Critical values for sulphur (S) in the southern region may be lower than originally thought. Established critical values are 4.5 mg/kg for wheat and 7 mg/kg for canola. These values are based primarily on WA data that use the KCl-40 S test.
Do these values apply in other regions? A project in the southern region found that despite low S (KCl-40 <3 mg/kg), there was limited yield response to applied S. This suggests that current critical values are too high.
Recent pot trial research in the southern region compared different soil S test methods and developed critical values for early biomass wheat and canola. Researchers used soils from the western, southern, and northern regions. Critical values using the KCl-40 soil test were 2.6 mg/kg for wheat and 2 mg/kg for canola (R2 = 0.46 wheat, R2 = 0.57 canola). The MCP-S soil test was more accurate, with critical values of 2.2 mg/kg for wheat and 3.6 mg/kg for canola (R2 = 0.73 wheat, 0.87 canola). These values are lower than reported in BFDC. BFDC values use mainly KCl-40 data from 0-10 cm which often underestimates the soil available S pool.
Critical values have not yet been tested in the field.
Sulphur is a mobile nutrient and may be leached after rainfall. It can accumulate deeper in the soil, such as in clay subsoil. Collect deeper soil samples to gauge soil S stocks. Match soil test results with growth responses, plant tissue concentrations and uptake. Ideally, try both KCl-40 and MCP S soil tests to see which one works best on your soil type.
Sulphur deficiency can look like nitrogen (N) deficiency. Sulphur isn’t very mobile within the plant so look for yellowing in the newer growth. Nitrogen deficiency shows in the older leaves. In canola, the edges of younger leaves turn purple.
Before investing in a S fertilizer program, measure soil S levels and consider using test strips to monitor S status. Gypsum and some fertilisers supply S. 100 kg/ha of MAP/DAP will supply about 1.5kg S/ha. Sulphate of ammonia (21% S) can correct both N and S deficiency.
Mineralisation of organic S during spring can also be an important source of S supply to the crop.