regular boundary to:
[Sample PC010/2@20-30]
21– 34
7.5YR 5/4 (strong brown) with many medium distinct dark brown & black mottles; very fine
sandy clay loam; moderate medium subangular blocky; with moderate discontinuous clayskins;
many fine pores; moist & slightly firm; many fine roots; few fine hard quartz gravel & few
medium slightly hard weathered pegmatite stones; many coarse black manganese stains on
horizontal surfaces; HCl negative; clear slightly wavy boundary to:
[Not sampled]
34 – 80
6YR 5/6 (reddish yellow) with common coarse prominent dark grey mottles; medium sandy clay
loam; moderate medium subangular blocky; with common coarse dark topsoil coatings down
cracks; many fine pores; moist & slightly friable; rare fine roots; few fine hard quartz gravel &
Page 52of 73
few medium slightly hard weathered pegmatite stones; HCl negative; clear slightly wavy
boundary to:
[Sample PC010/3 @50-60]
80 - 130
9YR 5/6 (strong brown) with grey, yellow & orange weathered rock colours; loamy fine sand;
very weak medium subangular blocky breaking to single grain; abundant fine pores; moist &
extremely friable; no roots; common patches of soft weathered rock; HCl negative; diffuse
boundary to:
[Sample PC010/4 @ 90-100]
130 – 160+
In situ grey silver moderately weathered slightly hard obliquely bedded phyllite; some brown clay
down cracks; HCl negative:
[Not sampled]
Comment:
Juxtaposition of loamy sand over weathered phyllite may be due to colluviation or to pegmatite
intrusion in country rock. Lab clay contents seem improbably low.
SPAL analytical results for SSU
Profile PC010
SSU
Depth
SPAL
pH
No.
cm
Lab No
H2O
PC01
0
0-10
5081
5.4
/1
Survey area: Radhi
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
3.9
1.5
0.02
6
1.3
0.1
13.0
/2
10-20
5082
5.5
4.7
0.8
0.01
3
1.7
0.1
17.0
/3
50-60
5083
6.5
4.4
2.1
0.01
1
0.2
0.01
20.0
/4
90-100
5084
6.5
4.3
2.2
0.01
1
0.1
0.01
10.0
SSU
Exchangeable
No.
Ca
Mg
PC010
/1
5.7
2.9
K
Na
0.5
0.2
/2
7.2
3.5
0.5
/3
3.2
2.9
/4
3.4
3.0
SSU
Sand
No.
>1000
425micron
1000
PC010
/1
nd
nd
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
9.3
nd
11.2
nd
83
nd
0.1
11.3
nd
13.1
nd
86
nd
0.5
0.1
6.7
nd
9.8
nd
68
nd
0.4
0.1
6.9
nd
8.5
nd
81
nd
Silt
Clay
Texture
class
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
44.8
15.5
38.2
53.7
1.5
ZiL
/2
nd
nd
nd
nd
nd
42.7
8.0
43.4
51.4
5.8
ZiL
/3
nd
nd
nd
nd
nd
63.1
9.4
26.5
35.9
1.0
SL
/4
nd
nd
nd
nd
nd
69.7
14.8
18.0
32.8
0.8
SL
Page 53of 73
Profile:
PC026
Map unit:
BSX
Radhi soil class: Brown sand (RBS)
Soil Taxonomy: Typic Dystrustept (thermic, loamy over sandy, mixed)
FAO:
Dystric Arenosol
Survey area:
Location:
GPS:
Altitude:
Radhi
Two thirds way up Tongling traverse
Not available
ca 1650 m a.s.l
Climate:
21.10.1998; IC Baillie & Tshering Dorji
General:
Recent weather: Heavy rain
Site position:
Slope:
Site drainage:
Low mountain
Middle slope
45%, ca 1 km long, irregular rectilinear, aspect NNE (150)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Edge of secondary broadleaf forest
Oak with moderate ground cover of herbs & few ferns
Surface:
Litter:
Outcrops:
Stones:
Chekha phyllite, slate & quartzite
Bioturbation & colluvium
Continuous 0.5 - 3 cm leaf litter
None
Common flaggy phyllite & quartzite boulders up to 0.5 m
diameter
Cracks:
None
Roots:
None
Micro relief:
Medium stepping behind roots
Faunal activity: None
Other features: None
Cm
2-0
10YR 4/3 (brown); humic litter; moist & friable; HCl negative; earthworms seen; gradual regular
boundary to:
[Not sampled]
0 - 10
10YR 4/4 (yellowish brown) with few coarse distinct reddish yellow mottles; fine sandy loam;
weak fine subangular blocky breaking to moderate fine crumb; few medium & fine pores; moist &
friable; many fine & medium & few coarse roots; few fine slightly hard phyllite gravel;
earthworms seen; HCl negative; gradual regular boundary to: [Sample PC026/1 @ 0-10]
10 - 25
10YR 3/2 (very dark greyish brown) with common medium faint brown mottles; fine sandy loam;
moderate medium subangular blocky breaking to moderate fine crumb; few medium & fine pores;
moist & friable; few coarse & many medium & fine roots; HCl negative; clear regular boundary
to:
[Sample PC026/2 @ 12 - 22]
25 - 42
7.5YR 5/6 (strong brown) with few coarse distinct black organic mottles; coarse sandy loam;
moderate medium subangular blocky breaking to moderate fine crumb; few medium & fine pores;
moist & very friable; few coarse & many medium & fine roots; HCl negative; many medium
slightly hard weathered phyllite stones; gradual slightly wavy boundary to:
[Sample PC026/3 @ 30 - 40]
Page 54of 73
42 - 105+
10YR 6/4 (light yellowish brown) with no mottles; stony loamy medium sand; stony - single
grain; many medium & fine pores; moist & stony & loose; common fine roots; HCl negative;
abundant platy slightly hard grey & dark brown weathered phyllite stones:
[Not sampled]
Comment:
Although phyllite is main stone type, textures indicate substantial quartzite content
SSU
Depth
SPAL
pH
No.
