RELATIONSHIP BETWEEN IRON AND MANGANESE OXIDES AND PHOSPHORUS SORPTION IN SOILS IN THE STATE OF RIO GRANDE DO SUL, BRAZIL

The soils used for irrigated rice by flooding in the state of Rio Grande do Sul, Brazil, are derived from different source material and are subjected to temporal variations in oxidation and reduction conditions. These conditions contribute to variations in total concentrations and crystallinity of iron oxides and manganese, which are related to sorption and desorption processes, affecting P availability to irrigated rice. With the objective of verifying P sorption and availability to rice in soils from different parent materials, two studies were conducted. In the first, the total content of Fe and Mn and maximum P adsorption capacity (MPAC) by the Langmuir Isotherm were determined, and associated with the MPAC, the total contents of Fe and Mn, Fe-Ox6/Fe-dit ratio Mn-Ox6/Mn-dit ratio, clay and organic matter. In the second study, P sorption was quantified in four flooding periods. In the soils derived from basalt sediment (Luvisolo, Vertisolo and Chernosolo), MPAC was higher than in soils derived from granite and sandstone sediment (Planosolos and Gleisolos), due to higher total of Fe and Mn contents and Fe-Ox6/Fe-dit and Fe-Ox3/Fe-dit ratios. The association with MPAC was highest with Fe-Ox6/Fe-dit in all soils. P sorption was lower in the Planossolo than in soils derived from basalt sediment in all flooding periods.