Sulphate leaching and sulphur balances of an organic crop rotation on three Danish soils

Summary

Sulphate leaching may reduce the long-term possibility of maintaining the sulphur (S) supply of crops in low input farming systems. Sulphate leaching and S balances were investigated in an organic cereal crop rotation with barley (Hordeum vulgare), grass-clover (Lolium perenne/Trifolium repens and pratense), winter wheat (Triticum aestivum) and barley/pea (Pisum sativum) in mixture at three sites during the first 3 years after conversion to organic farming. Sulphate leaching decreased significantly during the 3 years, and the concentration of drainage water was lowered 1.85.5 times in the third year. Similarly, the S-content of the crops decreased significantly over the years, reflecting lower soil S availability. Sulphur application to spring barley at all sites in Year 2 and at one site in Year 3 increased straw yield by 23% (P<0.01) and grain yield by 13% (NS) on average. Significant fertiliser-S response was restricted to the two sandy soils. Sulphate leaching was quantitatively the most important item affecting the S balance, since excess S was lost by leaching. The S-balance indicated that no substantial S-deficit may be expected in the longer term provided excess S is maintained in the system and the availability is synchronised with plant needs. Otherwise it will in the longer term be necessary on sandy soils to apply S in a plant-available form to avoid the negative effects of S deficiency on yield and quality of crops.