cm
Lab No
H2O
PC02
6
0-10
5541
4.7
/1
Profile PC026
Survey area: Radhi
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
4.0
0.7
0.08
35
3.6
0.22
16.3
/2
12-22
5542
5.3
4.2
1.1
0.01
35
3.4
0.02
170.0
/3
30-40
5543
4.7
4.1
0.6
0.21
9
0.6
0.05
12.0
SSU
Exchangeable
No.
Ca
Mg
PC026
/1
0.3
0.2
K
Na
0.2
0.1
/2
0.0
0.2
0.1
/3
0.0
0.1
0.2
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
0.8
nd
14.7
nd
5
nd
0.1
0.4
nd
15.6
nd
3
nd
0.1
0.4
nd
5.9
nd
7
nd
Page 55of 73
Profile:
PC027
Map unit:
BSX
Radhi soil class: Brown loam (-brown sand) ( RBL -(RBS))
Soil Taxonomy: Dystric Haplustept (thermic-mesic, coarse loamy, mixed)
FAO:
Dystric Cambisol
Survey area:
Location:
GPS:
Altitude:
Radhi
Western boundary, top of Tongling traverse
Not available
ca 1850 m a.s.l
Climate:
Recent weather:
21.10.1998; IC Baillie & Tshering Dorji
General:
Sunny
Site position:
Slope:
Site drainage:
Low Mountain
Upper slope, just off crest of Shong Ri – Boma Ri interfluve spur
65%, ca 1 km long, strongly convex, aspect E (1000)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Sokshing broadleaf forest
Young even-aged oak stand with dense ground cover of mesophyllic herbs & ferns
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Chekha phyllite & quartzite
Bioturbation & colluvium
Discontinuous 0.5 cm leaf litter
None
Few hard angular quartz stones
None
None
Slight stepping behind roots
None
None
cm
0- 25
10YR 3/2 (very dark greyish brown) with no mottles; fine sandy loam; moderate medium
subangular blocky breaking to moderate fine crumb; many fine pores; moist & slightly firm; many
fine roots; earthworm seen; HCl negative; diffuse boundary to: [Sample PC027/1 @ 0-10]
25 - 160
10YR 3/4 (dark brown) with no mottles; medium sandy loam; moderate medium-coarse
subangular blocky breaking to moderate medium crumb; many fine pores & few very large
burrows below 100 cm; moist & very friable; common medium & fine roots; few medium hard
subangular quartz; HCl negative:[Samples PC027/2 @ 40 - 50 & PC027/3 @ 90 - 100]
Continued by auger
160 - 180
As above
180+
Unaugerable stones
Comment:
Very deep & uniform friable subsoil
Page 56of 73
pH
SSU
Depth
SPAL
No.
cm
Lab
H2O
No
PC02
7
0 - 10
5544
5.1
/1
/2
40 - 50
/3
90
100
-
Profile PC027
Survey area: Radhi
Diff
EC
mS/cm
Avail.
P ppm
Organi
cC%
Total
N%
C:N
KCl
4.0
1.1
0.01
35
2.7
0.21
12.8
5545
5.4
4.2
1.2
0.01
10
1.3
0.05
26.0
5546
5.3
4.1
1.2
0.01
10
1.0
0.07
14.2
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
SSU
Exchangeable
No.
Ca
Mg
PC027
/1
0.01
0.2
K
Na
0.3
0.1
0.6
nd
12.5
nd
4.5
nd
/2
Tr
0.1
0.1
0.1
0.3
nd
11.8
nd
2.4
nd
/3
0.2
0.1
0.1
0.1
0.5
nd
10.1
nd
5.0
nd
SSU
Sand
No.
