For three ecological mixed arable-dairy farms on sandy soils in the Netherlands the nutrient balances for N, P and K have been assessed on the basis of inputs and outputs of products, manure, fodder and straw. For nitrogen the assessment of N-fixation and NH4volatilization from deep litter stables offer new problems because of the special farming methods in ecological agriculture. The nitrogen surpluses for the organic farms are 83 kg N per ha against 444 kg N per ha for conventional extensive dairy farms on sandy soils. The average nitrogen efficiency for the three organic farms is 31%, conventionally this was 12% in 1986. The leaching of N has been estimated on the basis of soil moisture analysis for most fields of the separate farms. The average N loss by leaching was 20 kg N*ha−1*yr−1. The average nitrogen content of shallow ground water was 11 mg NT*1−1, which will be lower in deeper layers as a result of denitrification. The results of the research over three consecutive years showed possibilities for improvement in farm management that will be necessary to meet recommendations for improvement of groundwater quality in the future.