Soil sulphur status following long-term annual application of animal manure and mineral fertilizers.

Summary

In agricultural systems with low S inputs, crops rely on the release of S from organic forms in the soil. In the Askov long-term experiments, started in 1894 on both sandy and loamy soils, was investigated the soil S status following long-term application of animal manure and mineral fertilizers in the growing season of 1995. In a field trial with oil-seed rape (Brassica napus, L.) soil analysis, leaf tissue analysis, yield and S-removal in plant material was used to characterize differences in availability of soil S. One half of all plots had 63 kg S/ha as gypsum applied. Fertilization with animal manure or NPK fertilizer increased the content of soil organic C compared with unfertilized plots. This increase was followed by a similar build-up of organic S in the sandy soil. Although dry matter yields were uneffected, the S uptake in harvested crop parts increased considerable by S application. The amount of N and S in harvested seeds and straw were closely related, but with higher S uptake per unit N were S was applied. Soil and plant analysis both indicated that critical levels of S concentrations were reached, and that S application was capable of raising S concentrations well above the critical level. Since no additional mineralization from residual organic S took place, it was concluded that the residual S effect from successive annual applications of animal manure or mineral fertilizers did not increase significantly the level of soil S available for a crop like oilseed rape, with a short growing season, compared with unfertilized plots.