The use of sewage sludges as nitrogenous fertilizers for grassland

Abstract

The value of liquid anaerobically digested (LDS), liquid activated (LAS) and pressed cake (PC) as nitrogen fertilizers for grassland was compared to that of ammonium nitrate (AH) in a field trial containing 16 treatments applied to 1.5m x 10m plots replicated four times at each of two sites over two years. The grass was harvested four times per year.
The availability of the nitrogen in LDS, LAS and PC based on the nitrogen yield response was 46%, 50% and 16% respectively on a sandy soil and 35%, 48% and 20% respectively on a clay soil. Gaseous losses may' have reduced the availability of the nitrogen in the liquid sludges on the clay soil. The water in the sludges had little effect on the availability of the nitrogen.
The effect of the sludges on the dry matter yield and digestibility and the concentrations of crude protein, water soluble carbohydrate, nitrate-nitrogen, calcium and magnesium in the grass was similar to that of AH supplying the same available nitrogen. The LDS slightly raised the concentration of potentially toxic elements in the grass, probably due to the foliar retention of sludge solids. A laboratory experiment found the foliar retention to vary with the sludge type and solids content.
The sludges increased the concentrations of organic matter, potentially toxic elements, and extractable phosphate, and the earthworm density in the soil. The AH acidified the soil more than the sludges.
The response of a grass/clover sward in 30 cm diameter pots in the field to the sludges and AN was lower than that of the pure grass sward.
The response to the sludges depended upon the availability and rate of release of the sludge nitrogen.
The speed of release of sludge nitrogen justifies considering liquid sludges as fast-acting fertilizers although the variation in the nitrogen availability reduces their value for grassland management.