A COMPARISON OF EXTRACTION METHODS FOR EVALUATING FE AND P IN FLOODEDRICE SOILS

Poorly crystalline Fe in soil has been shown to affect Fe and P availa bility. Oxalate extractable Fe, a measure of poorly crystalline Fe oxi des, has not been compared to soil test methods for Fe and P in rice s oils. Twenty eight soils used for rice production were incubated under aerobic and anaerobic soil conditions and extracted for Fe and P with ammonium oxalate, ammonium acetate-EDTA (AA-EDTA), ammonium bicarbona te-DTPA (AB-DTPA) and DTPA. Citrate-dithionite extractable Fe and Fe c ontent of rice plants in a greenhouse experiment were also determined. Soils used in this experiment had a large amount of poorly-crystallin e Fe oxide. In some soils, poorly-crystalline Fe constituted 60% of th e citrate-dithionite extractable Fe. The amount of extractable Fe and P increased significantly under anaerobic conditions. The relationship s between extractants showed that DTPA Fe was highly correlated to AB- DTPA Fe and oxalate Fe was highly correlated to AA-EDTA Fe. There was no relationship between Fe and P extracted by AB-DTPA, while there was a better relationship with ammonium oxalate and AA-EDTA extractants. Poorly-crystalline Fe and P extracted by ammonium oxalate were correla ted.