Environmental and economic implications of some slurry management options

Weather-driven simulations which take account of ammonia volatilization, soil water and soil nitrogen dynamics have been carried out to demonstrate the extent of losses of nitrogenous components which pollute the environment, following the land spreading of animal slurry (liquid manure). Alternative slurry management policies concerning selection of the size of store and spreading method were considered in these simulations. Losses were estimated of nitrate leached to held drains, volatilized ammonia and nitrous oxide released by denitrification. An economic analysis was carried out taking account of equipment costs and potential savings in costs of N, P and K mineral fertilizer. Results showed environmental benefits from a large store, enabling slurry to be spread at the optimum time for plant growth, compared with a small store requiring extensive winter spreading, and from spreading using an injection system rather than surface spreading with a splash plate. With the small store, benefits were shown from a single annual application on each area of land rather than repeated applications over the same area. Benefits took the form of increased utilisation of slurry nitrogen by growing plants (particularly on grassland rather than cereal crops), reduced mineral fertilizer requirements, and reduced pollution of both water and air. On economic grounds, results showed that neither an increased store size, nor injection rather than surface spreading, could be justified in terms of fertilizer cost savings. Only a small proportion of the additional costs of injection rather than splash plate spreading could be recouped in reduced fertilizer requirements, whereas for a large compared with a small store up to half the additional costs could be recovered in this way. However, repeated spreading of slurry on the same land area was shown to be a bad practice from an economic standpoint, as well as being damaging to the environment. (C) 1998 Silsoe Research Institute.