A COMPARISON OF THE LEACHABLE INORGANIC NITROGEN-CONTENT IN ORGANIC AND CONVENTIONAL FARMING SYSTEMS

Soil samples collected from 26 organic and 550 conventional farms during autumn 1990 through the national Grid Net Survey (GNS) and a simultaneous supplementary survey on 8 organic farms have been analyzed in order to estimate the risk of nitrogen leaching. The average nitrate N (nitrogen) content in the soil (0-75 cm) was very similar in samples from organic farms (31kg/ha) and conventional farms using manure (29 kg/ha), but superior to that of conventional farms not using manure (22 kg/ha). Samples collected from fields on organic farms cultivated with grass with clover or alfalfa were very small (12 kg/ha) compared to samples from fields cultivated with cereals (57 kg/ha) and samples from bare fields following cereals (48 kg/ha). The average nitrate N content was higher in samples from clayey soil (40 kg/ha) than from sandy soils (15 kg/ha). The nitrate N content in samples from regions with a high percolation level during the autumn (August to December 1990) was lower than in samples from regions with a low percolation level.