Efficiency of Residual Phosphorus of 16-20-0 Fertilizer in Some Paddy Soils

Abstract

In Bang Len silty clay of marine and brackish water deposits on former tidal flat area of the central plain with total iron 2.71% Fe and "active" iron 0.83% Fe, the residual phosphorus from 16 - 20 - 0 fertilizer for one rice-growing season had the same efficiency (20 - 33%) as that of the readily applied 16 - 20 - 0 fertilizer, while the efficiency of P-residuum of this fertilizer after two successive growing seasons was decreased distinctly to 13 - 15%, especially that of low application rate (100 mg P per culture). After three successive rice-growing seasons, the P-residuum became negligible (2 - 8%). In Saraburi loamy soil of semi recent alluvium on low terraces of the central plain which composite minerals had been weathered for some time, with total iron as high as 3.72% Fe and "active" iron 1.35% Fe, the P-residuum of the 16 - 20 - 0 fertilizer for one rice-growing season had the efficiency only 9 - 13% whereas that of the readily applied fertilizer was 17 - 26%. After two rice-growing seasons and over, P-residuum were as low as 2 - 8% which was negligible.