Oxidation dynamics and leaching behavior of elemental sulfur fertilizers in tropical soils with contrasting textures

Sulfate in soil is readily leached in humid tropical environments and sandy soils with low organic matter content. Elemental sulfur (ES), an alternative source of sulfur (S), requires microbial oxidation to sulfate (S‐SO 4 2− ) to become available to plants. Due to its low solubility, ES oxidizes slowly, enabling a gradual S release. Two experiments evaluated the S oxidation dynamics in tropical soils using ES‐based fertilizers. Five fertilizers were tested: ammonium sulfate (AS), UREA‐ES14 (micronized elemental sulfur urea‐based fertilizer 14) (micronized, 14.3% S), UREA‐ES75 (micronized elemental sulfur urea‐based fertilizer 75)‐granulated (75% S), UREA‐ES75‐ground (75% S), and elemental sulfur pastille (ES pastille, 90% S), along with a control. The first assessed S oxidation rates over 180 days of incubation (13 samplings); the second analyzed S‐SO 4 2− distribution in soil after simulated rainfall using polyvinyl chloride columns. In Experiment I, oxidation rates ranged from 22% to 32% for UREA‐ES14, UREA‐ES75‐granulated, and UREA‐ES75‐ground. The ES pastille showed the lowest oxidation rate, ranging from 3% to 9%. In Experiment II, ES‐based fertilizers retained higher S‐SO 4 2− concentration in the upper soil layers, indicating lower leaching than AS. After simulated rainfall of 110 mm, 50%–55% of S‐SO 4 2− from AS remained in the layer 0.00–0.05 m, compared to 58%–83% for ES‐based fertilizers. The results suggest that the rate of ES oxidation is positively influenced by longer incubation periods and offers residual soil effects. However, the oxidation rate to S‐SO 4 2− is limited and concentrated in soil surface layers, affecting long‐term bioavailability to plants. ES‐based fertilizers exhibited lower leaching potential, supporting ES uses as a slow release of S in tropical soils.