>1000
425micron
1000
PC027
/1
nd
nd
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
43.2
16.9
16
/2
nd
nd
nd
nd
nd
45.0
11.2
/3
nd
nd
nd
nd
nd
42.6
24.5
Silt
Clay
Texture
32.9
24.0
SCL
3.9
15.1
39.9
CL
4.5
29.0
28.4
SCL
Page 57of 73
Profile:
Pd016
Map unit:
BLX
Provisional Radhi soil class:
Soil Taxonomy:
FAO:
Survey area:
Location:
GPS:
Altitude:
Radhi
250 m SE of Radhi – Phongme feeder road
270 22.17’ N’, 910 42.70’ E
1740 m asl
Climate:
20.4.1998, Tsheten Dorji
General:
Warm temperate, P = 1500 mm p.a
Recent weather: Light shower
Site position:
Low mountain
Midslope of spur
Slope:
Site drainage:
30%, 1 km +, convex, aspect NWN (3500)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Fallow wetland
Fragaria spp, Plantago erosa, Rubus elliptica & grasses
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Brown clay (RBC)
Typic Haplustalf (thermic, clay, mixed)
Haplic Luvisol
Amphibolite with gneiss (Chekha)
Colluvium
None
None
Common hard subangular amphibolite boulders
Irregular, 1 cm wide
None
Irregular cattle poaching, 4 cm deep
None
None
Cm
0 - 14
2.5Y 5/3 (light olive brown) with common fine faint yellow brown & dark brown mottles; strong
silty clay loam; weak medium subangular blocky; common fine & medium pores; moist & friable;
common fine paddy roots; many medium hard angular amphibolite stones; HCl negative; gradual
[Sample Pd016/1 @ 0 – 10 cm]
14 - 46
2.5Y 4/2 (dark greyish brown) with common fine faint dark, yellow, orange & grey mottles; silty
clay loam; strong medium & fine subangular blocky; weak discontinuous clayskins; common fine
& medium pores; moist & slightly friable; few fine roots; common fine & hard subangular &
angular amphibolite boulders; few manganese stains; HCl negative; clear regular boundary to:
[Sample Pd016/2 @ 20 – 30 cm]
46 - 90+
7.5YR 4/4 (brown) with common medium distinct grey & dark mottles; clay; moderate medium
subangular blocky; weak discontinuous clayskins; many fine pores; moist & slightly firm; rare
fine roots; common fine & bouldery hard subangular & angular amphibolite; many manganese
stains; HCl negative; gradual regular boundary to:
[Sample Pd016/3 @ 60 – 70 cm]
Comments:
The increase in clay content with depth and the clayskins makes these soils Luvisols (Alfisols),
though some of the cutans may be pressure faces. Lab textures are wrong or due to poor dispersion
of sesquioxidic ‘silts’ in top two horizons. No convincing plough pan.
Page 58of 73
SPAL analytical results for SSU Profile Pd016
Survey area: Radhi
SSU
Depth
SPAL
pH
No.
cm
Lab No
H2O
Pd016
/1
0-10
5085
5.3
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
3.8
1.5
0.01
3
1.1
0.1
11
/2
20-30
5086
6.3
4.4
1.9
0.01
1
0.8
0.5
2
/3
60-70
5087
6.5
4.6
1.9
0.01
2
0.5
0.41
12
SSU
Exchangeable
No.
Ca
Mg
Pd016
/1
5.4
3.8
K
Na
0.1
0.1
/2
7.2
4.4
0.1
/3
11.2
4.6
0.2
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
9.4
nd
10.3
nd
92
nd
0.1
11.8
nd
14.9
nd
79
nd
0.1
16.1
nd
18.8
nd
87
nd
SSU
Sand
No.
>1000
425micron
1000
Pd016
/1
nd
nd
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
46.6
19.6
27.0
/2
nd
nd
nd
nd
nd
41.3
22.0
/3
nd
nd
nd
nd
nd
35.5
7.3
Silt
Clay
Texture
46.6
6.7
L
36.3
58.3
0.3
SL
19.7
27.0
37.5
CL
Page 59of 73
Profile:
Pd020
Map unit:
HX
Radhi soil class: Humic loam (RHL)
Soil Taxonomy: Humic Dystrustept (mesic, fine loamy, mixed)
FAO:
Siltic Umbrisol
Survey area:
Location:
GPS:
Altitude:
Shetimi area
Buna Ri - Shong Ri interfluve above Tephu Gompa
270 19.65‘ N, 910 43.65‘ E.
2720 m asl,
Climate:
General:
Recent weather: Sunny
Site position:
Main interfluve spur
Upper slope crest
Slope:
Site drainage:
30%, ca 200 m +, convex, aspect NW (3200)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Pasture
Fragaria spp, Trifolium repens, Rumex nepalensis, thistles & grasses
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Tirkhola (Chekha) phyllite & quartz
Colluvium
0.5 cm discontinuous grass leaf
None
None
None
None
Much stepping due to slipping
None
None
Cm
0 - 21
7.5YR 3/1 (very dark grey) with no mottles; silty clay; moderate medium subangular blocky; weak
discontinuous organic cutans; abundant fine & medium pores; moist & friable; abundant fine and
few coarse roots; HCl negative; ants, few earthworm; few charcoal; gradual regular boundary to:
[Sample Pd020/1 @ 0 – 10 cm]
21 - 46
7.5YR 4/2 (brown - dark brown) with abundant medium & coarse distinct grey & dark brown
mottles; silty clay loam; moderate - weak medium subangular blocky; weak discontinuous organic
cutans; common fine & few medium & coarse dark in-filled pores; moist & slightly friable; many
fine & few medium roots; HCl negative; gradual slightly wavy boundary to:
[Sample Pd020/2 @ 30 – 40 cm]
46 - 88
10YR 4/4 (dark yellowish brown) with no mottles; very fine sandy loam +; moderate medium
subangular blocky; weak discontinuous organic cutans; common fine & few medium & coarse infilled pores; moist & slightly friable; rare fine roots; few fine phyllite stones; HCl negative;
gradual regular boundary to:
[Sample Pd020/3 @ 60 – 70 cm]
88 - 210 +
2.5Y 5/4 (light olive brown) with weathering rock colours of dark brown, yellow brown & grey;
weathered phyllite (hand textures as gravelly fine sandy loam); weak subangular blocky; weak
discontinuous organic cutans; common fine, medium & few coarse in-filled pores; moist & stony;
common hard - soft weathered phyllite stones; HCl negative: [Not sampled]
Page 60of 73
Comments:
Characteristic of deep humic topsoil of grasslands, at top of geog. Nearby augering had dark
topsoil 40 cm deep. This soil is gullying very rapidly (the ‘eye’ of the gully cut back 8 + metre in
2 years). The gully is not headwards extension of Shong Ri, and it fades out downslope as a wide
open well-vegetated drainage depression with no channel. Organic carbon contents in subsoil
confirm OM mobility but topsoil value seems too low.
