Effects of Fresh and Residual Calcined Phosphates on Flooded Rice in Acid Sulfate soils

Abstract

Pot and field experiments were conducted during the wet seasons 1985 and 1987 to evaluate the relative effectiveness of a calcined phosphate fertilizer, Phospal-34 (PP-34), on flooded rice in acid sulfate soils. The experiments comprised 1) 2 consecutive pot trials and 3 consecutive field trials to assess the effects on rice of fresh and residual P from 2 sources, PP-34 and Monoammonium phosphate (MAP) and 4 rates of fertilizer phosphorus and 2) 2 consecutive field trials to investigate the effects on rice of fresh and residual P from fertilizers containing PP-34. Rice grown on acid sulfate soils in both pot and field experiments showed marked response to fresh and residual phosphates applied either in the form of PP-34 or MAP. In 3 consecutive field trials, the response to residual phosphate of the second rice crop was the highest. Averaging from the 3 crops, grain yield increase due to the added-P was about 30 percent. The effectiveness of fresh and residual PP-34 were comparable to those of P containing in Monoammonium phosphate. As a straight P fertilizer, PP-34 can be used for flooded rice in acid sulfate soils as an alternative source to highly water-soluble P fertilizer, MAP. Moreover, for the formulation of mixed fertilizers for rice, a fertilizer containing 40% available P derived from PP-34 and 60% deriving from Diammonium phosphate (DAP) was as agronomically effective as DAP. It is probable that PP-34 can be used to partially substitute other highly water-soluble P materials in the mixed fertilizer produce.