The effects of rice growing on soil phosphorus immobilization

Abstract

Representative rice soils of the Murrumbidgee and Coleambally Irrigation Areas, New South Wales, were analysed for phosphate (P) sorption characteristics and active iron. Samples analysed were taken before and after flooding (in the field and glasshouse) and from rice bays, irrigation control banks and nearby, non-irrigated fencelines. Previously flooded soils showed greater P sorption capacities than their non-flooded soil counterparts. It was indicated that soils that had previously grown rice required more P for upland cropping than non-rice soils. P sorption was significantly positively correlated with active (acid ammonium oxalate extractable) iron. It was proposed that prolonged waterlogging mobilized iron from relatively well ordered iron oxides of low P sorption capacity but on oxidation formed disordered iron oxides with a large capacity to immobilize P.