Thiosulfate (S2O32-) fertilizers are clear liquids that provide a source of sulfur (S) and can be used in a variety of situations. They also contain other nutrients including nitrogen (N) as ammonium (ATS), potassium (KTS), calcium (CaTS) or magnesium (MgTS).
Ammonium thiosulfate (ATS) is the most commonly used S-containing fluid fertilizer. Manufacturers produce it by reacting sulfur dioxide, elemental S and aqueous ammonia. Other common fluid thiosulfate fertilizers are similarly produced.
Thiosulfates are highly soluble in water and are compatible with many other fluid fertilizers. ATS is commonly mixed with urea ammonium nitrate (UAN) to produce a widely used fertilizer with the analysis 28-0-0-5 (5 percent S).
After application to soil, most of the thiosulfate quickly reacts to form tetrathionate, which subsequently converts to sulfate. Thiosulfate isn’t generally available for plant uptake until soil microbes convert it to sulfate. In warm soils, this process is largely complete within one to two weeks.
Thiosulfate is a chemical “reducing agent,” and it also produces acidity after oxidation of the S. Due to these properties, thiosulfate molecules create unique effects on soil chemistry and biology. For example, a band application of ATS has been shown to improve the solubility of some micronutrients. However, growers should follow local guidelines for maximum placement rates in seed rows.
Thiosulfate can slow the rate of urea hydrolysis, slow the conversion of urea to ammonium (NH4+), and reduce loss of ammonia (NH3) as a gas when ATS is mixed with UAN. This inhibiting effect likely comes from the formation and presence of the intermediate tetrathionate rather than the thiosulfate itself. Nitrification, the conversion of NH4+ to nitrate, also slows in the presence of ATS. Although the initial pH of thiosulfate fertilizers is near neutral, thiosulfate oxidizes to form sulfuric acid, and the NH4+ in ATS will form nitric acid, thus resulting in slight soil acidification in the application zone.
Growers can apply thiosulfates through surface and overhead irrigation systems, sprinklers, and drip irrigation. Many of them are blended into foliar sprays to provide a rapid source of plant nutrition (though this is not recommended with ATS).
Sulfur deficiencies afflict crops throughout the world. Thiosulfates make valuable S fertilizer materials because they’re easy to handle and apply, require minimal safety precautions and are compatible with many other common fertilizers. However, these fertilizers should not be mixed with highly acidic solutions since this will cause the thiosulfate molecule to decompose and subsequently release harmful sulfur dioxide gas.
Thiosulfate materials serve many industrial applications. In photographic processing, for example, they bind silver atoms present in film or paper. Sodium thiosulfate is used in water treatment systems to remove chlorine.
It is also used for gold extraction, since it forms a strong complex with this metal in a nontoxic process.
Source: Nutrient Source Specifics (No. 8), International Plant Nutrition Institute.