Liquid manure management to maximize its fertilizer value and minimize pollution

Land spread liquid manure from housed livestock is both a potential environ mental pollutant and a valuable source of plant nutrients to replace chemic al fertilizer, but typical farming practice neither minimises pollution nor maximises fertilizer value of manure. With view to providing decision supp ort, a modelling approach has been adopted to study the effects of manure m anagement options on two forms of pollution, that transported by surface ru noff(overland flow), and nitrate leaching to field drains or deep groundwat er. The Swedish weather-driven simulation models SOIL (soil water) and SOIL N (soil nitrogen dynamics) have been adapted for this purpose, including im plementation of drainage theory, and calibration/validation for site-specif ic soils and cropping in Scotland and Ireland. Ten year simulations were carried out with a range of management options in different climatic regions. Results show that surface runoff is minimised by confining spreading manure in winter to fields with low runoff susceptib ility because of a high profile conductivity. There is limited scope for re ducing runoff pollution by selecting days with suitable soil and weather co nditions, and this is applicable only to fields with a narrow (intermediate ) range of profile conductivities. Nitrate leaching (as well as N losses by volatilization and denitrification) increases non-linearly with organic an d mineral N application and varies crucially with application date. Results have been incorporated into decision support software for managing manure in terms of selection of fields for spreading, the quantity and timi ng of field applications, and also applications of complementary mineral fe rtilizer. Fields are classified in terms of their susceptibility to surface runoff, with defined spreading windows plus daily decisions about conditio ns being suitable for spreading for fields with intermediate susceptibility only. Nitrate leaching is minimised by making decisions about the quantity of manure applied to match nutrients in the manure to the requirements of the crop. An accurate knowledge of the nutrient composition of manure is re quired for this, with the computer system communicating with a modified man ure tanker equipped to carry out online sensing of nutrients and to give an even spread at a specified application rate.