SSU
Depth
SPAL
No.
cm
Lab No
Pd020
/1
0-10
5567
Survey area: Radhi
pH
H2O
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
5.4
4.4
1.0
0.02
2
1.7
0.17
10
/2
30-40
5568
4.8
3.8
1.0
0.05
1
5.0
0.34
15
/3
60-70
5569
5.3
4.5
1.2
0.02
3
2.1
0.18
12
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
SSU
Exchangeable
No.
Ca
Mg
Pd020
/1
0.2
0.7
K
Na
0.8
0.1
1.8
nd
29.0
nd
6.2
nd
/2
0.2
0.1
0.1
0.1
0.5
nd
22.6
nd
2.3
nd
/3
0.2
Tr
0.2
0.1
0.5
nd
16.7
nd
2.9
nd
TEB
SSU
Sand
No.
>1000
425micron
1000
Pd020
/1
nd
nd
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
36.3
14.1
25.7
/2
nd
nd
nd
nd
nd
33.2
18.0
/3
nd
nd
nd
nd
nd
33.1
12.8
Silt
Clay
Texture
39.0
23.9
SCL
27.8
45.0
21.0
ZC
26.5
39.3
27.6
ZC
Page 61of 73
Profile:
Pd021
Map unit:
YX
Radhi soil class: Yellow loam (RYL)
Soil Taxonomy: Dystric Haplustept (mesic, fine loamy, mixed)
FAO:
Dystric Cambisol
Survey area:
Radhi
Climate:
General:
Cool temperate, P = 2000 mm p.a.
Recent weather: Sunny
Site position:
Low Mountain
Mid slope
Slope:
Site drainage:
18%, ca 1 km +, rectilinear, aspect N (3500)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Broadleaf forest pasture
Oak, rhododendron, broadleaf stumps, all severely cut; highly overgrazed; many
white umbelliferous herbs
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Phyllite & quartz
Chheka Colluvium
None
Phyllite & quartzite in gully floor
Quartz stones in gullies
None
None
Multiple gullies to 1 m deep & much cattle poaching
None
None
Cm
0 - 23
7.5YR 4/3 (brown) with no mottles; very fine sandy clay loam; moderate coarse crumb; weak
discontinuous organic cutans; common fine pores; moist & friable; abundant fine and few decayed
coarse roots; HCl negative; earthworms seen; diffuse boundary to:[Sample Pd021/1 @ 0 – 10 cm]
23 - 38
10YR 4/4 (dark yellowish brown) with no mottles; fine sandy clay loam; moderate medium
subangular blocky breaking to moderate medium crumb; weak discontinuous clayskins; common
fine & medium pores; moist & friable; few fine & rare medium & coarse decayed roots; HCl
negative; gradual regular boundary to:
[Sample Pd021/2 @ 25 – 35 cm]
38 - 78
10YR 4/2 (dark greyish brown) with no mottles; silty clay loam; moderate coarse breaking to
medium subangular blocky; weak discontinuous clayskins; common fine & few medium & coarse
pores; moist & slightly friable; rare fine roots; few fine hard – soft phyllite; HCl negative; gradual
[Sample Pd021/3 @ 60 – 70 cm]
78 - 98+
7.5YR 5/4 (brown) with no mottles; silty clay loam; moderate medium subangular blocky; very
weak discontinuous patchy clayskins; common fine & coarse pores; moist & slightly friable;
common hard - soft weathered phyllite; HCl negative; coarse pores are made by ants:
[Not sampled]
Comments:
Characteristic of brownish - reddish yellow medium textured soils common above 2000 m at
Radhi. This profile appears to be in polyphasic colluvium, with 38-78 cm as a possible buried
topsoil, partly corroborated by Organic carbon data.
Page 62of 73
Survey area: Radhi
SSU
Depth
SPAL
pH
No.
cm
Lab No
H2O
Pd021
/1
0-10
5570
5.5
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
4.5
1.0
0.01
2
5.0
0.75
7
/2
25-35
5571
6.1
5.8
0.3
nd
1
2.7
0.25
11
/3
60-70
5572
6.9
5.7
1.2
nd
2
1.9
0.15
13
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
SSU
Exchangeable
No.
