Effectivity of phosphate fertilizers for flooded rice

Resumo

Low levels of available phosphorus (P) in soils is a common constraint on rice (Oryza sativa, L.) growing area of Argentina and Brazil. Since P fertilizers are required for crop production, direct utilization of phosphoric rock might be an economical option for farmers. The aim of this paper was to compare the direct and the residual effect of two P sources on rice grain yield, P absorption and P soil availability. Seven site-year combination of fertilizer experiments were carried out on acid soils in Corrientes, Argentina, from 1990 to 1993. Treatment rates, 0; 13; 27 and 40 kg ha^-1 of P were applied as Triple superphosphate and Rock phosphate of North Carolina. Two sites were conduced during two consecutive years, to evaluate the residual effect of previous-year P applications. Both sources behaved similarly at equal rates of applied P, either for direct or residual effects, that suggest a possible cheap substitute to traditional P sources. The fertilizer rate that gave the biggest contrast with the control, averaged across year-sites crops, were 13 kg ha^-1 of P, increasing grain yields from 5.13 to 5.95 Mg ha^-1. A mean of 38% of this rate is recovered in biomass, but about 60% of the P from fertilizer is exported by the grain harvest, remaining the difference in soil and crop residues.