Changed recommendations for the use of phalaris on acid soils

Transcription

Changed recommendations for the use of phalaris on acid soils
Changed recommendations for the use of phalaris on acid soils
Richard C. Hayes 1,3, Guangdi D. Li 1, Richard A. Culvenor 2
NSW Department of Primary Industries, Wagga Wagga Agricultural Institute, Pine Gully Road, Wagga Wagga, NSW 2650
CSIRO Agriculture Flagship, GPO Box 1600, Canberra, ACT 2601
3
Corresponding author Email: [email protected]
1
2
Abstract
Phalaris (Phalaris aquatica L.) has been cultivated in Australia as a forage grass for over a century. Since the
1970’s it has developed a reputation for being sensitive to soil acidity with conventional wisdom typically
advising against its use on acid soils. However, several recent studies have called this view into question
and have prompted a revision of the recommendations for the use of phalaris on acid soils. The release of
new phalaris cultivars, first with cv. Landmaster in 1996 and later cv. Advanced AT in 2008, which have
demonstrated superior tolerance to aluminium (Al) toxicity needs to be acknowledged. Yet, even prior to
these developments it was evident that phalaris performance on acidic soils far exceeded contemporary
expectations. We now have documented evidence of phalaris persisting in the field beyond pH and Al
thresholds at which it is deemed suitable, and of phalaris persisting significantly better in strongly acid soils
than species perceived to be highly tolerant of acidity, such as cocksfoot (Dactylis glomerata L.). Indeed,
phalaris is endemic in many acid soil environments across the Tablelands of south-eastern Australia. It is
suggested that phalaris should be considered for inclusion in new plantings on acid soils not only for its
acid tolerance, but also for its drought tolerance which increases the resilience of pasture swards. Of course,
any new plantings of phalaris in strongly acid soil environments should always use more tolerant cultivars
following lime application. However, being a deep-rooted species, phalaris requires adequate soil nutrition
and depth, and is less well suited to shallower soils which may also be acidic.
Key words
Aluminium toxicity; cocksfoot; drought; persistence; Landmaster; Advanced AT; soil depth
Introduction
Phalaris is one of the most productive and drought-tolerant pasture grasses used for livestock production on
permanent grazing lands of south-eastern Australia. However, during the 1970’s it developed a reputation for
being sensitive to acidic soils and in particular to aluminium (Al) toxicity (Cregan 1980). Field observations
were supported by experiments in solution culture which demonstrated that phalaris was more sensitive to Al
toxicity than species such as cocksfoot (Culvenor et al. 1986). Lime experiments conducted in pots (Helyar
and Anderson 1971) and in the field (Ridley and Coventry 1992) showed phalaris to be more responsive
to lime than other more tolerant species adding weight to the argument that phalaris was less suited to
acidic soils than other commonly used pasture species. A consequence of this collective experience is that
agronomy recommendations typically advocate against the planting of phalaris on strongly acid soils due to
its perceived poor adaptation (e. g. Anon. 2004).
However, the perception that phalaris is unsuitable to highly acidic soils seems counter-intuitive given
that its area of adaptation in south-eastern Australia, usually in higher rainfall (600+ mm) Tablelands
environments, coincides with a landscape that is renowned for high levels of soil acidity (Scott et al. 2000).
More recent experiments conducted by the present authors have cast doubt over the wisdom of conventional
recommendations suggesting that phalaris may indeed be a wise choice for some acid soil environments,
particularly given the recent development of more acid/Al tolerant phalaris cultivars available on the
Australian commercial market since 1996. The current overview paper aims to synthesise key findings from
a range of recent experiments with a view to revising recommendations for the use of phalaris on acid soils.
Field experiments
The need to amend recommendations for the use phalaris on acid soils is highlighted by two field
experiments conducted from 2004-2008 which showed phalaris persisting on acutely acidic soils under
drought where more tolerant species failed completely. In this context it implies that conventional
recommendations are in some instances leading to reduced resilience of improved pastures and actually
© 2015 “Building Productive, Diverse and Sustainable Landscapes “
Proceedings of the 17th ASA Conference, 20 – 24 September 2015, Hobart, Australia. Web site www.agronomy2015.com.au
costing growers money. The first experiment was conducted on an acutely acidic soil (pHCaCl2 to 4.1 with
Al saturation comprising up to 42% of the effective cation exchange capacity in the surface 0.4 m) near
Goulburn, NSW (Hayes et al. 2010) and the second experiment was conducted at Gerogery, NSW, where
soil in the surface 0.2 m had a pHCaCl2 as low as 4.1 with Al saturation up to 23% (Hayes et al. 2015). Both
experiments included phalaris cv. Landmaster and cocksfoot. cv. Currie, but tall fescue (Festuca arundinacea
Schreb.) cv. Demeter at the Goulburn site and cv. Fraydo at the Gerogery site. Tall fescue and particularly
cocksfoot are both considered more tolerant of soil acidity than phalaris, but in both cases in contrast to
phalaris, failed to persist, despite the acidic nature of both soils. Relative persistence was determined by
assessing basal frequency on two 1 m ×1 m fixed quadrats (divided into 100 squares) per plot, where the
number of squares containing the bases of a sown perennial species was counted (Table 1). In both instances,
phalaris responded positively to lime.
