Metals accumulate in sewage sludges from domestic and industrial sources. Where such sludges are used in agriculture, the metals may contaminate land to such an extent that the quality and yield of crops is affected. Control of the industrial sources of sludge applications will avoid these problems. The paper reviews available information in the chemistry of the most significant potentially toxic metals in soils and their effects on crops including contamination of the food chain ending with man. The paper emphasises the large body of information available but underlines the need for more data if greater certainty is to evolve in the guidelines produced for the agricultural use of sludge and trade effluent control.

A knowledge of the value of the phosphorus in a sewage sludge relative to a conventional phosphorus fertilizer such as superphosphate can provide a strong economic incentive for farmers to accept sewage sludge as an application to agricultural land. A study of the literature showed a range of values. These appeared to be modified by method of assessment, by type of sludge, and by its metal content. An incubation study of soil-sludge mixtures showed that changes occurred in the values for solubility of phosphorus in 0.5 M sodium bicarbonate. A study of six crops and three soils indicated that the most suitable crop to indicate availability of phosphorus in sludge was spinach ( Spinacea oleracea cv.) with ryegrass ( Lolium perenne cv.) as the next best. Soil of any texture was suitable provided the initial bicarbonate solubility of phosphorus in the soil was less than 10 mg P kg -1 . Data from the three studies carried out indicated that phosphorus in fresh undigested sludge was as available as that in superphosphate. Phosphorus in the digested sludge used was 60% as available as that in superphosphate.

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