Effectivity of phosphate fertilizers for flooded rice

Low levels of available phosphorus (P) in soils is a common constraint on r ice (Oryza sativa, L.) growing area of Argentina and Brazil. Since P fertil izers are required for crop production, direct utilization of phosphoric ro ck 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 y ield, P absorption and P soil availability. Seven site-year combination of fertilizer experiments were carried out on acid soils in Corrientes, Argent ina, from 1990 to 1993. Treatment rates, 0; 13; 27 and 40 kg ha(-1) of P we re applied as Triple superphosphate and Rock phosphate of North Carolina. T wo sites were conduced during two consecutive years, to evaluate the residu al effect of previous-year P applications. Both sources behaved similarly a t equal rates of applied P, either for direct or residual effects, that sug gest a possible cheap substitute to traditional P sources. The fertilizer r ate that gave the biggest contrast with the control, averaged across year-s ites 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 t he difference in soil and crop residues.