It is important to understand where sulfur that is utilized for crops comes from in order to determine where to best target fertilizer application, writes Daniel Kaiser, University of Minnesota Extension Soil Fertility Specialist. In Minnesota, sulfur was not recommended for many crops grown on medium and fine textured soils. Numerous studies were conducted during the 1970’s, 80’s, and 90’s with little to no positive benefits shown except for a limited number of studies where corn was grown on eroded soils. Over the past 10-15 years reports increased as to sulfur deficiencies and research has found that sulfur may be needed for crops most sensitive to sulfur deficiency.
Soil tests for sulfate-sulfur only account for a small fraction of the total amount of sulfur in the soil. Soil organic matter is a large storehouse of sulfur with as much as 95% of the total sulfur contained in organic matter. Sulfur in organic matter must be mineralized to sulfate before it can be taken up and assimilated by plants. It is within this process of mineralization which can explain, in many instances, why responses to sulfur fertilizer have increased.
The total amount organic matter in a soil has a significant impact on whether sulfur fertilizer is required. Soils with low soil organic matter have a smaller pool of sulfur that can be mineralized. A general rule of thumb is that somewhere between 3-5 lbs of S, as sulfate, can be mineralized per acre per year for each percent organic matter in the top six inches of the soil. Soils with 3.0% or less organic matter have demonstrated a more consistent need for some sulfur to be applied for corn or alfalfa. Targeting fields that fit within this range is justified as yield can be greatly increased with as little as 10-15 lbs of S broadcast on an annual basis.