Summary. An economic analysis was undertaken using
pasture yield data from 8 selected sites from the National Reactive Phosphate
Rock Project, that encompassed 7 different performance scenarios for North
Carolina phosphate rock. The aims were to determine whether the use of North
Carolina phosphate rock in place of single superphosphate might result in a
positive financial benefit in the 4th year, and after 4 years of annual
applications of fertiliser. The analysis was carried out using annual P
applications of North Carolina phosphate rock and single superphosphate, that
resulted in pasture yields equivalent to 50, 70 or 90% of the maximum
yield response of single superphosphate in the 4th year. Annual pasture dry
matter yields, produced by these fertiliser applications, were converted to
stocking rates, and dollar incomes were derived by applying appropriate gross
margins. The analysis was also undertaken to determine the financial benefit
from large, year-1 applications of North Carolina phosphate rock. Single
superphosphate was priced at $168/t while North Carolina phosphate
rock plus sulfur was priced at $180/t.
The economic analysis found that a positive financial benefit with North
Carolina phosphate rock occurred for only one scenario where the agronomic
performance of North Carolina phosphate rock and single superphosphate were
equivalent in the 4th year of annual fertiliser application. For 3 other
scenarios where the performance of North Carolina phosphate rock and single
superphosphate were also equivalent in the 4th year, the economic performance
of North Carolina phosphate rock was poor due to (i) a ‘lag’
effect where pasture yield with North Carolina phosphate rock was generally
less than that with single superphosphate in years 1–3, and/or to
(ii) a seasonal effect where the autumn–early winter pasture responses
with North Carolina phosphate rock were less than those with single
superphosphate, necessitating a reduction in annual stocking rates on the
North Carolina phosphate rock-fertilised pasture. A key finding was that
large, single, year-1 applications of North Carolina phosphate rock generally
overcame these seasonal and/or yearly lag effects, and led to positive
financial benefits from North Carolina phosphate rock applied in this way.
Annual applications of North Carolina phosphate rock were economically viable
at sandy, high rainfall sites where water-soluble P from single superphosphate
would be readily leached from the root zone, provided that the soil P status
was adequate and conditions were conducive to reactive phosphate rock
dissolution. However, North Carolina phosphate rock was not an economically
viable fertiliser to apply annually at (i) non-leaching sites where the soil P
status was low and marked yield penalties occurred in the first few years of
North Carolina phosphate rock use, (ii) where the soil had a very high
P-sorption capacity, and (iii) where North Carolina phosphate rock dissolution
was restricted by low rainfall or high pH. The inclusion of estimated residual
P value had only a small impact on the economic outcome for scenarios which
had not already performed poorly due to seasonal or lag effects.
Phosphorus status of pastoral soils where reactive phosphate rock fertilisers have been used
