Integrating manures, slurries and biosolids as nutrient sources in arable crop rotations

Abstract

Without careful management, manures can cause loss of yield and quality as a result of both under- and over-fertilising. In addition to possible financial penalties, organic manures represent a major potential source of both point source and diffuse pollution. Better information is required to provide the agricultural industry with confidence in their use. Focused applications of manures can benefit crop yields. This project, therefore, focused on (a) a better quantification of the N mineralisation from dewatered sludge cake and composted products (i.e. materials from which fertiliser value was predominantly driven by organic N release) and (b) shifting applications of liquid manures (sludges and slurries) from autumn to spring, thereby increasing their N fertiliser value.

Three separate, but linked, activities formed the project so that understanding was obtained at all operational levels: detailed, small plot experiments, supporting laboratory experiments to further understand N release from the organic fraction of solid manures and a demonstration phase, scaling up on large (semi-field scale) plots, covering use, application techniques, crop effects and economic aspects.

The small plot experiments using the solid ('high dry matter') materials targeted single season and rotational aspects. These were undertaken at two sites over three years (Gleadthorpe, Nottinghamshire and Emley, Yorkshire). The N fertiliser replacement value (FRV, % of N applied), was not affected by manure application rate, but differed considerably between materials: 15% of the organic fraction in the first year for fresh dewatered cakes and 5% for composted materials. There were also substantial residual effects in the second and third years. Breakdown was related to thermal time.

The data will be used to improve existing recommendation systems. The slow release N increased leaching in the second winter after application at large manure application rates (750 kg/ha N), because N continued to mineralise after crop uptake ceased. There was an indication that the slow release N also increased grain N content. Laboratory incubations were a good guide to whether the materials were likely to be 'slow' or 'quick' mineralisers.

The small plot experiments using liquids focused on seasonal aspects only, using sites only for one season. Experiments were undertaken on two sites per year for three years, i.e. six site-years (three sites each at Bridgets, Hampshire and Coven, Staffordshire). The experiments clearly demonstrated that liquid manures could be top-dressed to cereal crops, thus reducing the nitrate leaching risk and increasing their fertiliser value. Practically, there was a wide window of application, with similar responses from applications during early tillering through to stem extension. Fertiliser value was linearly related to the ammonium-N content of the liquid manures.

Three sites were successfully used to demonstrate principles of good fertiliser management at the semi-field scale (Bedale, N. Yorks and Coven, Staffs in 2000 and Sleaford, Lincs in 2002).