What happened to all that sulfur that naturally came from the atmosphere?

Sulfur as a crop nutrient

One of the things about farming in this century is that farmers can no longer rely on the soil providing all the sulfur a crop needs. Sulfur is probably the most overlooked nutrient into today’s crop production systems. It is a plant nutrient.

Since the Industrial Revolution, atmospheric sulfur deposition is been an important source of sulfur into agriculture. The key sulfur compounds in the atmosphere are sulfur dioxide and aerosol sulfate. Both of these wash out of the atmosphere into the soil. Deposition of sulfur oxides play an important role in circulating sulfur in the soil and keeping it available to plants and microorganisms.  However, with increased environmental regulations that have been imposed in North America and across the world, this has led to reduced atmospheric sulfur deposition which has created sulfur deficiency scenarios. With the introduction of the Clean Air Act back in the 1970s there is much less sulfur deposition than a few decades ago.

Additionally, there are a number of factors that are also contributing to an increase in sulfur deficiencies globally:

  • More sulfur is removed from the soil as a result of an increase in agricultural production by increasing fertilizer use and production intensity.
  • Less sulfur is being added to the soil due to the increasing proportions of high-analysis fertilizers that are now sulfur free (urea, diammonium phosphate, and potassium chloride).
  • Decreasing use of animal manures, composts and sulfur-containing fertilizers, such as single superphosphate and ammonium sulfate.

Across the Midwestern U.S., sulfur depositions vary from less than 5 pounds per acre in the west to 10 to 12 pounds per acre in the east. And the average soil, having only 2 to and 3 percent organic matter, will contribute just another 5 pounds of sulfur. Yet crops routinely require 20 to 40 pounds of sulfur per acre today. This means that it’s highly likely that your sulfur levels will be below what’s required.

The sulfur (S) in calcium sulfate dihydrate, a.k.a. gypsum (Formula: CaSO4 + 2H2O; the ‘dihydrate’ merely meaning there are 2 water molecules attached, making it more soluble) will supply immediately available sulfur when applied and watered in. To put this into a practice, 100 lbs of calcium sulfate (gypsum) applied per acre will provide about 18 pounds of sulfur per acre.

Dr. Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at djdavidson@goodearthminerals.net or call 402-649-5919.