Superphosphate fertiliser application and cadmium accumulation in a pastoral soil

At Winchmore, single superphosphate fertiliser has been applied annually to grazed pasture plots since 1949. To determine whether or not cadmium (Cd) in the fertiliser had accumulated in the soil, samples were collected at intervals (0–0.075, 0.075–0.15, 0.15–0.25, 0.25–0.50, 0.50–0.75 and 0.75 –1.00 m) to a depth of 1 m, the Cd concentration and bulk density measured and Cd density calculated. The relationship between Cd density and depth was curvilinear and the slope or curvature increased with increasing fertiliser application rate. Nearly 100% of the Cd stock was located in a depth of 0–0.5 m and 36% in the uppermost 0.075 m depth. Leaching calculations using Burn’s equation accurately estimated the measured Cd stock’s vertical distribution. Estimating the inputs to soils from fertiliser and atmospheric deposition and the loss by leaching and outwash accounted for the measured Cd stock. We concluded Cd accumulated in these soils at a rate determined by the fertiliser’s Cd concentration and application rate.