In extensive agricultural systems, i.g. organic farming, nutrient balance calculations are of high importance. Common calculation models for nitrogen fixation of grain legumes are either of far too much complexity especially for use in agricultural practice or of too low accuracy. Measurements in the literature of grain yield, harvest index, N2 fixation, N content, N surplus, Nmin content of the soil, etc., were compiled. Correlation analyses were then carried out with Vicia faba L. and Pisum sativum L. data sets from conventional and organic field trials per-formed in Central European. Grain yield or N output, Nmin content before sowing, and the N harvest index proved to be the most effective driving variables for developing a calculation model for plant nitrogen fixation and the surplus amount. All the variables mentioned are listed in farmers’ plot card indices, with the exception of the N harvest index, an essential variable which is not detectible by the farmer. Therefore, the N harvest index was indirectly determined through the effects of grain yield and Nmin content using non-linear multiple re-gression analyses. Comparing calculations between common and the improved forms of mod-els showed significantly better conformity between measured and calculated datasets of grain legumes
