DISTRIBUTION OF HEAVY-METALS IN RICE FARMING SOILS

The distribution of Cd, Co, Cu, Ni, Pb, Zn, Mn, and Fe in soils dedica ted to rice crops have been studied. Total content and extractable fra ction have been determined by acid digestion and extraction with EDTA, respectively. A method of selective sequential extraction (SSE) has b een applied to determine the fraction of the metals bound to organic m atter, soluble plus exchangeable, precipitated with carbonates, associ ated to Fe and Mn amorphous oxides, precipitated with sulfides, and th e residual; all the elements show their most important fraction as pre cipitates, mainly as forms of sulfides and carbonates. The forms avail able by plants and bound to the organic matter are less represented, e xcept for Cu and Ni.