Two field experiments were conducted from 1991 to 1996 on clay soils in central Sweden to provide information for improving soil fertility, minimizing N leaching, and increasing the benefits of ley to subsequent crops in organic farming. The results show that it is possible to calculate the amount of N mineralized during the 2-year period following incorporation of ley-crop residues, based on the proportion of incorporated organic matter stabilized in the more resistant humus fractions (i.e., the humification coefficient, calculated to be 35–40%), C:N ratios of the ley biomass, and ley age (humification appears to be higher in older crop residues). The fractions of potentially mineralizable N that are actually mineralized in the first and second years after ley incorporation vary depending on ley age and botanical composition and climatic conditions.
