Sulfur in soils

Sulfur (S) deficiency of crops, which has been reported with increasing frequency over the past two decades on a worldwide scale, is a factor that reduces yield and affects the quality of harvested products. Especially in Western European countries, incidence of S deficiency has increasingly been reported in Brassicaceae. For this reason, more attention should be paid to the optimization of S‐fertilizer application, in order to cover plant S requirements whilst minimizing environmental impacts. In soils, S exists in inorganic and organic forms. While sulfate (SO$ _4^{2-} $), which is a direct S source for plants, contributes up to 5% of total soil S, generally more than 95% of soil S are organically bound. Organic S is divided into sulfate ester and carbon‐bonded S. Although not directly plant‐available, organically bound S may potentially contribute to the S supply of plants, especially in deficiency situations. Sulfur turnover involves both biochemical and biological mineralization. Biochemical mineralization, which is the release of SO$ _4^{2-} $ from the ester sulfate pool through enzymatic hydrolysis, is controlled by S supply, while the biological mineralization is driven by the microbial need for organic C to provide energy.