Heavy metal solubility increases markedly with decreasing soil pH. Organic matter also has a strong influence on metal availability. This effect of pH is now generally recognised, but any modification of metal availability due to organic matter is more difficult to predict. We present some data from three field trials involving sewage sludge application to New Zealand pastoral soils.

At the first site, a sandy soil received large dressings of undigested sludge, resulting in acidification, and mobilisation of Zn down the soil profile and into pasture herbage. Intensive liming reduced plant uptake of Zn from as high as 600 mg kg⁻¹ to <200 mg kg⁻¹, and markedly reduced soil solution Zn.

At the second site, soil was amended with heavy metal (Cu, Ni and Zn) – spiked sewage sludge at concentrations up to and exceeding New Zealand soil limits. Initially, soil pH values ranged from 5.3 to 5.8 and considerable uptake of metals by pasture plants was observed. Some soil biological properties were adversely affected at the highest metal concentrations (especially Zn). Subsequent liming markedly reduced plant uptake of Ni and Zn, and negative impacts on biological properties were alleviated. Uptake of Cu was unaffected by pH.

At the third site, soil was amended with compost manufactured from sewage sludge, wood waste and green waste. Although total metals (Cd, Cr, Cu, Ni, Pb and Zn) were markedly elevated – total Cu to the current New Zealand soil limit concentration –compost application enhanced soil fertility and productivity, and reduced Cd uptake by plants.