Sulfur is an essential element in the life processes of all living things, including microorganisms, higher plants, animals and man. It is an important part of the proteins needed to sustain life in all biological organisms.
Sources of Sulfur in Soils
The sulfur found in Nebraska soils that is used for plant growth is derived from several sources. Below are some of these sources and their reactions in the soil.
Organic matter. About 95 percent of the total sulfur content of most soils is contained in the organic matter. As this soil organic matter is broken down or decomposed, the organic sulfur is mineralized into the sulfate form (SO4=-S). Sulfate-sulfur can then be taken up by the roots of growing plants. Breakdown of all the sulfur in organic matter does not occur in a single year, but rather is a continuous process requiring a considerable amount of time. The return of crop residues to the soil adds to the pool of sulfur. Nebraska soils containing over one percent organic matter would rarely have any sulfur deficiency problems.
Soil minerals. Several minerals in the soil contain sulfur in different forms. As these minerals are weathered or broken down, part of the sulfur is transformed to sulfate-sulfur.
The atmosphere. Most fuels that are burned for heat, power and transportation contain some sulfur. When these fuels are burned, the sulfur escapes as sulfur dioxide gas (SO2). Sulfur dioxide dissolves in rainwater and reaches the soil as sulfate-sulfur. Sulfur dioxide in the atmosphere is highest in industrialized areas. The air of Nebraska (excluding the Omaha area) contains only small amounts of sulfur dioxide. The amount of sulfur dioxide in the atmosphere is being reduced by pollution control regulations.
Pesticides. Some pesticides contain sulfur; however, their contribution to the sulfur in the soil is relatively small.
Fertilizers. In the past, commercial fertilizers supplied considerable sulfur in addition to the nitrogen, phosphorus, and potassium. For example: 0-20-0, which contains 12 percent sulfur, has been replaced by 0-46-0, which contains about 1 percent sulfur. As the fertilizer products become more concentrated and refined, the analysis of the active ingredient increases and sulfur content becomes very low.
Irrigation water. This is an important source of sulfur in some areas of Nebraska. Sulfur is present in the irrigation water as sulfate-sulfur. Irrigation water in most of Nebraska contains enough sulfur to meet requirements for this nutrient in crop production. Irrigation water from the Sandhills and bordering areas, however, contains little or, in some instances, no sulfur. According to NebGuide G174, Fertilizer Suggestions for Corn, irrigation water with six ppm SO4-S or greater will reduce UNL sulfur recommendations by half, depending on the soil test and soil texture.
Sulfur Reactions in Soil
The form of sulfur taken up by plants is sulfate-sulfur and must be constantly replenished. Sulfur from all sources must either be in the sulfate form or be converted to the sulfate form before it can be used by plants. As mentioned above, organic matter is the major reservoir of sulfur in most soils. Except for sulfur fertilizers and irrigation water, the other sources listed contribute minor amounts.
Sulfur can be removed from soils in several ways. Uptake by plants is a major pathway for removal of soil sulfur. Leaching is the second major pathway. Sulfate-sulfur is similar to nitrate-nitrogen, in that both are negatively charged and not held tightly by clay particles (which are also negatively charged). Therefore, sulfur in the soil water can move downward through the soil.
Sulfate-sulfur can also be converted to gaseous sulfur compounds under waterlogged conditions. This is rare in Nebraska and of minor concern.
Sulfur Uptake by Plants and the Need for Sulfur Fertilizers
Removal of sulfur from the soil varies with the type of crop grown. A 150 bu/acre corn crop will remove about 14 lb. sulfur per acre. Alfalfa will remove approximately 6 lb. for each ton produced. Wheat yielding 40 bu/acre will remove about 12 lb. of sulfur/acre.
As with other nutrients, there is also concern about adequate plant levels of the nutrient. Adequate levels of sulfur have been tentatively established for only corn and alfalfa. For alfalfa, whole plants that have a sulfur content of less than 0.20 percent are generally considered to be deficient in this nutrient. Adequate levels of sulfur have not been established for whole corn plants. The adequate levels have been suggested for the leaf opposite and below the ear at time of silking. The suggested adequate concentration for this leaf is set at the range of 0.18 to 0.20 percent. It’s important to point out that the time of sampling of these corn leaves is critical. The leaves should be collected when silks are emerging. Leaves collected at other growth stages cannot be used to evaluate adequate levels of sulfur. The sulfur concentration in plant tissue changes with maturity and adequate levels have not been established for other growth stages of corn.
