Sulfur use is becoming more common as deficiencies increase in many areas.
The most common forms of sulfur used in fertilizer are elemental sulfur and SO4. Thiosulfate forms of sulfur are also commonly available in many regions.
Sulfur plays two important roles in agriculture: as an essential nutrient required for proteins and enzymes and as a soil amendment for improving alkaline soils.
Many crops require 10 to 25 pounds of sulfur each year. While this is not as much as other nutrients, the frequency of crop sulfur deficiency has been steadily increasing because many fertilizers do not routinely contain sulfur.
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As well, deposits of airborne sulfur, often associated with acid rain, have decreased with the decrease in air pollution in many industrial areas.
Although sulfur exists in many different chemical forms in nature, plants primarily absorb it in the SO4 form. The SO4 molecule carries a negative charge, so it moves freely with soil moisture. As a result, SO4 concentrations are sometimes greater with increasing depth in the soil below the root zone.
Several excellent sources of plant-available SO4 provide immediate crop nutrition: potassium-magnesium sulfate, ammonium sulfate and potassium sulfate.
Elemental sulfur is unavailable to plants because it cannot be directly taken up by roots. However, when elemental sulfur is added to soil, it gradually becomes oxidized to the plant-available SO4 form.
Many factors control the transformation of elemental sulfur to SO4. Because this conversion is done by soil microbes, several environmental and physical conditions govern how quickly the change takes place.
In general, sulfur oxidation takes place most rapidly in warm and moist soil. However, field application should take place some time before the plants have a need for SO4.
The physical properties of elemental sulfur are also important. Small sized particles have the most surface area and the most rapid reaction. However, fine particles of sulfur can be difficult to apply.
Fertilizer manufacturers have developed useful techniques in which fine sulfur particles are clumped together with expandable clay to form a pellet that disintegrates in the soil.
Elemental sulfur is highly acidifying after it is oxidized in the soil. It’s commonly used to treat high pH soil or to amend calcareous soils loaded with harmful concentrations of sodium. These specific sulfur application rates should be calculated with the aid of a crop adviser.
Thiosulfate has also become a popular source of sulfur nutrition for crops. Thiosulfate generally converts to SO4 within a few weeks in typical summer growing conditions. Thiosulfate has been shown to have beneficial effects on nitrogen transformations and may offer unique benefits for plant metabolism.
There is no reason to risk yield loss from sulfur deficiencies. When the need for sulfur is suspected, many excellent materials are available to meet crop needs.
