Sulfur (S) is an essential secondary macronutrient for plant health, ranking as the fourth most required nutrient after nitrogen, phosphorus, and potassium in many cropping systems. It is a fundamental building block for numerous organic compounds, playing a pervasive role in the biological chemistry of plant life from germination to maturity. Although classified as “secondary,” a lack of sulfur can limit growth and yield as severely as a deficiency in the other major nutrients. Plants primarily absorb sulfur from the soil as sulfate ions (\(\text{SO}_4^{2-}\)), making its availability a direct factor in plant nutrition.
Sulfur’s Essential Role in Plant Growth
Once absorbed, sulfur’s primary function is its incorporation into the amino acids cysteine and methionine, which are foundational components for nearly all plant proteins. Cysteine is the first stable organic sulfur compound created during assimilation and acts as the precursor for other sulfur-containing molecules. Methionine is essential for protein synthesis and metabolism.
Sulfur is necessary for the synthesis and activation of various enzymes that regulate plant metabolism and energy production. It is an integral part of coenzymes like Coenzyme A and vitamins such as biotin and thiamine, which are required for metabolic pathways. Though not structurally part of the chlorophyll molecule, sulfur is required for its formation, impacting the plant’s ability to photosynthesize and generate energy.
In oilseed crops like canola and soybeans, sulfur directly influences the oil content and quality of the seed. Furthermore, it plays a specific role in the synthesis of specialized compounds that contribute to the characteristic flavors and odors of crops such as garlic, onions, and cabbage. Adequate sulfur nutrition enhances the efficiency of nitrogen utilization, as the two nutrients are metabolically linked, affecting overall plant productivity and the protein content of the harvested product.
Modifying Soil Acidity
Elemental sulfur (\(\text{S}^0\)) is used in soil management as an agent for lowering soil pH, distinct from its role as a plant nutrient. This acidification process is biologically mediated, requiring the activity of specialized bacteria like Thiobacillus. These microbes oxidize the elemental sulfur, converting it into sulfuric acid (\(\text{H}_2\text{SO}_4\)).
The sulfuric acid reacts with alkaline components in the soil, such as calcium carbonate, effectively lowering the pH of the soil solution. This transformation depends on environmental factors; oxidation occurs more rapidly in warm, moist soils with high microbial activity. The rate of oxidation is slowed by large particle size and is generally faster in alkaline soils, which provide a greater buffering capacity.
Lowering the pH in highly alkaline soils is beneficial because it increases the solubility and plant availability of micronutrients like iron, manganese, and zinc, which are often unavailable at high pH levels. For example, reducing the soil pH by one full point may require applying hundreds of pounds of elemental sulfur per acre. This technique is a long-term soil amendment, as the microbial oxidation process is slow and continuous.
Forms of Sulfur and Soil Availability
Sulfur exists in the soil in several forms, but only one is directly available for plant uptake. The vast majority, about 90 to 95 percent of the total sulfur, is contained within organic matter. This organic form must undergo mineralization, a microbial process of decomposition, to be converted into the plant-available inorganic form.
The form that plants absorb is the sulfate anion (\(\text{SO}_4^{2-}\)), which is highly water-soluble. Because it carries a negative charge, sulfate does not bind readily to the negatively charged soil particles and is therefore highly mobile, making it susceptible to leaching below the root zone, especially in sandy soils or areas with high rainfall. The reverse process, immobilization, occurs when microorganisms consume the sulfate, incorporating it back into their organic biomass, temporarily making it unavailable to plants.
Elemental sulfur (\(\text{S}^0\)) is another important form, but it is not plant-available upon application. It must first be oxidized into sulfate by soil microbes, acting as a slow-release source of nutrition. The balance between these forms—organic, elemental, and sulfate—is dynamically managed through the sulfur cycle, shifting based on microbial activity, temperature, moisture, and organic matter present.
Recognizing and Addressing Sulfur Imbalances
Sulfur deficiency is often first observable in the plant’s newest growth, manifesting as a general yellowing, or chlorosis, of the younger leaves. This occurs because sulfur is relatively immobile within the plant, meaning the plant cannot easily relocate the nutrient from older leaves to support the growth of new tissue. This symptom contrasts with nitrogen deficiency, which typically causes yellowing in the older, lower leaves first.
Deficient plants may also exhibit stunted growth, thin stems, and delayed maturity, with specific crops like canola showing unique symptoms such as purpling or cupped leaves. For oilseed and legume crops, a lack of sulfur can significantly reduce both oil and protein synthesis. Addressing an imbalance involves applying sulfur in a suitable form, which depends on the desired speed of availability.
For immediate correction of a deficiency, water-soluble sulfate sources like ammonium sulfate or gypsum (calcium sulfate) are effective because the sulfur is immediately available for plant uptake. For a long-term, season-long supply, elemental sulfur is often incorporated into fertilizer blends, providing a sustained release as it slowly oxidizes in the soil. However, the mobility of the sulfate form means that a continuous supply is necessary throughout the growing season to prevent mid-to-late-season deficiencies.