Developments in some aspects of reactive phosphate rock research and use in New Zealand
Authors: Bolan N. S. , Hedley M. J. 
Published: August 2004

Summary. There has been over 50 years of use and
research into the agronomic effectiveness of reactive phosphate rocks (RPR)
directly applied to New Zealand pastures. In recent years RPR-carrying
fertilisers made up about 16% of phosphatic fertiliser sales in the
North Island of New Zealand. Most is applied, as maintenance fertiliser, to
hill country sheep and beef farms. Use has been recommended on soils with pH
<6 and in annual rainfall regimes >800 mm. This is based on the poor
performance of Sechura phosphate rock in summer dry areas receiving <750 mm
of rainfall annually.
Phosphate rocks that have more than 30% of their total phosphate
soluble in 2% citric acid have been classed as ‘reactive’
and suitable for direct application. More recent research indicates that
extraction with 2% formic acid, or a dissolution test performed in a
simulated soil solution at a fixed pH, will provide improved measures of RPR
quality.
Field trials, undertaken by the New Zealand Ministry of Agriculture and
Fisheries [MAF; now AgResearch Crown Research Institute (CRI)] and
others, to evaluate the relative agronomic effectiveness of RPR versus soluble
P fertilisers in adequate to marginally P-deficient soils have proven to be a
painstaking task. Long periods (3–6 years) of fertiliser withdrawal were
required for pasture growth on some soils to become significantly responsive
to applied P. Only then did differences between P sources become significant.
This problem has encouraged efforts to relate measurements of the extent of
RPR dissolution in soils to their agronomic effectiveness. Three main
modelling approaches have been used to achieve this objective: Kirk and Nye
(1986a, 1986b,
1986c); Sinclair et al.
(1993a); and Watkinson (1994b).
These models are reviewed and their explanation of RPR dissolution in mowing
trials tested. Components of each model have then been combined to produce
models to predict the agronomic effectiveness of RPR.
The development of P tests for soils receiving RPR-containing fertilisers is
reviewed. Separate Olsen P test–yield response calibration curves are
required for soils fertilised with soluble P fertilisers and soils fertilised
with sparingly soluble P sources or soluble P in the presence of heavy lime
applications. Whereas alkaline P tests such as Olsen or Colwell underestimate
the amount of plant-available P in these soils, acid P tests such as Bray 1
are likely to overestimate the available P. Tests involving cation and anion
exchange resin membranes appear to be more appropriate for soils with unknown
histories of soluble P and RPR use and may permit the use of single
calibration curves.
Trends observed in Olsen P soil test values, from farms on the North Island of
New Zealand that have a history (3–15 years) of RPR use are presented. A
predictive dissolution model is used to explain these trends but it is evident
that spatial and temporal variation in soil test results on farmers’
paddocks will be a major constraint to the precision to which this or similar
models may be used. The model, however, may provide the basis for sound advice
on the strategic use of RPR for direct application to New Zealand pasture
soils. It may prove useful in explaining the variation in RPR effectiveness in
a wider range of climates and soils.