Phosphorus in Sewage Sludges as a Fertilizer

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.