Sulfur Cycling in Soils

by | Jul 23, 2018 | Ag

Yield benefits for many crops from sulfur application are well known and fairly well understood. How sulfur cycles in the soil is generally understood, but not discussed very frequently. Questions surrounding sulfate’s leaching potential and how it is incorporated and released from organic matter are still common.

Sulfur is susceptible to leaching

Like nitrate, sulfate is a negatively charged ion (anion). Since soils typically carry a net negative charge, sulfate does not adhere to soil particles like positively charged ions (cations) like calcium, magnesium, and potassium. Due to this negative charge, both nitrate and sulfate are susceptible to leaching. But to what degree?

Most soil scientists think that sulfate has slightly less leaching potential than nitrate. This is mostly due to larger molecular mass and nearly constant cycling by bacteria in soils with organic matter at or above 3%. However, leaching potential is still high, which should drive decisions regarding the best sulfur fertilizer choice and application timing. It is estimated that only 10-15% of sulfur present in soils is in the sulfate form, while the remainder is in organic form.

Sulfur can become immobilized

Also like nitrate, sulfur exists in various chemical forms in the soil and can become immobilized in soil organic matter. When the right conditions exist, sulfur will be chemically incorporated into organic matter (immobilization), and when opposite conditions exist, sulfur will be chemically released from organic matter and available for crop uptake (mineralization). What conditions favor each scenario? And how do things like sulfur fertilizers and crop residue affect the status of sulfur in the soil?

Drivers of sulfur cycling

Calcium Products has recently developed and released a set of variable rate pH maintenance equations that are based on the most recent soil test data. The rates are on a sliding scale with a max application rate of 360 lbs/A. The higher rates are applied to the most acidic areas, with the theory that they will be the most prone to acidification during the remainder of the soil testing cycle. These rates are also flexible with different target pH values and will adjust automatically as you change that value. 

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