Sulphur (S) is a major constituent of seed protein and as a result, mustard plants have a higher S requirement during crop flowering, but will utilize S from the soil throughout the growing season until seed filling is complete. Plants deficient in S will show yellowing of the newest leaves, stunted growth and smaller petals on pale yellow flowers. Unlike N, S is not mobile in the plant. As a result, deficiency symptoms will be seen in new leaves, flowers and pods. When deficiencies are marginal, visual symptoms may not be present, but yield losses can still be severe. Missing pods, small pods and missing and/or small seed will be the symptoms.
Mustard needs a constant supply of S throughout the growing season with an increased requirement at flowering, as S is a major constituent of seed protein. For mustard production, the general S recommendation is 15 lbs/acre of actual S as ammonium sulphate in the Brown and Dark Brown soil zones and 20 lbs/acre of actual S in the Grey and Black soil zones
Mustard is very sensitive to the salt effect when ammonium sulphate is placed with the seed. Ammonium sulphate can be broadcast or side or mid-row banded. If S was missed during seeding, ammonium sulphate can be broadcast anytime up to first flower to rescue the yield of mustard. However, the earlier the ammonium sulphate is broadcast the better the chance of yield recovery, providing there is some rain to dissolve and move the ammonium sulphate into the soil for access by the roots.
Recommendations are based on the application of ammonium sulphate, not elemental S. If elemental S is applied to a field, it can take two or more years of adequate moisture for some conversion to the plant-available sulphate form.
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