Sulphur (S)

Low levels of available sulphate-S may occur in any non-saline soil in Manitoba. Sulphur (S) deficiencies are most frequently found on well-drained and grey wooded soils. Soil testing is the best available tool for determining S fertilizer needs. Testing should be done to a 24” depth to account for sulphate not at the surface, but still available for crop use.

Sulphate concentrations within a field can vary, depending upon soil type and slope position. On rolling land, sample hilltops, mid-slope positions and low-lying areas separately. Sandy, coarse textured soils should be sampled separately from heavier soils. This is important since it is not uncommon for low lying, heavy soils to contain many times more sulphate-S than light-textured hilltops. Sampling a variable field as a whole would typically result in a recommendation that no S fertilizer is needed, yet crops in some areas may be highly S-deficient. For this reason an “insurance application” of S fertilizer may be advisable on variable soils or where high value, high S-demanding crops, such as canola, are to be grown.

Available sulphate levels are often low following the breaking of a perennial legume or grass-legume stand, due to their high S removal rates (Table 1).

Sulphate forms of S fertilizer, primarily ammonium sulphate and liquid ammonium thiosulphate, are equally effective when applied as a surface application, banded or incorporated. Elemental S must be oxidized by soil micro-organisms to form sulphate before plants can use it³⁶. Elemental S should be applied at least one year before it is needed by the crop and left on the surface as long as possible before incorporation, as rainfall and weathering help disperse the fertilizer granule and speed the conversion to the sulphate form.