Sulfur?

Sulfur is one of the essential elements for crop growth and reproduction. Its most important role is in the production of protein, as part of two amino acids: cysteine and methionine. Further, sulfur is associated with the assimilation of nitrogen into protein, so sulfur deficient plants have low protein contents despite an adequate supply of nitrogen. Sulfur deficiency symptoms in plants often appear similar to nitrogen deficiency, with stunting and yellowing of the plant, although nitrogen deficiency shows most strongly in the new growth while sulfur deficiency will usually affect the whole plant.

The crops with the highest requirement for sulfur are those with the highest protein contents, and particularly proteins containing high levels of cysteine and methionine. Among commonly grown crops in Ontario, canola has by far the highest requirement, followed by hard red wheat, and alfalfa. The question is whether these crops require extra sulfur, above what is already available from the environment, for maximum yield and quality.

The largest source of sulfur to crops is the breakdown of organic matter. A secondary, but very significant source, is from atmospheric deposition. Sulfate is released by the burning of fossil fuel, either by industry or automobiles, and this is returned to the earth as acid rain, or by dry deposition as ammonium sulfate. Sulfur deficiency is common in the prairies, where dry conditions slow the breakdown of organic matter and there is little heavy industry. It is much rarer in Ontario, unless you are west of Lake Superior.

There is some concern that sulfur deficiencies will develop in Ontario because of the reduction in acid rain. Several trials have been carried out with canola over the past 3 years, which should show deficiencies sooner than any other field crop. There have been some plots that showed a yield response to sulfur, but the average response over all the trials was zero. For the moment, it is unlikely that adding sulfur will improve your bottom line.

If you do want to apply some sulfur, the most economical form is ammonium sulfate. This material is available in different grades, and the price varies accordingly. The raw product from the steel mills is a flaky powder, which is hard to handle but is competitively priced with urea. A granular form of ammonium sulfate has superior handling characteristics, but can cost twice as much or more per pound of nitrogen. In either form, it has the advantage over urea that it will not volatilize if left on the surface of the soil.

In any trial with ammonium sulfate, it is difficult to sort out whether yield effects are from the sulfur, or from more available nitrogen for the crop. If both urea and ammonium sulfate are surface applied, volatilization losses from the urea could result in a nitrogen deficiency in the urea treatment. Another common error is to ignore the nitrogen in the ammonium sulfate, and apply it over and above the regular nitrogen program. Is the yield effect from the 24 pounds of sulfur, or the extra 21 pounds of nitrogen? Even when the nitrogen effects are considered, careful measurement will be required to pick up any yield responses. They will be small and inconsistent, if previous trials are correct!