Table 1. Basal frequency (%) of phalaris, cocksfoot and tall fescue grown with (+L) and without lime following a
period of drought at Goulburn and Gerogery. (Adapted from Hayes et al. 2010; 2015)
Site
Goulburn
Gerogery
Phalaris
40.8 a
12.0 ab
Phalaris +L
55.2 a
19.2a
Cocksfoot
0.8 b
3.8 c
Cocksfoot +L
9.2 b
6.4bc
Fescue
2.0 b
5.0 bc
Fescue +L
13.2 b
5.8 bc
Values at each site followed by the same superscript letter are not significantly different at P=0.05.
The explanation for the superior performance of phalaris at these sites is likely due to its inherent tolerance
of drought and soil acidity. Both sites experienced severe drought conditions shortly after establishment in
2006 where drought was widespread across much of southern Australia. In both cases the basal frequency of
phalaris increased in response to more favourable seasonal conditions in later years (data not shown). At both
sites the phalaris swards were able to withstand periodic drought and recover, whereas the cocksfoot and tall
fescue failed to persist.
Solution culture experiment
A solution culture experiment was conducted in 2014 to assess the relative tolerance of phalaris, cocksfoot
and tall fescue seedlings to Al and Mn toxicity (Song et al. 2015). Phalaris genotypes were found to be more
sensitive to Al toxicity than cocksfoot genotypes in particular, with the decline in root length from the 0mmM
treatment compared to the 300 mM treatment being greater in phalaris and tall fescue. However, the absolute
root length of phalaris remained at least as large as that of cocksfoot or tall fescue (Table 2) and root length
of phalaris was greater at the nil Al concentration (i.e. more responsive to amendment).
Table 2. Seedling root lengths (mm) of phalaris, cocksfoot and tall fescue at 5 concentrations of Al, and the decline (%) from
nil to maximum Al concentration after 21 days growing in solution culture (Adapted from Song et al. 2015).
Species
Phalaris
Cocksfoot
Tall fescue
LSD0.05
Root length (mm) at 5 concentrations of Al in solution
0mM
50mM
100mM
200mM
300mM
%Decline
237.9
134.7
171.4
25.85
141.4
95.9
102.7
n.s.
128.5
90.5
84.6
n.s.
61.6
48.7
39.1
n.s.
43.5
44.6
30.7
n.s.
81.7
66.9
82.1
14.22
Advances in breeding Al tolerant phalaris genotypes
Improvement in acid soil tolerance became a priority in phalaris breeding after the reports of sensitivity to soil
acidity in the 1970’s. Landmaster was released in 1996 after selection for Al tolerance in solution followed by
several cycles of selection at field sites with skeletal, moderately acidic soils. Simultaneously, the discovery
that P. arundinacea was more Al-tolerant than P. aquatica led to a program of hybridisation and backcrossing
between the two species to transfer higher Al tolerance into a P. aquatica background (Oram et al. 1990).
Cycles of selection at highly acidic field sites resulted in the release of Advanced AT in 2008. Studies in Al
solution demonstrated Al tolerance in the order: Advanced AT > Landmaster > Holdfast > Sirosa, Australian
II (Requis and Culvenor 2004; Culvenor 2008). Establishment after a spring sowing at a strongly acid (pH
3.9) field site at Chiltern, Victoria, closely followed this order presumably reflecting success in growth of deep
roots into a drying subsoil high in Al, but not when sown in autumn in a year with good rainfall (Culvenor et
al. 2004). The resultant hypothesis that the effect of differential Al tolerance in phalaris on field performance
© 2015 “Building Productive, Diverse and Sustainable Landscapes “
Proceedings of the 17th ASA Conference, 20 – 24 September 2015, Hobart, Australia. Web site www.agronomy2015.com.au
was dependent on seasonal moisture and sowing time was confirmed (Culvenor et al. 2011). Advanced AT and
Landmaster were shown to establish and persist better than other cultivars on strongly acid soils (pH 3.9-4.2) but
Advanced AT was superior in establishment to Landmaster when sown in a drought year. Haling et al. (2010)
also showed better root growth by Advanced AT than Sirosa in a soil with pH 3.9 and Al (CaCl2 extractable Al)
of 18 μg/g indicating that Advanced AT was similar in Al tolerance to cocksfoot. A caveat to this observation is
that cocksfoot may still be better adapted than phalaris to low soil fertility which can coincide with acidity. The
higher seedling vigour of Sirosa partly compensated for its greater sensitivity to Al.