Ca
Mg
Pd021
/1
0.2
0.2
K
Na
0.2
0.1
0.7
nd
21.6
nd
3.1
nd
/2
0.2
0.1
0.1
0.1
0.5
nd
21.9
nd
2.3
nd
/3
0.1
0.1
0.1
0.1
0.4
nd
16.0
nd
2.3
nd
Page 63of 73
Profile:
Pd022
Map unit:
BLX
Soil Taxonomy: Typic Haplanthrept (thermic, loamy, mixed)
FAO:
Hydragric Anthrosol
Survey area:
Location:
GPS:
Altitude:
Radhi
Spur below Three Chortens on Phongme feeder road
270 22.36‘ N, 910 42. 25‘ E.
1480 m asl
Climate:
General:
Recent weather: Slightly cloudy
Site position:
Lower Mountain
Lower slope
Slope:
Site drainage:
15%, ca 2 Km +, rectilinear, aspect WNW (3000)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Paddy land, just harvested
Crassaiphylum cripidoides, Ageratum conizoides & Artemisia myriantha
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Microrelief:
Faunal activity:
Other features:
Phyllite & quartz
Colluvium
None
None
Few hard subangular gneiss & quartz boulders
1 – 2 cm wide cracks
None
3 – 5 cm deep irregular cattle poaching
None
None
Cm
0 - 25
2.5Y 4/2 (dark greyish brown) with abundant fine & medium distinct reddish brown mottles; very
fine sandy loam +; moderate medium subangular blocky; abundant fine pores; moist & friable;
abundant fine paddy roots; few medium hard phyllite & quartz stones; HCl negative; gradual
[Sample Pd022/1 @ 0 – 10 cm]
25 - 74
10YR 3/2 (very dark greyish brown) with abundant medium & coarse distinct dark brown, black
& orange mottles; fine sandy clay loam; moderate coarse breaking to medium subangular blocky;
weak discontinuous clayskins; abundant fine & few medium pores; moist & friable; few fine
paddy roots; common fine, medium & coarse quartz & granite stones; HCl negative; abundant
manganese stains; diffuse boundary to:
[Sample Pd022/2 @ 50 – 60 cm]
74 - 101
7.5YR 4/2 (brown) with no mottles; stony fine sandy clay loam; moderate coarse breaking to
medium subangular blocky; weak discontinuous clayskins; abundant fine & few medium pores;
moist & slightly firm; abundant coarse quartz & granite stones & boulders; HCl negative; gradual
[Sample Pd022/3 @ 90 – 100 cm]
101 - 130 +
5YR 4/3 (reddish brown) with abundant fine & medium distinct black, yellowish brown & reddish
brown mottles; stony clay loam; moderate coarse breaking to medium subangular blocky; weak
discontinuous clayskins; common fine pores; moist & slightly firm; rare fine roots; abundant
coarse quartz & granite stones & boulders; HCl negative; common manganese stains:
[Not sampled]
Page 64of 73
Comments:
Textures grades heavier with depth, but accompanied by increasing stone and boulder content.
This soil has reddish colours of pegmatite/granite parent material, as often found close to spurs on
lower & middle slopes. No sign of plough pan.
Survey area: Radhi
SSU
Depth
SPAL
pH
No.
cm
Lab No
H2O
Pd022
/1
0-10
5573
7.0
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
5.8
1.2
0.01
1
1.2
0.09
13.3
/2
50-60
5574
5.8
4.4
1.4
0.01
1
1.1
0.08
13.7
/3
90-100
5575
6.7
5.9
0.8
0.01
1
1.0
0.07
14.2
SSU
Exchangeable
No.
Ca
Mg
Pd022
/1
1.7
1.6
K
Na
0.2
0.1
/2
4.2
3.6
0.2
/3
4.3
3.3
0.2
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
3.6
nd
10.4
nd
34.5
nd
0.1
8.1
nd
12.3
nd
65.8
nd
0.1
7.9
nd
15.2
nd
52.0
nd
Fine earth granulometric.
SSU
Sand
No.
>1000
425micron
1000
Pd022
/1
nd
nd
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
43.9
16.6
21.7
/2
nd
nd
nd
nd
nd
39.9
13.5
/3
nd
nd
nd
nd
nd
35.6
10.6
Silt
Clay
Texture
38.3
17.8
L
22.2
35.7
24.4
ZL
24.0
34.6
29.8
ZCL
Page 65of 73
Profile:
Pd023
Map unit:
GX
Radhi soil class: Grey loam (RGL)
FAO:
Hydragric Anthrosol
Survey area:
Location:
GPS:
Altitude:
Radhi
Ca 50 m below Radhi – Phongme feeder road just west of Three Chortens spur
270 22.14’ N, 910 42. 17’ E.
