Changing How We Manage Sulfur – Shane VanFleet

While 2015 will rightly be remembered as the year when supplemental nitrogen applications paid huge dividends in much of the Corn Belt, post-planting applications of sulfur often returned strong results for growers as well. The questions many are asking is why sulfur application is more important now than in previous years and how can we solve the issue.

Sulfur deficiencies continue to arise for a number of reasons:

  1. As power plant emissions have decreased, the free sulfur (S) previously deposited via rainfall has declined dramatically. Many areas of the Midwest receive 10-20 lb. per acre less S from this source.
  2. Higher yields remove more S from our soils, requiring higher levels of replacement.
  3. Fewer sulfur-containing pesticides are used in today’s agricultural industry.
  4. The movement to reduce tillage reduces the mineralization of S into our soil mainly due to cooler temperatures.

The main source of sulfur for crops (other than inorganic fertilizers) comes via the mineralization of soil organic matter. However, the soil releases a limited amount of S via mineralization – roughly 2-5 lb. of sulfur are mineralized for every percent of organic matter. Therefore, if you have a 3% organic matter
soil and a good mineralization year, you might expect to mineralize roughly 10-15 lb. of S. However, a 200 bushel corn crop requires approximately 26 lb. of S. So, if we get 15 lb. via mineralization, where does the rest of the sulfur supply come from?  In the past, it generally came from rainfall deposits but since that source has nearly been eliminated we now must supplement the crop.

One consideration for fertilizing a crop is that S gets to the plant in the sulfate form (SO4-) mostly via mass flow of soil water. Since the SO4- ion is negatively charged like your soil and moves with soil water, it can move quickly through your soil profile – much like the nitrate molecule. As a result, we continue to see more deficiencies appearing. The other consideration is that the corn plant uses a little over 50% of S after tassel, so we need to make sure it is available later in the year when the corn plant needs it to help convert nitrates in the plant during the critical grain fill stage.