Identification of key factors for reducing N and P leaching from organic crop rotations

Leaching of nitrogen (N) and phosphorus (P) for different 6-year organic crop rotations was examined in separately tile-drained field plots on two different sites in southwest Sweden. On each site, two different farming systems, one with dairy cows and one without stock, were evaluated to identify parts of the crop rotations with the greatest risks of N and P leaching and to examine the scope for improvement. Although organic farming methods themselves already led to a reduction in nutrient leaching, critical periods in the crop rotation could nevertheless be identified for the two soil types. While P leaching is of major importance on clay soils, sandy soils are strongly susceptible to N leaching. From the present study it could be concluded, that key factors for reducing N and P leaching from clay soils are late ploughing, avoidance of early incorporation of clover-grass leys in order to sow a winter cereal, the use of undersown crops and an even distribution of nutrients within the crop rotation. For the sandy soil, where measures like undersown crops and spring tillage were already integrated in the studied crop rotation, potato cultivation and application of farmyard manure were identified as the main sources of N losses to the drainage water