1560 m asl
Climate:
General:
Recent weather: Slightly cloudy
Site position:
Mid Mountain
Midslope
Slope:
Site drainage:
21%, ca 1 km +, slightly convex, aspect NW (3220)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Paddy land, just harvested
Crassaiphylum cripidoides, Ageratum conizoides & Artemisia myriantha
Surface:
Litter:
Outcrops:
Stones:
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
Chekha phyllite & quartz
Colluvium
None
Few gneiss
Few hard subangular gneiss boulders
None
None
None
None
None
Cm
0 - 16
2.5Y 4/1 (dark grey) with abundant fine & few coarse distinct reddish brown mottles; very
gravelly very fine sandy loam; moderate coarse breaking to medium subangular blocky; abundant
fine pores; moist & friable; abundant fine & medium paddy roots; few fine phyllite & quartz
gravel; HCl negative; gradual regular – diffuse boundary to: [Sample Pd023/1 @ 0 – 10 cm]
16 - 35
10YR 3/2 (very dark greyish brown) with abundant fine distinct reddish brown mottles; stony very
fine sandy loam; moderate coarse breaking to medium subangular blocky; weak discontinuous
clayskins; abundant fine & few medium pores; moist & friable; common fine & few medium
paddy roots; many medium & coarse quartz & granite stones; HCl negative; clear wavy boundary
to:
[Sample Pd023/2 @ 20 – 30 cm]
35 - 100
10YR 4/4 (dark yellowish brown) with weathering colours of yellowish brown, orange, black,
grey & reddish brown; medium sandy loam +; weak coarse breaking to medium subangular
blocky; strong discontinuous clayskins; common fine & medium pores; moist & very friable; few
medium & coarse quartz, phyllite, granite very weathered stones; HCl negative; clear wavy
boundary to:
[Sample Pd023/3 @ 60 – 70 cm]
100 - 142 +
Mixed grey, yellowish brown, white, orange & reddish brown; very fine sandy loam+; weak
coarse breaking to medium subangular blocky; common fine, medium & few coarse pores; moist
& very friable; abundant soft weathered quartz, phyllite, granite stones & few granite boulders;
HCl negative:
[Not sampled]
Page 66of 73
Comments:
This soil is located in declivity on midslope and appears to be of metamorphic rater than intrusive
pegmatitic origin. The quartzites are clearly an important element in the parent materials. No sign
of plough pan.
SPAL analytical results for
SSU
Profile Pd023
Survey area: Radhi
SSU
Depth
SPAL
pH
No.
cm
Lab No
H2O
Pd023
/1
0-10
5576
7.1
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
6.1
1.0
0.01
6
1.8
0.14
12.8
/2
20-30
5577
5.1
4.4
0.7
0.01
1
1.1
0.09
12.2
/3
60-70
5578
5.4
4.2
1.2
nd
1
0.4
0.03
13.3
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
SSU
Exchangeable
No.
Ca
Mg
Pd023
/1
2.2
1.4
K
Na
0.3
0.2
4.1
nd
10.9
nd
37.4
nd
/2
3.9
2.0
0.3
0.1
6.4
nd
11.6
nd
54.7
nd
/3
3.2
2.3
0.3
0.2
6.0
nd
8.6
nd
70.0
nd
SSU
Sand
No.
>1000
425micron
1000
Pd023
/1
nd
nd
Silt
Clay
Texture
class
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
43.2
16.1
23.9
40.0
16.9
L
/2
nd
nd
nd
nd
nd
47.5
7.4
24.2
33.6
20.8
L
/3
nd
nd
nd
nd
nd
52.7
16.1
19.5
35.6
11.8
L
Page 67of 73
Profile:
PH036
Map unit:
GX
Radhi soil class: Grey loam (RGL)
WRB:
Hydragric Antrosol
Survey area:
Location:
GPS:
Altitude:
Radhi
Kardung, ca 400m below Radhi – Phomgme feeder road
270 21.58’ N, 910 42.26’ E
1530m a s l
Climate:
20.4.98, H B Tamang
General:
Recent weather: Light shower
Site position:
Low Mountain
Lower slope
Slope:
Site drainage:
20%, ca 1km long, concave, aspect NW (3200)
Good
Parent material:
Solid:
Drift:
Land use:
Vegetation:
Fallow wetland
Rice stubble
Surface:
Litter:
Outcrops:
Stones:
Chekha phyllite & quartzite
Colluvium
Cracks:
Roots:
Micro relief:
Faunal activity:
Other features:
None
None
Many medium angular phyllite stones & few
boulders
Thin cracking of clay cap
None
Cattle poaching 2 – 5cm depth
None
None
Cm
0 -20
2.5Y 6/2 (light brownish grey) with many medium distinct pinkish & yellow mottles; silty clay;
moderate medium & fine subangular blocky; common fine pores; common fine roots; moist &
friable; few fine hard quartz gravel; HCl negative; gradual regular boundary to:
[Sample PH036/1 @ 0-10]
20-50
2.5Y 5/2 (greyish brown) with common fine faint yellow brown mottles; silty clay; strong medium
subangular blocky; weak discontinuous clayskins; common medium & fine pores; few fine roots;
moist & firm; few fine hard quartz gravel; HCl negative; diffuse boundary to:
[Sample PH036/2 @30-40]
50-98
2.5Y 5/2 (greyish brown) with common fine distinct yellow brown mottles; fine sandy clay loam;
strong medium subangular blocky; weak discontinuous clayskins; few fine & medium pores moist
& firm; many medium quartz gravel; HCl negative; diffuse boundary to:
[Sample PH036/3 @ 65-75]
98-124
Mixed grey, brown and reddish brown; bouldery sandy loam; weak fine sub angular blocky; weak
continuous clayskins; many fine & medium pores; moist & stony, with interstitial slightly firm;
many quartzite and phyllite stones & boulders; HCl negative: [Sample PH036/4 @ 105-115]
Page 68of 73
Comment:
This is a typical landslip soil in re-entrant. Firm below 20 cm but no indication tat consistence
change is due to plough pan.
