Sulfur (S) is one of the 17 elements essential for plant growth, and the fourth most important after nitrogen (N), phosphorus (P) and potassium (K) in terms of amount required by crops. When considering nutrient sources, elemental S (ES) is the most concentrated form of S, making it appealing due to lower transportation costs (per lb of nutrient) when compared to sulfate-based fertilizers. Elemental S is not taken up by crops in the applied form, and must be converted in the soil to sulfate S before uptake by plants or microorganisms. This article describes how elemental S is oxidized in soil, and the factors that affect the rate of oxidation. Results from a model predicting rates of S oxidation in granulated fertilizers containing elemental S are provided to illustrate the key factors affecting oxidation in soil.
Key Factors Affecting Elemental S Oxidation
Oxidation of elemental S in soil is a microbial process requiring the presence of both water and oxygen (Fig. 1). The converted sulfate can be taken up by crops, taken up by microorganisms in the soil, or leached below the root zone in coarse-textured soils or high-rainfall areas. There is a wide variety of microorganisms in soil that are capable of oxidizing elemental S, including both bacteria and fungi; oxidation is not solely dependent on the presence of specific S-oxidizing organisms. The primary factors affecting elemental S oxidation are as follows:
Soil and Environmental Factors Affecting Elemental S Oxidation
Oxidation is more rapid in warm, moist soils with high organic matter (OM) contents. Oxidation reactions of elemental S are also faster in alkaline soils than in acidic soils (Fig. 2). Of the soil and environmental factors affecting oxidation rate, temperature and soil pH have the greatest effect.
Fertilizer Attributes Affecting Elemental S Oxidation
Oxidation of elemental S is also affected by the characteristics of the fertilizer, including the particle size of elemental S and/or its concentration in the fertilizer.
Oxidation is a surface-based process, and surface area increases dramatically as particle size decreases; therefore, particle size is one of the most important attributes affecting oxidation. When elemental S is dispersed throughout the soil, the oxidation of elemental S is faster as the particle size of the elemental S decreases (Fig. 3). In co-granulated elemental S fertilizers (i.e., in which elemental S particles are co-granulated with macronutrients [N, P, K]), the oxidation is reduced compared to the elemental S particles of the same size dispersed through soil. This is not because the macronutrients reduce the oxidation rate, but because of the reduction in surface area of elemental S available for oxidation when dispersed in soil. Therefore, the lower oxidation rate of co-granulated elemental S can be explained by a reduction in the surface area of S in contact with the soil.
Additionally, the ES concentration within a fertilizer granule affects the surface area and oxidation rate. Fertilizer granules with a high concentration of ES (Fig. 4A at 90%) have much smaller surface area than a co-granulated fertilizer with a low concentration of ES (Fig. 4B at 5%). An Excel-based model has been produced by the Fertilizer Technology Research Centre, University of Adelaide, that integrates all soil, environmental and fertilizer-granule factors that affect the oxidation of elemental S in soils, thus allowing predictions of oxidation rates in various locations with defined fertilizer types. An example of the model’s output is shown in Fig. 5. The oxidation of the S pastilles is predicted to be much slower than for the S in the MicroEssentials® granules. This is due to the much higher concentration of the S pastilles, resulting in less surface area exposed to the soil.
In conclusion, elemental S must be oxidized into the plant-available form of SO42- for uptake and is affected by soil, environmental and fertilizer factors. Elemental S oxidation is a microbial process that is largely driven by temperature and soil pH. Warmer climates and/or higher soil pH increase oxidation rate. Additionally, the fertilizer source and characteristics have a significant effect on oxidation rate. The individual particle size of ES distributed throughout the granule and the total ES concentration of the fertilizer granule are major contributors. Oxidation rate decreases as particle size increases and ES concentration within the fertilizer granule increases.