Effects of Sources of Phosphorus Fertilizer on Crop Yields in Rice-Soybean Double Cropping

Abstract

An effect of rock phosphate applied to rice on soybean seed yield in typical rice-soybean cropping system of the north was investigated on Low Humic Gley Soils (Typic Tropaqulfs) in a farmer’s field in Tambon Chomphu, Amphoe Muang, Changwat Lampang during July, 1983 April, 1984. Three randomized complete blocks with 4 treatments of 0, 37.5 kg P₂O₅/ha of triple superphosphate (TSP), 1250 and 2500 kg/ha of 3% P₂O₅ citrate soluble (30% total P₂O₅) rock phosphate were utilized to study the effect of fertilizer sources and rates on a local indica rice cultivar Sanpatong. A split plot design was subsequently superimposed on this phosphate-rice experiment with 4 sub-plot treatments consisting of 0, 18.75, 37.5, 75 kg P₂O₅/ha of TSP fertilizers on soybean cultivar SJ. 4. The results indicated no rice yield response to either from of phosphate fertilizer but showed a significant increase (p > 0.01) in the plants P-uptake levels and in the soil Bray II-P availabity levels when applied with both forms of phosphated fertilizer. Significant increase (P > 0.01) in soybean seed yield with the application of superphosphate fertilizer was obtained in the main check treatment and the residual TSP treatment. No response was observed from those newly TSP application to either rate of those residual rock phosphate. These results thus indicate that rock phosphate can be an effective source of phosphorus fertilizer in the rice-soybean double cropping system under some conditions of lowland soils.