Sulphate leaching in an organic crop rotation on sandy soil in Denmark

Sulphate leaching losses may reduce the long-term possibility of maintaining the S supply of crops in low input farming systems. Sulphate leaching and S balances were investigated in an organic dairy/crop rotation (barley [Hordeum vulgare] --> grass-clover [Lolium perenne/Trifolium repens] --> grass-clover --> barley/pea [Pisum arvense] --> winter wheat [Tritucum aestivum] --> fodder beer [Beta vulgaris]) from 1994 to 1998. The importance of climatic conditions and use of different organic manure types at different application rates for sulphate leaching were investigated and related to the concurrent nitrate leaching. As an average of years, sulphate leaching from the crop rotation was 20 kg S ha(-1), which was equivalent to 60% of the total input to the rotation. Sulphate leaching was highly variable over the years (4-45 kg S ha(-1) for the same crop) and closely related to the annual drainage volume (r(2) = 0.99; P < 0.01). The same relationship between drainage and nitrate leaching was not significant. No differences were observed in sulphate leaching between the organic manure types or the application rates, but significant differences were found in sulphate leaching between the different crops in the rotation. In the year with the largest drainage volume, there was a significant correlation between the S input in irrigation and sulphate leaching (r(2) = 0.69; P < 0.05). The S balance was slightly positive when averaged over the four years, as more S was imported than removed. Thus, immediate S deficiency may not occur, bur in the longer term a negative S balance must be expected in this crop rotation. However, even with a positive overall S balance, the S input is not necessarily synchronised with plant needs. In order to maintain a sufficient S supply in the future when further reductions in the atmospheric deposition are expected, it is important to reduce leaching losses of sulphate. (C)2000 Elsevier Science B.V. All rights reserved.