In most non-sandy Nebraska soils, the surface soil contains adequate sulfur for most crops. Also, these same soils contain substantial amounts of sulfur in the subsoils. The contribution of sulfur from both topsoil and subsoil for these soils eliminates the need for sulfur in a fertilizer program.
Shortages or deficiencies of sulfur are generally restricted to sandy soils. Research, however, has shown that applying sulfur will not increase crop production on all sandy soils. These research studies show that the organic matter content of the soil also must be considered.
Research in the Great Plains has shown that applying sulfur fertilizers has not increased crop yield on non-sandy soils for corn and alfalfa. These soils usually have a higher organic matter content than sandy soils. In addition, the non-sandy soils usually have higher levels of sulfate-sulfur. The combination of these two factors provides adequate amounts of sulfur for crop production.
The amount of sulfate-sulfur in the soil can be measured in the laboratory. This test is appropriate for sandy soils only. Fine-textured soils may test low in sulfate-sulfur; however, applying sulfur may not result in increased crop yields. Also, the organic matter content of the soil and the sulfur content of irrigation water must be known. This information provides a useful guideline for determining sulfur fertilizer needs.
Several fertilizers will supply sulfur for crop production. The phosphate fertilizer, 0-20-0, contains 12 percent sulfur; and 16-20-0 contains about 15 percent sulfur. Ammonium sulfate, usually sold as a nitrogen source, contains 24 percent sulfur. Zinc sulfate, a commonly used zinc source, contains 14 percent sulfur. Ammonium thiosulfate, a liquid material containing both nitrogen and sulfur, contains 26 percent sulfur. Sulfur can be purchased as powdered or granular gypsum that contains 17 percent sulfate-sulfur. Potassium-magnesium sulfate (sold as K-Mag or Sul-Po-Mag) contains 22-23 percent sulfur. Except for ammonium thiosulfate, all of the products mentioned are dry materials.
Sulfur can be used in the elemental form. Finely ground material must be used to be effective. This finely ground powder is commonly combined with clay or organic binding agents and formed into flakes or granules to improve handling. Prilled elemental sulfur formed without a binding agent is not an effective source of sulfur. When fertilizers containing elemental sulfur are added to soils, the elemental sulfur must be changed to sulfate-sulfur. Microorganisms in the soil carry out this process. This change occurs rather rapidly when soil temperatures are warm, but is slowed by cool soil temperatures in early spring. This process is very slow if the elemental sulfur is not finely ground.
Attention to the use of micronutrients in corn production runs in cycles. When a micronutrient is deficient, large crop responses to the applied micronutrients have been obtained. This has led to perhaps an over-stimulated interest in micronutrients. While some applications of micronutrients have increased crop yield greatly, many of the applications have not been economical. Micronutrients will increase yield when a deficiency exists. Any yield response has to pay for the cost of the micronutrient and its application.
Plants require micronutrients in very small amounts — ounces per acre. Excessive amounts of these nutrients, particularly copper, boron, and molybdenum, can be toxic to plants. Micronutrients should be applied only when the soil cannot provide sufficient amounts for good plant growth.
Micronutrients are found in soil minerals similarly to how phosphorus and potassium are found. As soil minerals weather, nutrients are released for plant use. However, most of the readily available micronutrients are made available to plants through the mineralization of organic matter.
Of the seven micronutrients, only three have been identified as being deficient in Nebraska soils. Zinc and iron deficiencies occur most commonly, but not in all Nebraska soils. Boron deficiency has been identified in the past in sugar beets grown in very sandy soils in north central Nebraska and in alfalfa grown on sandy soils with limited soil moisture. Boron deficiency is rare in Nebraska and has never been identified as a problem in crops of the grass family. Deficiencies of copper, manganese, molybdenum, and chlorine have not been documented in Nebraska soils.
Manganese availability increases as soil pH decreases; thus, manganese deficiency is not likely in the near future in Nebraska soils. Copper deficiency is most likely in organic soils or weathered sandy soils. Nebraska soils are young mineral soils and copper deficiency is not likely.
Molybdenum is needed in small amounts (one ounce per acre may be sufficient for several years). The lack of molybdenum is not a threat to crop production if a deficiency is ever identified in Nebraska. Chlorine deficiency has not been identified in Nebraska; however, Kansas and South Dakota have shown increased wheat yields with chloride fertilizers. Research in Nebraska is needed to study the influence of chloride on wheat. Response to chloride was associated with disease suppression.