Profile PH036
Survey area: Radhi watershed
SSU
Depth
SPAL
pH
No.
cm
Lab No
H2O
PH03
6/1
0-10
5088
5.3
Diff
EC
mS/cm
Avail.
P ppm
Organic
C%
Total
N%
C:N
KCl
4.9
0.4
0.02
1
1.0
0.1
10
/2
30-40
5089
6.7
4.6
2.1
0.01
1
0.5
0.1
5
/3
65-75
5090
7.1
4.8
2.3
0.01
1
0.8
0.01
80
/4
105115
5091
7.0
4.7
2.3
0.01
1
0.7
0.01
70
SSU
Exchangeable
No.
Ca
Mg
PH036
/1
4.0
3.1
K
Na
0.1
0.2
/2
4.2
3.3
0.01
/3
4.1
3.1
/4
4.8
3.4
TEB
Extr
Al
CEC
AmOAc
ECEC
BS%
AmOAc
EBS%
7.4
-
11.4
-
65
-
0.1
7.71
-
12.0
-
64
-
0.1
0.1
7.4
-
11.1
-
67
-
0.1
0.1
8.4
-
12.8
-
66
-
SSU
Sand
No.
>1000
425micron
1000
PH036
/1
nd
nd
212425
106212
50106
Total
sand
20-50
micron
2-20
Total
silt
nd
nd
nd
47.6
13.5
28.2
/2
nd
nd
nd
nd
nd
42.8
15.5
/3
nd
nd
nd
nd
nd
47.8
/4
nd
nd
nd
nd
nd
60.2
Silt
Clay
Texture
41.3
11.2
L
33.7
49.2
49.2
ZL
7.2
23.8
31.0
31.0
L
1.1
13.7
14.8
25.0
L
Page 69of 73
APPENDIX C:
C. I
SOIL CORRELATION
Soil classification and correlation in Bhutan.
Table 5.6 in the main report summarises the correlation of the Radhi soil classes with the two
main international systems of soil classification. This appendix discusses further the correlations
assigned.
The Soil Taxonomy (ST) was originally developed to meet the needs of soil survey in the
continental United States (Soil Survey Staff 1998). It has been extended since then, but it is still
stronger on temperate than on tropical soils. It is detailed and comprehensive. The FAO/ISRIC
World Reference Base (1998) system is more globally oriented, and is less detailed, but still
comprehensive. One of its advantages is that it uses more traditional and comprehensible soil
names.
Nepal has adopted the Soil Taxonomy, but previous soils consultants in Bhutan have preferred
the FAO system. At this stage it is not necessary for Bhutan to choose between them. It is
intended that, for the present, SSU will continue to use ad hoc local soil classes and names, and
will correlate these against both of the international systems. For the future, SSU is exploring
the possibility of adapting the multi–horizon approach of the FitzPatrick and the French
Referentiel Pedologique systems to Bhutan conditions (SSU Working Paper WP2, 1998).
C.2
General Criteria
Before assigning soils to classes in Soil Taxonomy, there are some general environmental
features of the area that need to be defined.
C.2.1 Soil moisture regime
In the absence of soil moisture data, soil moisture regimes (SMR) are approximated from rainfall
totals and distribution. It is assumed that all of the soils at Radhi have an ustic SMR, which is
defined in ST as having more than 90 consecutive dry days per year and a summer rainfall
distribution. However this may no be the case for the area around Shetimi at the top of the
geog. The lower temperatures and presumed higher rainfall there may make the SMR udic,
which may still be seasonal but is dry for less than 90 days consecutively.
Many of the soil in the lower part of the area are used for wetland rice. Their moisture regime
has seasonal artificially poor drainage. ST refers to these as anthraquic soil moisture conditions.
WRB is more specific in that it distinguishes the artificially gleyed topsoil (anthraquic horizon)
from the underlying mottled horizon (hydragric) that is affected by the artificially wet conditions
above. The naturally wet hillside gleys have aquic SMR’s.
C.2.2 Soil temperature regime
This is a criterion for classification at family level in Soil Taxonomy. In the absence of soil
temperature data, atmospheric temperatures are used. As discussed in 2.2 of the main report, the
lower part of the survey area has a thermic STR, with an annual mean between 150 C and 220 C
and a summer – winter difference greater than 50 C. The upper part has a mesic STR, with an
annual mean between 80 C and 150 C, but still with a summer – winter difference greater than
50 C. The thermic-mesic STR boundary is probably at about 2000m a.s.l.
Page 70of 73
C.2.3 Mineralogy class
This is another family criterion in Soil Taxonomy. Muscovite is a highly visible component in
many soils at Radhi, especially in the soils derived from the Chekha phyllite. However mica
contents are less than 40 % of the sand and gravel fractions, so that the soils do not qualify as
micaceous, and have to be classified in the mixed mineralogy class.
C.2.4 Particle size class.
This varies with stone content and fine earth texture, and is therefore varies for the different soil
classes at Radhi, with the full range from sandy to clay.
C. 3
Correlation of Radhi soils
C.3.1 Grey clays and loams (RGC & RGL)
Many of the uncultivated soils in these classes have been subject to recent mass movements and
their profiles show considerable recent mixing. These profiles probably qualify as Regosols,
mostly Haplic but with some Skeletic on the steep lower slopes, in WRB; and as Ustorthents,
mostly Typic but a few Lithic, in ST. In more stable uncultivated sites, the profiles are
sufficiently developed to be Skeletic or Haplic Cambisols in WRB; and to qualify as Lithic or
Typic Dystrustept, or Lithic, Dystric or Typic Haplustepts in ST.
