The overall aim of this study will be to evaluate the effects of established soil heavy metal concentrations (resulting from inorganic metal salt additions) on soil microbial activity, with regard to implications for agricultural productivity and long term soil fertility. Addition of inorganic salts (zinc, copper and cadmium carbonate) will provide a contrast to the main long term sewage sludge experiments (SP0125), where contaminated sludge cakes and metal-amended liquid sludges have acted as the metal carriers. The project will be undertaken collaboratively by ADAS, Rothamsted and WRc and experiments will be conducted at 3 sites in England with different topsoil compositions, for which metal dose-response curves have already been established. Experimental plots receiving different treatments (varying metal salt target concentrations and controls) will be managed under a ley/arable cropping regime and planted with the same grass cultivar and wheat variety. During harvest years 1998 and 2000, first cut grass yields and metal concentrations in the harvested dry matter will be determined at all 3 sites. In harvest years 1999 and 2001, wheat yields and metal concentrations in the grain will be determined at each of the sites. Soil microbial activity will be determined by assessing biomass carbon, soil respiration, rhizobia (most probable numbers) and by use of the Biolog™ microtitre plate system for identifying microorganisms. Soil chemical properties measured will be ammonium nitrate-extractable heavy metal concentration, soil organic carbon, cation exchange capacity, total N and soil mineral N. Metal bioavailability to crops, agricultural productivity and long-term soil fertility will also be evaluated; results from this part of the study will be compared with those obtained from the experiments involving addition of metal rich sewage sludge cakes and metal-amended liquid sludges (conducted in SP0125). The metal salt treatments will enable the importance of organic matter additions in mediating metal bioavailability to be evaluated. In the short term, the treatments will provide a “worst case” scenario in terms of likely metal bioavailability; in the longer term, the experiments will be of great value in allowing a comparison to be made of metal additions as inorganic salts with metal additions in the form of contaminated sludge cakes and metal-amended liquid sludges.