70 fields on a total of 17 farms in South East Norway, which were either ecological or in the process of converting to ecological farming, have been followed for a period of 2–3 years by sampling for soil mineral N content and crop N content. The nitrogen leaching potential was estimated from soil nitrate nitrogen content during late October, and was lowest on fields with plant growth in late autumn. On average leaching potential was lowest in leys (6 kg NO3ha−1). Undersown grain (13 kg NO3ha−1) and green fodder (14 kg NO3ha−1) had a low leaching potential. Turnips/vegetables (17 kg NO3ha−1), grain without undersown ley (30 kg NO3ha−1) and potatoes (33 kg N03ha−1) had a higher leaching potential. Fallow led to a much higher leaching potential (100 kg NO3ha−1). Autumn ploughing generally led to a higher nitrogen leaching potential than unploughed soils. A large proportion of legumes in a ley and in green fodder did not correlate with a higher leaching potential. Only in exceptional cases were large nitrogen losses found during the winter, even where the leaching potential was large. This was primarily due to the winter climate during the years of the study.