Phosphorus in sewage sludges as a fertilizer

A knowledge of the value of the phosphorus in a sewage sludge relative to a con-ventional 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 superphos-phate. Phosphorus in the digested sludge used was 60 % as available as that in superphosphate. Key Words—Sewage sludges; phosphorus; availability to crops; soil; assessment method; agricultural value; recycling, U.K. 1. Introduction an