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DISTRIBUTION OF IRON AND PHOSPHORUS IN FLOODED AND UNFLOODED SOIL PROFILES AND THEIR RELATION TO PHOSPHORUS ADSORPTION1

Phosphorus adsorption was investigated in two, paired soil profiles—flooded rice and irrigated pasture—to elucidate adsorptive changes of phosphorus as a consequence of long-term seasonal soil flooding. In the two concentrations studied, 10 and 50 ppm P, the adsorption of phosphorus was considerably higher in the flooded rice topsoils, 0 to 30 cm in depth, than in the upland topsoils. The differences in phosphorus adsorption gradually decreased as soil depth increased. High phosphorus adsorption in the rice topsoils was attributed to the higher content of amorphous iron resulting from reduction caused by seasonal soil submergence. Fractionation of phosphorus in the soil profiles revealed that prolonged seasonal flooding tends to increase the 0.1 N NaOH-extractable phosphorus and, concomitantly, to decrease the reductant soluble and calcium phosphate contents. The lower pH and reductive conditions prevailing in the flooded rice soils appear to be responsible for the transformations of calcium and crystalline iron phosphates to amorphous. types of phosphate precipitates. No significant eluviation of iron or phosphorus was observed in the flooded rice, as compared with adjacent pasture soils. The high clay content of these soils, coupled with a virtual lack of water movement through the profiles, may restrict the movement of these two elements.