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. Applications were made in autumn, winter and spring (following first silage cut), with raw sludge applied by injection and digested sludge both 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 tine-only treatment and an untreated control were also included. Nitrate leaching was measured following autumn and winter applications using porous ceramic cups. Average NO3--N concentrations in drainage water exceeded the EC limit for potable water of 11.3 mg/l following autumn injection of digested sludge at one site for both years. Average concentrations in drainage water for all other sludge treatments were below this limit. Up to 24, 11 and 6% of the applied total N was leached from injected digested, surface-applied digested and injected raw sludge respectively. Autumn applications resulted in greater leaching losses than winter application. Leaching losses were greater from digested than from raw sludge when both were applied by deep injection. Injection of digested sludge as compared to surface application exacerbated nitrate 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.