EVALUATION OF SOIL-PHOSPHORUS TESTS FOR FLOODED RICE SOILS UNDER OXIDIZED AND REDUCED SOIL-CONDITIONS

Soil tests have had limited success in predicting the response of rice (Oryza saliva, L.) to phosphorus (P) fertilizer. Phosphorus soil test methods have been developed under oxidized conditions without any con sideration of changes in available P due to flooding of rice soils. To determine the potential of various P test to predict P availability i n flooded soils 28 rice soils were incubated under oxidized and reduce d soil conditions and extracted for P with Bray 1, Olsen, ammonium ace tate-ethylenediaminetetraacetic acid (AA-EDTA) (Texas A&M test), resin , and water. Phosphorus response experiments were conducted on 19 soil s in the greenhouse and eight soils under field conditions. Soil extra ctable P, yield, and plant P were determined. Reducing conditions incr eased the amount of P removed by AA-EDTA and Bray 1 soil test methods. AA-EDTA and Bray 1-extractable soil P were highly correlated. Resin e xtracted the highest amount of P under oxidized conditions. The AA-EDT A soil test method overestimates the available P for high pH soils. Th e relationship between extracted P and rice plant growth parameters un der oxidized and reduced soil conditions indicated that soil P test me thods could be equally effective in predicting rice yield response to P fertilizer only when soils had very low extractable P.