The long-term effectiveness of reactive phosphate rock as a phosphate fertiliser for New Zealand pastures

Abstract

Sechura reactive phosphate rock (SPR) and triple superphosphate (TSP) have been compared as phosphate fertihsers for pastures in a series of 19 field trials dispersed throughout New Zealand, each continuing for 3-6 years. Both fertilisers were applied annuaIly at 4 rates. In the first 2 years SPR was much less effective at increasing dry matter yields than TSP, but its performance improved markedly with time. In year 6 SPR outyielded TSP at alI application rates for all sites combined. The relative performance of SPR and TSP differed markedly at different sites. Very high soil phosphate retention and a combination of low moisture and high pH appeared to be unfavourable for SPR performance. In most sites SPR was equal or superior to TSP by year 6. SPR substantially raised molybdenum concentration in clovers, and this may have contributed to its superior performance at some sites. As TSP and single superphosphate were equally effective, the good performauce of SPR and the relative cheapness of reactive phosphate rocks suggest an important role for the latter on New Zealand pastures. Keywords Sechura phosphate rock, reactive phosphate rock, RPR, superphosphates, phosphate fertilisers