The optimum timing, dose, and sulfur form used depends on the specific sulfur demand of a crop and application technique. Under humid conditions, the sulfur dose should be split in such a way that sulfur fertilization in autumn is applied to satisfy the sulfur demand on light, sandy soils before winter and to promote the natural resistance against diseases. At the start of the main vegetative growth, sulfur should be applied together with nitrogen. With farmyard manure, on an average 0.07 kg sulfur is applied with each kg of nitrogen. In mineral fertilizers and secondary raw materials, sulfur is available usually as sulfate, elemental sulfur, and sulfite. Sulfate is taken up directly by plant roots, whereas sulfite and elemental sulfur need prior oxidation to sulfate, whereby the speed of transformation depends on the particle size and dimension of the thiobacillus population in the soil (Figure 7.19) (337,338).
The main secondary-sulfur-containing raw materials from the flue gas desulfurization process are gypsum and spray dry absorption (SDA) products, which are a mixture of calcium sulfite and calcium sulfate in a mass ratio of about 8:1 (340). SDA products with fly ash contents < 8% may contain up to 68% calcium sulfite, whereas this percentage in products with fly ash contents between 20 and 85% will not exceed 47% (341). A phytotoxic effect of sulfite applied by SDA products was observed when it was used as a culture substrate and on soils with a pH < 4 (337). The time required for complete oxidation of sulfite is about 2 weeks (342). Sulfite oxidation proceeds faster with increasing oxygen content and soil pH, and decreasing soil moisture content (343,344). When sulfur was applied at rates of <80 kg ha"1 to exclusively satisfy the sulfur demand of agricultural crops, no negative impact on crop performance and subsequent crops in the rotation was detected (337,342,345,346).
In general, the efficiency of sulfur uptake by rape is highly dependent on the sulfur status of the shoots (Figure 7.20). There is a close relationship between the initial sulfur content and its increase
Specific surface (cm2 g 1) 250 500 750 1000
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