Case studies examining nitrogen supply dynamics were carried out on two biodynamic farms in Sörmland (for a 7 year rotation period on one farm and 3 years on the other) and on one biodynamic farm in Skåne (for 3 years) in Sweden. Each of the farm locations had a different set of climatic and soil conditions. Two conventional farms, one with and one without livestock, served as controls in each of the two areas. In this paper results from the biodynamic farm in Sörmland are presented. Changes in soil mineral nitrogen, nitrogen uptake by the crop and productivity were studied in relation to previous crops, crop rotations, and nitrogen fixation. A nitrogen balance and flow of nitrogen in the farm ecosystem is presented. For a farm to meet its nitrogen needs without resorting to the use of artificial fertilizers requires that nitrogen be effectively recycled within the farm with only small losses; that enough nitrogen-fixing crops be cultivated (legumes on a third of the area in the circulation); and that the intensity of livestock management is adjusted on the basis of the farm's feed production capacity.