Sulfur (S) is widely distributed throughout the world in many forms. Yet in some soils, insufficient S levels can’t meet crop needs. The good news: No such shortage applies to the many excellent S-containing fertilizer products that can address S deficiencies where they occur.
Sulfur is a relatively abundant element in Earth’s crust. Traditionally it’s been extracted as pure elemental S from volcanic deposits and salt domes. It’s now more commonly obtained as a co-product from processing fossil fuels. Coal, crude oil and natural gas typically contain between 0.1 and 4 percent S, which energy companies remove during refining or scrubbing of combustion gases. The agriculture industry uses a variety of common earth minerals as S sources for crops.
Elemental S has a fairly low melting temperature of 115 ºC (240 ºF), so it’s often transported and handled in a hot liquid state until transformed into final products. The majority of global S production is converted to sulfuric acid (H₂SO₄) for further processing. A major use of sulfuric acid is in production of phosphate fertilizers.
Elemental S is not water soluble, so soil bacteria (such as Thiobacillus) must oxidize it to sulfate (SO₄²⁻) before plant roots can take it up. The general reaction in soil is 2S + 3O₂ + 2H₂O→2H₂SO₄. Environmental factors such as soil temperature and moisture, as well as the physical properties of the S, govern the speed of this microbial process.
Plants use sulfate almost exclusively as their primary source of nutrition, which they convert to many essential constituents, such as proteins and enzymes. Various approaches have been used to enhance this conversion of elemental S to plant-available sulfate, one of which is manipulating fertilizer particle size. The speed of elemental S oxidation relates directly to the particle size: Large particles of S may require months or years of biological action before oxidizing significant amounts of sulfate; smaller particles’ greater surface area lets soil bacteria act quicker. Even so, while fine, dust-sized particles have the advantage of oxidizing quickly, they’re difficult to apply.
Another approach fertilizer manufacturers take to enhance the rate of S oxidation is to add a small amount of clay to the molten S prior to cooling and forming small pellets (“pastilles”). When added to soil, the clay swells with water and the pastille disintegrates into fine particles the soil bacteria can rapidly oxidize.
To further build oxidation efficiency, fertilizer companies take very thin layers of elemental S and incorporate them during fertilizer granule manufacturing. This S quickly oxidizes in soil and becomes available for plant uptake. This reaction can also create a positive impact on the plant availability of some micronutrients such as zinc (Zn) and iron (Fe) that become more soluble as the pH declines. Finely ground elemental S is also sometimes added to fertilizer suspensions. Farmers widely apply elemental S as a fungicide for crop protection where toxic hydrogen sulfide is evolved from the interaction of elemental S and the living fungal tissue.
Common environmental uses for elemental S and sulfuric acid include the reclamation of soils that contain excessive sodium and the treatment of some irrigation water.
Sulfur pastilles, containing small amounts of clay to enhance dispersion and oxidation
Sulfur comes in many forms to meet specific cropping requirements. Growers generally apply elemental S well in advance of crop demand because of the lag period between bacterial oxidation and conversion to sulfate. And since sulfate is an anion, it may be subject to leaching loss, similar to nitrate. Nevertheless, no adverse environmental impacts are associated with typical concentrations of sulfate in water.
Sulfur is widely used in many consumer products and industrial applications. It’s commonly converted to sulfate prior to use in textiles, rubber, detergents and paper, for example.
Source: Nutrient Source Specifics (No. 13), International Plant Nutrition Institute.