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).
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 by fertilization. Under sulfur-limiting growth conditions, root-expressed sulfur transporters are highly regulated and induced. Besides that, sulfur fertilization improved root growth and thus access to sulfate (53).
An insufficient sulfur supply will not only reduce crop productivity, diminish crop quality, and affect plant health, but it also will impair nitrogen-use efficiency (53,347). Under conditions of sulfur deficiency, nitrate and non-S-containing amino acids accumulate-actions which may reduce the nitrate reductase activity. Sulfur fertilization promotes nitrate reduction and thus restricts nitrate accumulation in vegetative tissues. In Table 7.9, the influence of an increasing sulfur supply on the nitrate reductase activity and nitrogen-use efficiency is shown.
The highest nitrate reductase activity occurred at a sulfur dose of 120 kg S ha-1 and the highest N-use efficiency at 160 kg S ha-1 (Table 7.9) (349). This result corresponds to an increase of 18.2 and 18.7%, respectively, for the two doses. In comparison, the net nitrogen utilization of oilseed rape and cereals was significantly increased by sulfur fertilization by about 7 to 16%. A sulfur application rate of 100 kg S ha-1 yielded the best results for oilseed rape during three consecutive years of experimentation (347).
The sulfur demands of agricultural crops vary highly, as do the recommended sulfur doses (Table 7.10). Recommended sulfur rates vary between 30 and 100 kg S ha-1 for oilseed rape, and between 20 and 50 kg S ha-1 for cereals (103,337,348). For other crops such as sugar beet, grassland, rice, and soybean, the highest crop productivity occurred at sulfur rates of 25, 40, 45, and 60 kg S ha-1, respectively (351-353).
Aulakh (364) gives a detailed overview of sulfur uptake and crop responses to sulfur fertilization in terms of yield and quality, with special attention being paid to crops grown in India. Sulfur fertilizer can be applied to the soil or given as foliar dressings. As the sulfur dose is limited when applied via the leaves, this form of fertilization can only be a complementary measure to correct severe sulfur deficiency. Usually, for foliar applications, either Epsom salts or elemental S are used. Calculated from changes in the sulfur uptake by seeds, only 0 to 3% of foliar-applied sulfate-S with Epsom salts was utilized, while 33 to 35% of sulfur applied as elemental sulfur product (Thiovit?) was utilized (338). Foliar-supplied sulfate moved into leaves much faster than elemental sulfur and was supposedly trapped in vacuoles so that it did not contribute to increased yield. The better results with elemental sulfur were explained by the fact that it needs to be oxidized before significant quantities can be absorbed by leaves. As oxidation is slow, sulfate supply from foliar-applied elemental sulfur fits better to the metabolic demand of the leaves and avoids excess sulfate concentrations in the cytosol and their deposition in vacuoles.
The problem of severe sulfur deficiency still exists on a large scale as the widespread regular appearance of macroscopic symptoms reveal, even more than 20 years after addressing this nutrient disorder (147). The reason is most likely the wide variation of official sulfur fertilizer recommendations in Europe (Table 7.11), recommendations, which only partly acknowledge site-specific features and productional peculiarities.
On-farm experimentation employing precision agriculture tools would be an ideal approach for setting up site-specific sulfur response curves (366).
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