Kinetics of Fertilizer Derived Cadmium In Agricultural Soils and the Factors That Affect Its Rate of Release.

Agricultural applications of phosphate fertilizers lead to the release of toxic metals present in the parent minerals. Of special concern are the comparatively high levels of cadmium (Cd) in some rock phosphate minerals and the potential for Cd to be released from fertilizers, become bio-available or to accumulate in soils. We measured the release kinetics of Cd from a commercial ammonium phosphate (16-20-0) fertilizer and subsequent Cd transfer to the mobile water phase or to soil surfaces in controlled sand-column and soil-column experiments. Replicate quantities of fertilizer particles were embedded in a clean sand or natural soil matrix in a shallow column and and flushed with sequential applications of water to simulate irrigation or precipitation events.  Effluent from the column was analyzed for pH, electrical conductivity (EC), Cd, and P. Cd release rose rapidly with 24 hr of intermittent contact with soil moisture, but then peaked and declined with only 10-20 % of the total Cd being released in the initial flush. In contrast, P release rose rapidly and remained fairly constant for a period of days until the P content of the fertilizer was depleted.  The pH in our studies varied relatively little in the effluent and does not explain the Cd release pattern. Soil-water EC and Cd release profiles track closely and results suggested first-order kinetics for the release of both Cd and P.  P appeared to affect Cd release through the formation of secondary Cd-phosphate complexes or solids and via pH buffering. EDTA additions to the eluent enhanced Cd release, indicating the solution activity of Cd is important in controlling Cd release.  However, release rates are not well correlated with the total mass loss of the fertilizer matrix.