Sulfur management for rapeseed

Canola or rapeseed (Brassica sp. L.) is a major oilseed, being grown on more than 31 million hectares worldwide. Rapeseed has a high concentration of S in its tissue and seed and a particularly high demand for S relative to its yield potential. Therefore, effective S management is an important part of rapeseed production. Sulfur deficiencies are becoming increasingly prevalent due to higher crop yields, decreasing aerial deposition of S and decreasing mineralization of S from soil organic matter. Seed yields of both open-pollinated and hybrid rapeseed are usually optimized with applications of 15–60 kg S ha−1 applied as sulfate, which can be applied effectively in the autumn or in the spring, and broadcast, banded or seed-placed, depending on the environment in which the crop is being grown. On coarse-textured soils in high-moisture areas, leaching of autumn-applied sulfate–S may reduce yield response as compared to spring-application. If deficiencies are observed during the growing season, application of sulfate–S as late as bolting to early flowering can be beneficial although yield will generally be lower than if the S had been available from the start of crop growth. Elemental S fertilizers need to oxidize to sulfate before they are available for crop uptake or leaching and will generally not supply sufficient available S to optimize rapeseed yield in the year of application, or possibly for several years, depending on the environmental conditions and the management practices used. Elemental S should be managed in a manner that increases particle dispersion and contact with soil microorganisms to hasten the oxidation process. Management of S fertilizer sources should consider both the short- and long-term impacts on crop yield, seed quality and economics of production. Soil testing to determine the requirements for S fertilizer is challenging due to the spatial and temporal variability in sulfate availability. Improved assessment of S availability needs to consider both the release of S from organic matter and the site-specific distribution of sulfate across the landscape.

► Sulfur deficiency is an increasing problem in many parts of the world. ► Sulfur deficiencies are particularly common in rapeseed due to the high S demand of the crop. ► Increasing yield potential of hybrid rapeseed may increase S demand ► Sulfate forms of S fertilizer are immediately available and good sources for use in the year of application ► Elemental S must oxidize to sulfate before it is plant-available and the conversion may be too slow under many conditions for adequate plant-availability in the year of application ► Soil testing is challenging for sulfur because of high spatial variability.