New Zealand Journal of Agricultural Research abstracts
Defining the relationships between pasture production and soil P
and the development of a dynamic P model for New Zealand pastures: a
review of recent developments
D. C. Edmeades
agKnowledge Ltd
P.O. Box 9147
Hamilton, New Zealand
A. K. Metherell
Ravensdown Fertiliser Co-operative Ltd
P.O. Box 1049
Christchurch, New Zealand
J. E. Waller
AgResearch Ltd
P.O. Box 2123 Hamilton, New Zealand
A. H. C. Roberts
Ravensdown Fertiliser Co-operative Ltd
P.O. Box 608
Pukekohe, New Zealand
J. D. Morton
Ballance AgriNutrients Ltd
P.O Box 65, Rolleston
Christchurch, New Zealand
Abstract A database was constructed comprising
records from 2255 pasture phosphorus (P), potassium (K) and sulphur (S)
field trials, of which 1799 included one or several rates of
P. Subsets of this data were selected based on predetermined
criteria to define the relationships between relative pasture
production and available soil P (0–75 mm, Olsen P in
µg P cm–3 soil)—the P
production functions—for the major soil groups in New Zealand.
These relationships, and their 95% confidence intervals, were defined
using Bayesian statistics. For those soil groups for which there was
sufficient data, the production functions were well defined and gave
reasonably precise estimates of the relative pasture yield for a given
Olsen P. For example, for the volcanic soils, the relative pasture
production is most likely (P < 0.05) to be in the range
88–94% at Olsen P 25 and 98–100% at Olsen P 50. The shape
of the production functions was similar for all soil groups—the
relative pasture production increased with increasing Olsen P up to an
asymptote—except the pumice soils and the podzols, which showed
irregularities. The production function for the podzols was also
flatter. There was good agreement between the empirically derived
production functions and those generated from a dynamic P model. The
Olsen P level required to achieve 97% maximum production was estimated
for all soil groups. These ranged from 10 to 45 depending on soil
group. The critical Olsen P levels were related to the soil anion
storage capacity (ASC, a laboratory measure of P buffer capacity) and
to soil volume weight (g cm–3 of sieved and dried soil), although not strongly. The field measured P buffer capacity (ΔPF)—the amount of soluble fertiliser P (kg P ha–1)
required above maintenance to increase the Olsen P (0–75 mm)
level by 1 unit—was estimated for selected trials. There was
reasonable agreement between these estimates and those derived from the
P model (ΔPM), and these results indicated that
ΔP decreases with increasing Olsen P. The results imply that
factors other than those related to soil chemical properties affect the
relationship between soil P and pasture production. The factors which
determine the relationship between pasture production and soil P are
defined and discussed. These were assigned to two categories: those
factors which affect the ability of the soil to supply P for plant
uptake and those that affect the ability of the plant to acquire soil
P. It is concluded that further progress towards improving our
ability to predict pasture responses to fertiliser P will depend on
quantifying the latter effects. Based on these results and the
development of a dynamic P model, an econometric P model was developed
for New Zealand pastures which enables consultants to quantify the
likely agronomic, financial and investment effects of any given
fertiliser strategy on a given farm or block within a farm. This was
not previously possible but is essential for the sustainable use of P
fertilisers in pastoral farming.
Keywords database; economics; fertiliser; field
trials; nutrients; modelling; Olsen P; pasture; pasture production;
phosphorus; production functions; soil tests
A05055; Received 23 October 2005; accepted 14 March 2006; Online publication date 16 May 2006
New Zealand Journal of Agricultural Research, 2006, Vol. 49: 207–222
0028–8233/06/4902–0207 © The Royal Society of New Zealand 2006
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