Fall 2007, No. 3
AN OVERLOOKED NUTRIENT…ARE YOU KEEPING TRACK OF SULFUR?
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 between 10 to 25 lb of S each year. While this is not as much as some other nutrients, the frequency of crop S deficiency has been steadily increasing since many fertilizers do not routinely contain S and deposition of air-borne S has decreased.
Although S 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 rootzone. There are several excellent sources of plant-available SO4 that will provide immediate crop nutrition. These include materials such as potassium-magnesium sulfate, ammonium sulfate, or potassium sulfate.
Elemental S is totally unavailable for plant uptake since it can not be directly taken up by roots. However, when elemental S is added to soil, it gradually becomes converted (oxidized) to the plant-available SO4 form.
The transformation of elemental S to SO4 is controlled by many factors. Since this conversion is done by soil microbes, several environmental and physical conditions govern how quickly this change takes place. In general, S oxidation takes place most rapidly in warm and moist soils. But field application should take place some time before the plants have a need for SO4.
The physical properties of elemental S are also important. Small-sized particles have the most surface area and the most rapid reaction. However, fine particles of S can be difficult to apply. Fertilizer manufacturers have developed useful techniques where very fine S particles are clumped together with expandable clay to form a pellet which disintegrates in the soil.
Elemental S is highly acidifying after it is oxidized in the soil. It is commonly used to treat high-pH soils or to amend calcareous soils loaded with harmful concentrations of sodium. The specific S application rates should be calculated with the aid of a crop adviser.
Thiosulfate has also become a popular source of S nutrition for crops. Thiosulfate generally converts to SO4 within a few weeks in typical summer growing conditions. Thiosulfate has also been shown to have beneficial effects on N transformations and may offer some unique benefits for plant metabolism.
There is no reason to risk yield loss from S deficiencies. When the need for S is suspected, there are many excellent materials that are available to meet crop needs.
—RLM—
For more information, contact Dr. Robert L. Mikkelsen, Western North America Director, IPNI, 4125 Sattui Court, Merced, CA 95348. Phone: (209) 725-0382. E-mail: rmikkelsen@ipni.net.
Abbreviations in this article: S = sulfur; SO4 = sulfate.