11 December 1998

Bid to discover the nutrient value of cake

ANNUAL dressings of digested sewage sludge cake, augmented with bag nitrogen, give arable crop yield responses over and above those expected from its nutrient value alone, judging by the results of an ongoing replicated plot trial at ADAS Bridgets.

Started in autumn 1995, it is on a shallow chalkland site previously cropped continuously with cereals. Hunter winter wheat was grown in the trials first two years and Apex oilseed rape in 1997-98.

This Southern Water-funded work aims to determine the nutrient and soil conditioning value of the cake applied as single or annual dressings of around 25t/ha (10t/acre). This supplies an estimated 250kg/ha of nitrogen of which about 40kg is available in the first year and 20kg in the second and proportionately less from then on.

Yield responses to the straight cake were compared with those of six levels of inorganic nitrogen, ranging from 60 to 360kg/ha (48 to 288 units/acre), and 25t/ha of cake plus sufficient inorganic nitrogen to provide the same total as the straight inorganic nitrogen treatments.

"Were surprised at the contribution made by the cake – its more than we expected on the basis of its nutrient content," declares ADAS management consultant Gordon Hickman. "One probable reason is the improved soil organic matter level, resulting in better soil structure and workability. The cakes trace elements and sulphur content also help."

Application of cake, prior to ploughing, in successive autumns, plus bag nitrogen, have given the best yield responses. This years top Apex plots, yielding 4.1t/ha (1.7t/acre), received 120kg/ha (96 units/acre) of bag nitrogen in addition to an estimated 70kg/ha from the autumn 97 cake and the residue from the previous two dressings.

That is 11% better than the highest yielding straight bag nitrogen plots 3.7t/ha (1.5t/acre) which had received 240kg/ha (192 units/acre).

SLUDGEBENEFITS

&#8226 ADAS Bridgets trial on shallow chalk site.

&#8226 Run since 1995.

&#8226 Yield responses greater than nitrogen effect alone.

&#8226 Soil conditioning role?

&#8226 Trial to check long-term role.