It is also required for modulation and nitrogen fixation of legume. As the sulfate ion, it may be responsible for activating some enzymes. In soil, sulfur is present primarily in the organic form, which becomes available when organic matter decomposes. In this form, it is subject to leaching as well as microbial immobilization. Fertilizer impurities also contribute to the total supply of available sulfur in soils.
Atmospheric deposition supplies a considerable amount of plant-available sulfur.
For rural areas of Michigan, the amount will generally vary from 8 to 15 pounds per acre annually, depending on proximity to an industrial emission source. Precipitation within several miles of certain industrial sites may contain 10 to 20 times as much sulfur as precipitation in more rural areas. Sulfur dioxide in the atmosphere can also be absorbed through the leaves of plants. Once absorbed, it is rapidly converted to the sulfate ion. Because of the many light-colored sandy soils in Michigan, more intensive cropping systems and increased use of fertilizers low in sulfur, one might expect sulfur deficiency to be widespread.