Effect of Cogranulation on Oxidation of Elemental Sulfur: Theoretical Model and Experimental Validation

Abstract

Most studies on elemental sulfur (Es) oxidation have focused on small Es particles mixed through soil, even though commercial Es fertilizers are usually in granular form. Although it has been recognized that cogranulation of Es decreases its oxidation rate, no attempt has been made to quantify this effect. We developed a conceptual model that predicts the "effective diameter" (the diameter of Es particles mixed through soil that would oxidize at the same rate as the granulated Es) by taking into account the effect of granulation on the effective surface area available to the Es in the granule cavity after the soluble macronutrient compound in the fertilizer has diffused away. To validate the model, the oxidation rate was determined for Es-fortified monoammonium phosphate fertilizer with varying Es content (20-250 g kg −1 ), Es particle diameter (25 or 60 mm), and granule diameter (1.8 or 3.4 mm). The time to reach 50% oxidation varied from 17 d for small granules at the lowest Es content to 210 d for the large granules with the highest Es content. In agreement with the model predictions, reducing Es particle size did not affect the oxidation rate except at the lowest Es rate, whereas reducing granule size increased the oxidation rate. Predicted and observed oxidation rates were in good agreement, indicating that the model describes the effect of granulation with reasonable accuracy. This model may assist in improving formulation of Es-containing fertilizers and guiding fertilizer recommendations.Abbreviations: ES, elemental sulfur; MAP, monoammonium phosphate; TSP, triple superphosphate. E lemental sulfur (ES) is a common S fertilizer source. Elemental S is insoluble in water and hence is not susceptible to leaching and also has the benefit of lower transport/handling costs because it is the most concentrated form of S. However, ES only becomes available to plants when it is oxidized to sulfate (Boswell and Friesen, 1993). Knowledge of the factors that affect ES oxidation is hence required to develop fertilizer recommendations for ES-containing fertilizers.The oxidation of ES particles is affected by biological, fertilizer, and soil-related factors (Germida and Janzen, 1993). Because ES oxidation is a biologically mediated process, it is strongly influenced by temperature. Janzen and Bettany (1987a) assessed ES oxidation in six soils in Saskatchewan, Canada, at temperatures ranging between 3 and 30°C and found Q 10 values (factor increase per 10°C increase) ranging from 3.2 to 4.3. Oxidation of ES is a surficial process, and the rate of oxidation is therefore proportional to the specific surface area of the ES particles. Soil properties also affect the oxidation rate, likely through their effect on the composition and abundance of the microbial community. For instance, Zhao et al. (2015) found that the surface-based oxidation rate of ES in 10 Australian soils ranged between 5 and 52 mg cm −2 d −1 and was positively correlated with the pH and organic matter content of the soil.

Sulfur Uptake from Fertilizer Fortified with Sulfate and Elemental S in Three Contrasting Climatic Zones

“…Co-granulated ES oxidizes more slowly than ES particles of the same size mixed through soil, due to a reduction in the effective surface area. The "effective diameter" of MES fertilizers is around 200 µm, i.e., the ES in MES fertilizers oxidizes at the same rate as ES particles with a diameter of 200 µm mixed throughout the soil [42]. The average temperature in the first cropping season was 17 • C for the Canadian site and 26 • C for the Brazilian site (Figure 2).…”

Long-term fate of fertilizer sulfate- and elemental S in co-granulated fertilizers

“…where d gran is the diameter of the fertilizer granule and f ES the fraction of ES in the fertilizer (Degryse et al 2016b).…”

“…Best-fit parameter values were obtained by least-square regression of measured versus predicted percentage of plant S derived from fertilizer (%Sdff). For the site in Canada, also a lag-time for the oxidation was fitted (Degryse et al 2016b), because the negligible contribution of fertilizer ES to the uptake at the early growth stage in the first year could not be explained without a lag-period and because no convergence was obtained without including a lag-time. It is noteworthy that Chapman (1989) observed an increase in the lag period of ES oxidation with decreasing temperature.…”

“…Commercial products usually consist of ES cogranulated with macronutrient fertilizers or ES:bentonite pastilles, and are much slower to oxidize (Chien et al 2011). Degryse et al (2016b) showed that the oxidation rate of ES cogranulated with monoammonium phosphate (MAP) was inversely proportional to the ES content and to the granule size, which could be mathematically explained based on the surface area of ES particles in contact with soil.…”

Application method influences the oxidation rate of biologically and chemically produced elemental sulfur fertilizers

“…The oxidation of ES in soil is a microbially mediated process influenced by the fertilizer's form and application method and also by abiotic parameters such as climate and soil type (Chapman, 1996; Germida & Janzen, 1993; McCaskill & Blair, 1989). Elemental S particles are often bound to bentonite or other macronutrients to form granular fertilizers because of the explosion risk of fine ES particles dispersed in the air (Boswell, Owers, Swanney, & Rothbaum, 1988; Degryse, da Silva, Baird, & McLaughlin, 2016c). Different formulation and granulation methods are used in fertilizer manufacturing that can affect the disintegration of the granules and the release of S to plants.…”