Cadmium in soil solutions from a transect of soils away from a fertiliser bin

The effects of high inputs of phosphate fertiliser on Cd concentrations were studied in soil solutions extracted from topsoils. Soils were sampled along a transect at distances of 1-100 m away from a fertiliser bin. The transect was sampled four times during 1 year. Soil solutions were analysed for Cd, pH, major cations and anions, and other heavy metals (As, Cr, Cu, Pb). For one of the transect samplings, soil total Cd, Cr, Cu, Pb and P were also measured. Cd speciation in the soil solutions was calculated by the GEO-CHEM-PC computer program. Chemical composition varied substantially along the transect, and also between samplings, indicative of seasonal effects and the influence of a fresh application of superphosphate fertiliser during the year. Application of fertiliser decreased soil solution pH and increased the levels of heavy metals in soil solution. Generally, soil total Cd, Cr, Cu, Pb, and P decreased with increasing distance from the fertiliser bin. Correlations between P and the four heavy metals were: P and Cd (R2 = 0.978), P and Cr (R2 = 0.712), P and Pb (R2 = 0.538), P and Cu (R2 = 0.267). Less than 1% of the total Cd in the soil samples was found in the soil solution. The free metal ion Cd2+ accounted for 55-90% of solution Cd. Of the complexed species of Cd, the chloride and sulphate complexes were usually the most important, even when nitrate and phosphate concentrations were relatively high. The presence of As, Cr, Cu and Pb had no effect on Cd speciation.