Möjligheter att minska kväveutlakningen genom att anpassa kvävegödslingen till variationer inom stråsädesfält
Due to excess precipitation that cause runoff, all soils in Sweden naturally leaches nitrogen.High leaching from arable land is an environmental problem when the runoff reacheswaters. In the international agreement, Baltic Sea Action Plan, the aim is to reduce therunoff of nutrients. Sweden has agreed to reduce its load of e.g. nitrogen to the Baltic Sea.One of the proposed measures is an increased use of site-specific N-fertilization, which hasbeen shown to increase nitrogen use efficiency and therefore also reduce the nitrogen runoff.Either yields are increased with the same total amount of nitrogen, or the total quantityof nitrogen is reduced with preserved yield levels, which is profitable for the farmers.The dynamics of nitrogen leaching differs between top dressing applications above orbelow the economical optimum. There is a consensus about a difference in the dynamics,but not about its magnitude. The curve that describes the leaching is steeper above optimumlevels which lead to an increased leaching. Therefore it is important to achieve anoptimal application, but that is not easily done when optimum can vary greatly betweenyears, between fields and within fields due to differences in clay and organic matter content,topography and the history of cultivation.The objective of the present work is to review the possibilities to reduce nitrogen leachingby adjusting the nitrogen fertilizer rates according to variations within fields croppedwith cereals.Calculations of the differences in leaching between uniform applications and sitespecificones were done with the leaching model used in the application STANK in MINDdeveloped for advisors by the Swedish Board of Agriculture. Clay content and differentdegrees of within-field variation in nitrogen fertilizer demand over the fields are factorsthat among others affect the reduction in leaching, and were therefore accounted for in thecalculations. The results showed that the reduction in leaching by using site-specific Nfertilization,with an average optimum rate of 100 kg N/ha, varied between 0,5-3,8 kg N/hafor a sandy soil (< 5 % clay) and 0,2-1,6 kg N/ha for a soil with high clay content (>40 %)due different degrees of within-filed variation. The highest reduction in leaching is reachedfrom adjusting the fertilization to the field average demand, but adjusting it sitespecificallywithin the field will reduce the leaching further. The first step should be toobtain the correct average level, but site specific fertilization may be a good way to do this.The results imply that increased knowledge of within field variations by means of sitespecificN-fertilization is profitable for both the famers and the environment.