Phosphorus leaching from liquid digested sewage sludge applied to sandy soils

The aim of the experiment was to determine the importance of liquid digeste d sewage sludge ('sewage sludge') as a source of phosphorus leaching from s andy soils in the UK. The sewage sludge was applied to uncropped loamy sand and sandy loam monolith lysimeters (1.2 m deep) annually for 3 years. The application rate was sufficient to supply approximately 60 kgP/ha annually. An application equivalent to four times this amount was also applied to tw o other loamy sand lysimeters to test the effects of excessive amounts. Tot al P (TP), total dissolved P (TDP) and molybdate-reactive P (MRP) concentra tions were measured in the drainage water and were compared with leaching f rom untreated control soils. After two sewage sludge applications, Olsen's extractable P content of the topsoil had increased: a P surplus of c. 100 kg/ha was required to raise th e topsoil concentration by 5 mg/kg. Sewage sludge applications at the lower rate did not increase P concentrations in the drainage. Phosphorus concent rations were generally small and were, as an average of both soil types and nil and lower rate sludge applications, 12, 20 and 34 mug/l for MRP, TDP a nd TP respectively. Phosphorus leaching losses from the higher rate (excess ively large sewage sludge applications) were more variable but not signific antly (P > 0.05) different from the other treatments. Average concentration s were 9, 23 and 50 mug/l for MRP, TDP and TP respectively. Leaching from s ludge biosolid applications at operational rates to sandy soils moderately well supplied with P is not a major P loss pathway.