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

In agricultural systems with low S inputs, crops rely on the release of S f rom organic forms in the soil. In the Askov long-term experiments, started in 1894 on both sandy and loamy soils, soil S status following long-term ap plication of animal manure and mineral fertilizers was investigated 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 mat erial was used to characterize differences in availability of soil S. One h alf of all plots received 63 kg S ha(-1) as gypsum. Long-term fertilization with animal manure or NPK fertilizer increased the content of soil organic C in both soils and of organic S in the sandy soil compared with unfertili zed plots. Although dry matter yields were unaffected, the S uptake in harv ested crop parts increased considerably after S application. The amounts of N and S in harvested seeds and straw were closely related, but the N:S rat io decreased when S was applied. Soil and plant analyses both indicated tha t critical levels of S concentrations were reached, and that S application was capable of raising S concentrations well above the critical level. Beca use no additional mineralization from residual organic S took place, it was concluded that the residual S effect from long-term annual applications of animal manure or mineral fertilizers did not significantly increase the le vel of soil S available for crops with a short growing season, such as oil- seed rape.