Review of Australian phosphate rock research
Authors: Lewis D. C. , Bolland M. D. A. , Gilkes R. J. , Hamilton L. J. 
Published: August 2004

Summary. Most of the research on the effectiveness of
phosphorus (P) fertilisers in Australia has involved comparing phosphate rock
(PR) or partially acidulated PR (PAPR) with superphosphate (SP) or other
water-soluble P fertilisers. There are many estimates of effectiveness
(current relative effectiveness or CRE) which compared freshly-applied
(current) PR and freshly-applied (current) SP. The CRE values for PR range
from <0.1 to 2.5, with a mean value for apatite PR of 0.26 and 0.43 for
calcined calcium iron aluminium PR (Calciphos). As measured in field
experiments in the years after application, and using current SP as a basis
for comparison, the residual effectiveness of PR (residual value or RV) is low
and constant for up to 11 years after application. Phosphate rock is
5–30% as effective as current SP. The average value of RV for SP
declines by about 40% in the first year after application, followed by
a further 15% in the second year, and a further 30% over the
remaining 6 years. Values of relative effectiveness and RV, and the rate of
decline in RV differ substantially between sites and sometimes between plant
species.
Laboratory studies of reactions between PR and soil have shown that the poor
effectiveness of PR is primarily due to the limited extent and rate of
dissolution of these fertilisers compared with the almost complete and rapid
dissolution of water-soluble P fertilisers. Many Australian soils are only
moderately acid (pH in water >5.5) with low pH buffering capacities and
they cannot quickly contribute a large supply of hydrogen ions to promote
rapid dissolution of PR. Soils are commonly sandy and have low water-holding
capacities; in the strongly seasonal Mediterranean climate of south-western
and southern Australia, the fertilised surface soil rapidly dries between
rains thereby restricting PR dissolution. This restricted dissolution
contributes to the poor agronomic effectiveness of PR fertilisers. Studies in
Western Australia have shown that the effectiveness of current and residual PR
relative to current SP generally decreases with increasing level of
application. Therefore, relative to current SP, PR fertilisers become less
effective per unit of PR as more is applied to the soil. Consequently, PR
fertilisers frequently cannot support the same maximum yield as current SP.
Published work indicates that PR fertilisers cannot be regarded as economic
substitutes for SP for most agricultural applications in Australia. However,
much Australian research has used low reactive PRs in conditions that are not
likely to favour even highly reactive PRs. The soils dry out between rains
during the growing season and have insufficient hydrogen ions to cause rapid,
extensive dissolution of even reactive PR. Research elsewhere has suggested
that reactive apatite PRs can be as effective as SP for suitable soils and
environments. These are soils that remain wet for the whole growing season and
which contain sufficient hydrogen ions to cause rapid dissolution of reactive
PR. Laboratory studies, in which there is no P leaching, on 254 different
soils collected from throughout south-western Australia showed that 29 soils,
all collected from >800 mm average annual rainfall areas, dissolved
>40% highly reactive North Carolina PR, suggesting that in the field
these soils could be suitable for highly reactive PRs. Insufficient research
has been conducted in the high rainfall areas of Australia, where the
environment is more likely to favour highly reactive PR, and PAPR made from
highly reactive PR. Therefore, a national program was undertaken in 6
Australian states to identify circumstances under which PRs, including
reactive PR and PAPR made from reactive PR, may be economic fertilisers for
acidic soils in the high rainfall areas of Australia where agricultural
production is largely based on pasture production.