Thresholds of phalaris on acid soils
The interaction of environmental factors with Al tolerance clearly complicates the setting of thresholds for
the use of phalaris. In a drought year in the field study of Culvenor et al. (2011), establishment was related
to Al tolerance at sites with pH in the range 3.9-4.2, AlCaCl2 15-25 μg/g and Al saturation 30-55% of CEC to
0.50 m depth. In contrast, all phalaris cultivars established well at a site where pH averaged 4.4 and AlCaCl2
ranged from 3-7 μg/g and Al saturation from 12-22 % to 0.50 m depth. It was concluded that Advanced AT
can be established more reliably than Landmaster in soils where a substantial proportion of the depth to 0.50
m was pH 4.2 or less and Al saturation >30%. All cultivars should tolerate exchangeable Al of 20% of CEC
and pH >4.2. Advanced AT and probably Landmaster should still be suitable where layers with exchangeable
Al of 30-50% and AlCaCl2 of 15 μg/g occur in the upper 0.50 m but with the recognition that they may be able
to tolerate higher levels of Al in restricted depth intervals. For example, phalaris established, produced and
persisted well on a soil averaging pH 4.2 and Al saturation ranging from 28-62% to 0.50 m depth but CaCl2
extractable Al averaging only 7 μg/g (Culvenor et al. 2007). Further consideration of AlCaCl2 as a measure of
Al toxicity potential is warranted (Bromfield et al. 1983).
Effects of Al tolerance on persistence in the study of Culvenor et al. (2011) were parallel but smaller than those
on establishment, and tolerance to grazing may be more important for persistence. Thus, both Advanced AT and
Landmaster benefit significantly in persistence from rotational rather than continuous grazing at high stocking
rates under good soil fertility (Culvenor and Simpson 2015). Culvenor et al. (2004) found that control cultivars
and families related to Advanced AT persisted adequately on strongly acid soils if they established well although
survival was lower on a low fertility soil derived from sedimentary material on the Southern Tablelands. They
concluded that nutritional factors, such as soil P buffer capacity, were clearly important for phalaris on acid
soils and require closer attention. Analysis of soil to depth (≥0.50 m) is also important since acidity at depth can
influence phalaris persistence. Culvenor et al.(2004) found that persistence was excellent on a soil where average
pH of 4.0, AlCaCl2 of 16 μg/g and Al saturation of 41% were restricted to the upper 0.30 m with pH increasing to
4.5-5.2 from 0.30-0.60 m depth. However, persistence was lower on a soil with less available Al in the upper
0.30 m but in which strong acidity extended to at least 0.60 m (pH 4.0, AlCa 16 μg/g, Al saturation 40%).
Discussion and conclusion
The complete failure of ‘acid-tolerant’ species, such as cocksfoot and tall fescue, but superior persistence of the
‘acid-sensitive’ phalaris at highly acidic field sites at Goulburn and Gerogery highlights the need to challenge
conventional wisdom that phalaris is not suitable for acid soils. Indeed, phalaris has proven its resilience
on a range of acidic soils. The current recommendations derived from previous solution culture and liming
experiments were merely reflecting the highly responsive nature of this species which could achieve greater
increases in yields than other species such as cocksfoot. This high level of response should not be interpreted as
an indication of poor suitability to an acidic or high Al soil environment. Results from the latest solution culture
experiment (Song et al. 2015) suggested that the increased ‘sensitivity’ of phalaris genotypes to Al toxicity
did not necessarily infer lower ‘suitability’ to acid soil environments. A phalaris seedling with a root length
of 44 mm is likely to have at least as great a chance of accessing water and nutrients in a soil environment as
a tall fescue seedling with a root length of 31 mm. Indeed, it might be argued that at lower Al concentrations,
longer seedling root lengths in phalaris may give that species an advantage over tall fescue or cocksfoot in less
extreme acidic soil environments (Table 2). Genetic improvement of phalaris since the 1970’s has led to the
development of more Al-tolerant cultivars, particularly Landmaster and Advanced AT. These cultivars have
extended the acid-soil thresholds at which phalaris might be reliably grown, with good establishment achieved
at pH 4.2 and >30% Al. The majority of newly sown pastures would fall within these thresholds meaning that
phalaris is now a viable candidate for most new perennial grass plantings.
We recommend that phalaris cultivars such as Advanced AT or Landmaster be used where soil is acidic.
Use of these cultivars is anticipated to guard against establishment failure in drier than average seasons as
© 2015 “Building Productive, Diverse and Sustainable Landscapes “
Proceedings of the 17th ASA Conference, 20 – 24 September 2015, Hobart, Australia. Web site www.agronomy2015.com.au
supported by the experience of Culvenor et al. (2011). For optimal results, lime should be applied prior to
the establishment of newly sown pastures on acidic soils, thereby also limiting risk of establishment failure
but also capitalising on the highly responsive nature of phalaris to lime. We recognise that persistence of
these phalaris cultivars is further improved by maintaining high levels of nutrition in soils and by imposing
a rotational grazing management regime. Phalaris is a deep-rooted species which requires adequate soil
nutrition and depth, and is less well suited to shallower soils which may also be acidic.
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© 2015 “Building Productive, Diverse and Sustainable Landscapes “
Proceedings of the 17th ASA Conference, 20 – 24 September 2015, Hobart, Australia. Web site www.agronomy2015.com.au

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