Although sulfur is described as a secondary plant nutrient, largely because it is not found to be deficient as often as nitrogen, phosphorus, and potassium, it is actually just as important as any of the major nutrients. In fact, many crops contain approximately equal amounts of sulfur and phosphorus.
However, sulfur is not always available in the soil and may need to be applied to reach required amounts. Many farmers underestimate the importance of sulfur. Sulfur is linked to the availability of elements to the extent that sulfur is an enabling element to primary nutrients. For instance, sulfur, as a constituent of nitrate reductase enzyme, is involved in the conversion of nitrate into organic nitrogen. Sulfur deficiency, consequently, interferes with nitrogen metabolism, which explains why sulfur deficiency can lead to nitrogen deficiency in many crops.
Sulfur transformations in the soil are very similar to those of nitrogen. Most sulfur in the soil is unavailable as part of soil organic matter. Sulfur is tied up by bacteria during the decomposition of crop residues rich in carbon. Sulfur availability is made possible through microbial activity (oxidation) by bacteria of many species. One well-studied species is Thiobacillus. The bacteria works fastest in warm temperatures above 23 degrees C, with plent of oxygen. It is thought that most soils contain Thiobacillus but soils that have had years of heavy chemical application will have diminished populations. It is also believed that the regular application of sulfur will promote the existence of these bacteria in the soil. Therefore, it stands to reason that smaller applications with higher frequency would be more effective that one large application.
Other conditions that can cause sulfur deficiency in the soil are cold temperatures and soils that are water-logged. In these conditions, available sulfur turns into the sulfide (unavailable) form until soil temperature rises or soil aeration improves. Once Sulfur can combine with oxygen, it re-forms available sulfate sulfur. Because of this, sulfur applications tend to be more effective when made in the spring, rather than in the fall.
Sulfur is necessary for formation of proteins, oils and amino acids. Sulfur is a component of the amino acids cysteine, cystine, and methionine. These amino acids are among the “building blocks” of protein. Therefore, a shortage of sulfur can retard protein synthesis. Cysteine and methionine both play critical roles in cell metabolism. In general, proteins play a role in chlorophyll formation as well as nodulation in legumes.
Plants store proteins to provide carbon, nitrogen, and sulfur resources for subsequent growth and development. The storage and mobilization cycles of amino acids that compose these proteins are critical to the life cycle of plants. Protein storage and mobilization serve many different functions in the plant. For example, stored protein provides building blocks for rapid growth upon seed and pollen germination. Similarly, protein reserves in vegetative cells provide the building blocks for seed and fruit set during reproductive growth and for rapid expansion of vegetative growth after periods of dormancy.
In agriculture, proteins stored in seeds and vegetative tissues account for much of the protein consumed directly as food by humans and livestock.
Therefore, sulfur is a critical component in improving the health and productivity of all crops.