Plants absorb sulfur in the SO4= (sulfate ion) form. Sulfur is a part of several amino acids and is, therefore, a component of most proteins. Plants can continue to absorb sulfate in excess of plant requirements, as long as it continues to be available in the soil. However, plants cannot translocate extra sulfate to younger tissue if the soil levels become too low.

In my opinion, sulfur is one of the more neglected macronutrients in alfalfa production. In our area sulfur is needed in alfalfa in amounts second only to canola, and if you haven’t included sulfur in your fertilizer program for several years, you may benefit by adding it to your next application program. As crop production has greatly increased over the past few decades, it appears that the application of sulfur, as with some of the micronutrients, has not kept up. There is a great deal of difference between sulfur needs and an increase in production from added sulfur. Consider for a moment that sulfur deficiency most often shows up on sandy soils (leaching of sulfur), and low organic matter soils (reduced mineralization). Consider also that much sulfur, as well as nitrogen and phosphorus, is added to the soil through organic matter decomposition, and thus liberates these minerals for the crop. This decomposition process is driven by soil organisms, which in turn are partly controlled by the moisture and temperature of the soil.

Conclusions? An answer might be that the rate of mineralization (liberation of S) could be much slower than normal due to our unseasonably moist, cool conditions. Even though in past years your alfalfa crop may not have shown what may be sulfur deficiency, you may benefit from a little added sulfur. Many of our soils in the Red River Valley are high to very high in organic matter. High organic matter and good moisture and temperature can translate to higher mineralization of sulfur for the crop. However, higher rates of mineralization may add only adequate sulfur and not enough to give you greater production. I would also like to note that soil tests for sulfur are questionable at best, partly due to their failure to estimate mineralization of S throughout the season.