SEWAGE-SLUDGE APPLICATIONS TO GRASSLAND - INFLUENCE OF SLUDGE TYPE, TIME AND METHOD OF APPLICATION ON NITRATE LEACHING AND HERBAGE YIELD

Sewage sludge applications were made to two freely draining grassland sites of contrasting rainfall in SW England in 1991/92 and 1992/93. Ap plications were made in autumn, winter and spring (following first sil age cut), with raw sludge applied by injection and digested sludge bot h as a surface application and injection at application rates of 140 m (3)/ha and 125 m(3)/ha for raw and digested sludge respectively. A tin e-only treatment and an untreated control were also included. Nitrate leaching was measured following autumn and winter applications using p orous ceramic cups. Average NO3--N concentrations in drainage water ex ceeded the EC limit for potable water of 11.3 mg/l following autumn in jection of digested sludge at one site for both years. Average concent rations in drainage water for all other sludge treatments were below t his limit. Up to 24, 11 and 6% of the applied total N was leached from injected digested, surface-applied digested and injected raw sludge r espectively. Autumn applications resulted in greater leaching losses t han winter application. Leaching losses were greater from digested tha n from raw sludge when both were applied by deep injection. Injection of digested sludge as compared to surface application exacerbated nitr ate leaching losses. Herbage yields and N recoveries were better from later sludge applications, with the exception of injection treatments, where spring sludge injection during a prolonged dry period resulted in yield reductions as compared to autumn injection.