Assessing the leaching of cadmium in an irrigated and grazed pasture soil.

To decrease the concentration of the toxic metal cadmium (Cd) in topsoil, and the human food chain, many countries have limited the Cd concentration allowed in phosphorus (P) fertilisers. However, to inform those limits we need accurate estimates of Cd leaching from established farming systems. Different soil layers were sampled to 2000 mm depth of a long-term trial that had applied 22.5 kg P ha yr for 67 years to grazed pastures that received either no irrigation or were irrigated when soil moisture fell below 10 or 20%. The annual yield of Cd leaching from the top 150 mm of soil to the 151-250 mm layer was between 1.1 and 1.8 g ha with Cd leaching increasing with the frequency of irrigation. The rate of Cd accumulation measured to 2000 mm was within the mean and standard error estimated for treatments using a mass balance approach. Estimates of annual Cd leaching loss were like those established from field trials measuring leaching events over a year (0.3-1.8 g ha) with a similar rate of P application (9-24 kg P ha yr). Using a Cd leaching rate of 1.8 g ha yr and P applications rates of 22.5 kg P ha, topsoil Cd concentrations may stop increasing if Cd concentrations in P fertiliser can be maintained at < 72 mg Cd kg P.