DISTRIBUTION OF HEAVY-METALS IN RICE FARMING SOILS

The distribution of Cd, Co, Cu, Ni, Pb, Zn, Mn, and Fe in soils dedicated to rice crops have been studied. Total content and extractable fraction have been determined by acid digestion and extraction with EDTA, respectively. A method of selective sequential extraction (SSE) has been applied to determine the fraction of the metals bound to organic matter, soluble plus exchangeable, precipitated with carbonates, associated to Fe and Mn amorphous oxides, precipitated with sulfides, and the residual; all the elements show their most important fraction as precipitates, mainly as forms of sulfides and carbonates. The forms available by plants and bound to the organic matter are less represented, except for Cu and Ni.