Factors affecting the oxidation of elemental sulfur in soils

The use of elemental sulfur (S⁰) to alleviate widespread S deficiencies in agricultural soils is limited by the unpredictability of its oxidation to plant available sulfate. Here we review the biological, fertilizer and soil-related factors that control S⁰ oxidation. Sulfur oxidation in soil is mediated primarily by microorganisms, and thus it is the size, composition and activity of the microbial community which dictate oxidation rates. Because S⁰ oxidation is a biological process, it is strongly influenced by factors directly affecting microbial activity including soil temperature, water potential, and aeration. In many soils these factors represent the primary constraints to S⁰ oxidation. Oxidation is also influenced by the effective surface area of the S exposed to microbial activity. Thus oxidation is favored by reducing the particle size and abundant populations of heterotrophic bacteria and fungi capable of oxidizing S⁰, thus the availability of organic substrates from residue additions or root exudates may also affect S oxidation. Previous application of S⁰ may increase oxidation rates in many soils, presumably by stimulating S⁰ oxidizing populations. The large number of factors that govern S⁰ oxidation account for the variability in oxidation rates among soils, climatic regions, and agronomic practices. Many of these factors are subject to agronomic control, however, and it should be possible to devise S fertilizer strategies that exploit the slow release characteristics of S⁰ to meet crop demands efficiently in a variety of conditions.