However most of these soils have greatly modified by agricultural terrace construction and
prolonged use for wetland rice. The human modifications qualify the affected soils as Hydragric
Anthrosols in WRB. ST has a relatively new great group known as Haplanthrepts. So far (1998)
this group has not been subdivided to allow for the particular conditions of flood irrigation. This
is contrast to other groups in ST, such as the Anthraquic Ustorthents or Anthraquic Haplustepts.
We provisionally extend this concept to identify (Anthr) Aquic Haplanthrepts, with the initial
‘anthr’ dropped because it is repetitious. The current version of ST (1998) makes no provision
for Anthraquic Dystrustepts.
C.3.2 Brown loams and sands (RBL & RBS)
Similar considerations apply to the brown soils on the middle slopes and on the lower slope
spurs. However the proportion of wetland terraces is lower on these soils and the Anthrosols
(WRB) and Anthraquic (ST) taxa are of minor importance. Also these soils have higher
proportions of quartzite in their parent materials and are less prone to mass movements. Few of
the uncultivated soils in these classes are so recently disturbed as to qualify as Regosols (WRB)
or Entisols (ST). The main correlations are with Dystric Cambisols (WRB); and Typic
Dystrusrepts and Dystric and Typic Haplustepts (ST).
C.3.3 Brown clay (RBC)
The amphibolitic parent materials of these soils give them good base status. In the profile
described in detail, there are clayskins and an increase in clay content with depth, so that the
subsoils in at least some of these soils are probably argillic horizons. This qualifies these soils as
Haplic Luvisol (WRB) and Typic Haplustalf (ST). There are also shallower and less developed
profiles which are Eutric Cambisols (WRB) and Typic Haplustepts (ST).
Page 71of 73
C.3.4 Yellow loam (RYL)
Similar considerations apply to the yellow loams except that they are acid and of low very base
status. The more developed profiles in these soils qualify as Umbric and Haplic Acrisols (WRB)
and Typic Haplustults. However most of these soils lack a true argillic horizon and therefore
qualify as Dystric Cambisols (WRB) and Typic Dystrusutepts (ST).
C.3.5 Humic loam (RHL)
These soils have deep dark topsoils. They are acid and base deficient and are therefore
Umbrisols in WRB. Sinclair Knight (1983) designated these as Phaeozems but the modern
definition of this group requires a base saturation of more than 50% (FAO 1998). The humic
topsoil is differentiated only at subgroup level in Soil Taxonomy, so that these soils mostly
qualify as Humic Dystrustepts.
C.3.6 Hillside gley (RHG)
These soils are clearly Gleysols in WRB, but the absence of chemical data prevents them being
assigned to a sub-unit. In Soil Taxonomy they are mostly Aquepts although some of those in
very recent gully deposits or spring slump areas may be Aquents. Most of them will be in the
Endo- rather than Epi- great groups.
Page 72of 73
APPENDIX D:
SSU SOIL SURVEY REPORTS
The Soil Survey Unit (SSU) was set up by an Agreement signed in September 1996 by the Royal
Government of Bhutan (RGOB) and Danish International Development Assistance (Danida).
This is the second semi-detailed soil survey undertaken by the Project. Its training objective was
to increase the Soil Surveyors' experience of the factor–based pedogenic approach to the
planning, fieldwork, mapping and interpretation involved in soil surveys with only limited
fieldwork.
The completed and on-going soil surveys and reports by SSU are listed in Table D1.
Table D.1
SSU main soil surveys and reports
SSU No.
Title
Status December 1999
1 & 1(a)
General & Technical reports of detailed soil Final, distributed 7/98
survey of Yusipang RNR-RC
2 & 2(a)
General & Technical reports of detailed soil Final, distributed 9/98
survey of Bathpalathang site, Jakar RNR-RC
SS 3 & SS General & Technical reports of detailed soil Final, distributed 12/98
3(a)
survey of Bajo RNR-RC
SS 4 & SS General & Technical reports of detailed soil Final, distributed 3/99
4(a)
survey of Khangma RNR-RC
SS 5 & SS General & Technical reports of semi-detailed Final, distributed 4/99
5(a)
soil survey of Lingmutey Chhu watershed
SS 6 & SS General & Technical reports of semi-detailed This report, distributed 12/99
6(a)
soil survey of Radhi geog
SS 7 & SS General & Technical reports of semi-detailed Draft distributed for feedback
7(a)
soil survey of Lame Gompa Research Forest
SS 8
Report on soils of Merak and Sakten
Final, distributed 11/99
SS 9 & SS General & Technical reports of semi-detailed Fieldwork still in progress
9(a)
soil survey of arable lands of middle Tsang
Chhu valley
SS 10 & SS General & Technical reports of semi-detailed Final, distributed 11/99
10(a)
soil survey of arable lands of Nyakulumpa
valley, Punakha
SS 11 & SS General & Technical reports of detailed soil Draft in progress
11(a)
survey of Royal Botanic Garden, Serbithang
Page 73of 73

technical report on the semi-detailed soil survey of radhi watershed

Transcription

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