Sulfur is part of every living cell and required for synthesis of certain amino acids (cysteine and methionine) and proteins. Sulfur is also important in photosynthesis and crop winter hardiness. In addition, sulfur is important in the nitrate-reductase process, during which nitrate-nitrogen is converted to amino acids.
Sulfur in Soil
Sulfur is supplied to plants from the soil by organic matter and minerals, but it is often present in insufficient quantities and at inopportune times for the needs of many high- yielding crops. Most S in the soil is tied up in the organic matter and cannot be used by plants until it is converted to the sulfate (SO4-2) form by soil bacteria. That process is known as mineralization.
Sulfate is mobile in the soil, just as nitrate-
nitrogen is mobile, and in some soils, it can be leached beyond the active root zone with heavy rainfall or irrigation. Sulfate may move back upward toward the soil surface as water evaporates, except in the sandier, coarse- textured soils that may be void of capillary pores. This mobility of sulfate-sulfur makes it difficult to calibrate soil tests and to use them as predictive tools for sulfur fertilization needs.
Sulfur tends to be held by clay soil particles
more than nitrate-nitrogen. When early spring rains occur, soils with a sandy topsoil, but containing relatively high amounts of clay in the subsoil, may have sulfate-sulfur leached out of the topsoil but retained in the subsoil. Therefore, crops grown on these types of soils may show early S deficiency, but as the roots penetrate into the subsoil, the deficiency may disappear. On deep sandy soils with little or no clay in the subsoil, plants will likely respond to sulfur applications.
In the field, sulfur deficiency and nitrogen deficiency are often easily confused. Symptoms of both deficiencies may appear as stunted plants, with a general yellowing of leaves. Sulfur is immobile within the plant and does not readily move from old to new growth. With sulfur deficiency, yellowing symptoms often first appear in younger leaves, whereas with nitrogen deficiency, the yellowing appears on the older leaves first. In less severe situations, visual symptoms may not be noticeable.
The best way to diagnose a deficiency is with a plant tissue analysis that includes an assay for both sulfur and nitrogen. Sulfur concentrations in most plants should range from about 0.2 to 0.5 percent. Desirable total nitrogen to total sulfur ratios have been considered, and range from 7:1 to 15:1. Wider ratios may point to possible sulfur deficiency, but should be considered along with actual N and S concentrations in making diagnostic interpretations.
When sulfur is deficient, nitrate-nitrogen may accumulate. This can pose significant health threats to grazing ruminants or those consuming hay high in nitrates. When nitrates accumulate in the plant, seed formation can be inhibited in some crops such as canola. Balancing sulfur with nitrogen nutrition is important to both plant and animal health.
Crops such as hybrid bermudagrass, alfalfa and corn that have a high dry- matter production generally require the greatest amount of sulfur. Also, potatoes and many other vegetables require large amounts of S, and have produced best when S is included in the fertility program. Without adequate S fertilization, crops that receive high rates of nitrogen may develop sulfur deficiencies.