PHOSPHORUS RELATIONSHIPS IN FLOODED RICE SOILS WITH LOW EXTRACTABLE PHOSPHORUS

Rice (Oryza sativa L.) grain yields on some flooded soils are not incr eased by P fertilizer despite their low soit test P status determined on air-dry soils by soil test methods such as Bray, Olsen, or Texas A& M. Conventional soil test methods apparently do not accurately assess the capacity of these soils to supply P to rice grown under flooded co nditions. To test the possibility of an Fe-P association, oxalate extr actant (which would extract noncrystalline Fe and its associated P) wa s used to provide a more accurate measure of available P in flooded so il. Phosphorus response experiments were conducted on 10 rice soils un der greenhouse and field conditions and related to the oxalate-extract able Fe and P, P uptake, P adsorption, and P desorption under aerobic and anaerobic conditions. Oxalate-extractable Fe significantly increas ed under anaerobic conditions in all soils including the soils that we re nonresponsive to P and had low levels of extractable P using conven tional extractants. Phosphorus adsorption and desorption data confirme d the importance of oxalate extractant in predicting P availability fo llowing anaerobiosis. The inability of Bray 1, Olsen, and Texas A&M so il test methods to accurately predict P availability in flooded rice s oil appears to be due to their failure to extract the active reductant -soluble P fraction. The measurement of P associated with poorly cryst alline Fe by oxalate extractant is a useful soil test method for predi cting the availability of P in flooded rice soils.