AGRONOMY, MODELING AND ECONOMICS OF REACTIVE PHOSPHATE ROCKS AS SLOW-RELEASE PHOSPHATE FERTILIZERS FOR GRASSLANDS

Reactive phosphate rocks (RPRs) from Sechura, Peru (SPR) and North Car olina, USA (NCPR) were compared with triple superphosphate (TSP) as ph osphate (P) fertilizers for permanent grass/clover pastures in four fi eld trials in New Zealand. Trial sites ranged in initial pH (in water) from 5.7 to 6.3 and in rainfall from 712 to 1338 mm yr-1. SPR and NCP R were used in the unground 'as-received' state. Fertilizers were appl ied annually for six years. Pasture was harvested by frequent mowing, and herbage dry matter (DM) yields were measured at each cut. Herbage P concentrations were measured at each cut in two trials and on most c uts in the other two. For all sites combined, DM production from RPRs was initially significantly less than from TSP but it improved relativ e to TSP with time. Substitution values of RPR relative to TSP, denote d by S.V. (TSP/RPR) and defined as the ratio of P in TSP to P in RPR r equired to produce the same plant response during a specified period o f time, were estimated by relating yields from RPR treatments to the y ield response curve for different application rates of TSP. For the fo ur trials combined, S.V. (TSP/SPR) increased from 0.32 in year 2 to 0. 85 in year 6. S.V. values for NCPR were similar. The site which had th e lowest S.V. values (average 0.20) for total production over six year s was the site with highest pH (6.3) and lowest rainfall (712 mm). Cor responding S.V values for the other sites were 0.50 to 0.78. Herbage P concentrations showed a similar pattern of RPR performance relative t o TSP to that shown by DM production except at the highest application rate where TSP always supported much higher herbage P concentrations than RPR. The pattern of DM production from RPR relative to TSP was ex plained on the basis of a model involving soil P pools of undissolved fertilizer P and plant-available P respectively, with the hypothesis t hat P dissolved from RPR entered the plant-available P pool and was us ed with the same efficiency as P entering by dissolution of TSP. Model predictions of substitution values using directly measured RPR dissol ution rates agreed well with observed substitution values.The advantag e of RPRs in comparison to soluble P fertilizers for permanent pasture s was considered to lie in their lower price and not in greater nutrie nt efficiency. Economic advantage was calculated in terms of the retur n on investment from establishing and maintaining a pool of RPR in the soil large enough to release the required annual amount of plant-avai lable P compared with the cost of annual applications of soluble P fer tilizer.