Elemental sulfur performance depends on source, soil type, and application timing

Declining atmospheric sulfur (S) deposition together with increasing crop yield potential has increased S removal, contributing to greater risk of S deficiency across U.S. cropping systems. Elemental S is a concentrated S source that can supply S over a growing season. Crops take up S primarily as sulfate; therefore, elemental S requires microbial S oxidation before plant uptake can occur. Because S oxidation is driven by soil microorganisms and environmental conditions, timing of plant-available S depends on source, soil type, and soil temperature following application. 

Study overview

In Oregon, S oxidation was evaluated in Quincy fine loamy sand (mixed, mesic Xeric Torripsamment) and Walla Walla silt loam (coarse-silty, mixed, superactive, mesic Typic Haploxeroll) using laboratory incubation temperature regimes designed to simulate fall/winter and spring soil temperature patterns based on long-term (30-year) climate averages for Hermiston (Quincy soil) and Pendleton (Walla Walla soil) over a five-month lab incubation.

Fertilizer sources included micronized elemental S (reference material), monoammonium phosphate combined with micronized elemental S, monoammonium phosphate combined with micronized elemental S and ammonium sulfate, bentonite-bound prilled elemental S, and monoammonium phosphate as a non-S control (Figure 1 and Table 1). All elemental S sources were applied at an equivalent target rate of 100 lb S/ac (50 mg S/kg S equivalent) to allow direct comparison of S oxidation among fertilizer sources.  

Selected soil properties appear in Table 2 to provide context for interpretation of soil effects. 

Elemental S source and S oxidation potential

Across soils and temperature regimes, bentonite-bound prilled elemental S showed limited S oxidation during the evaluation period across soils and temperature regimes (Figure 2). Aggregation within prills reduced exposed elemental surface area until physical breakdown occurred. Prilled elemental S functioned more as a longer-term S source than a contributor to immediate sulfate availability. Earlier application timing or pairing with sulfate-S may help support early crop uptake where needed.

Monoammonium phosphate combined with micronized elemental S also showed limited S oxidation compared with non-granulated micronized elemental S used as a reference material (Figure 2). Granulation of micronized elemental S with MAP likely reduced exposed elemental surface area, resulting in slower S oxidation. Monoammonium phosphate + micronized elemental S contributed to S supply over time rather than rapid sulfate availability. Earlier application timing or inclusion of sulfate-S may help with early plant S nutrition needs.

Addition of ammonium sulfate supplied immediate sulfate-S while elemental S continued undergoing S oxidation over time (Figure 2). Co-granulated materials containing sulfate-S provided both early sulfate availability and continued S supply over a growing season.

Soil pH and texture effects on S oxidation

Elemental S oxidized more rapidly in Quincy fine loamy sand and more slowly in Walla Walla silt loam across evaluated fertilizer sources (Figure 2). Differences between soils indicate that soil type influenced elemental S oxidation processes. Quincy soil represented an alkaline, coarse-textured soil while Walla Walla soil represented a slightly acidic, finer-textured soil, suggesting that both soil acidity/alkalinity and soil physical conditions may contribute to differences in S oxidation. Acidic soil conditions corresponded with slower S oxidation although soil texture and associated aeration may also influence biological processes involved in S oxidation. Because soil pH and texture differed simultaneously between evaluated soils, independent effects of acidity/alkalinity and texture cannot be separated within this study. Additional research comparing soils with similar pH but differing texture, and soils with similar texture but differing pH, would help clarify which factor contributes more strongly to S oxidation responses.

Elemental S applied under simulated spring conditions began at cooler soil temperatures (approximately mid-30s °F on February 12) but experienced steadily increasing temperatures through simulated March, April, and May conditions, reaching approximately 70 °F (Figure 2). Increasing soil temperatures into biologically active ranges corresponded with continued S oxidation over time and resulted in earlier sulfate availability compared with the fall regime. Soil temperatures increasing from near-freezing conditions in late winter to approximately 70 °F by early summer corresponded with faster S oxidation, even though initial temperatures were cooler at application.

Although fall application began under warmer soil temperatures than spring application, prolonged exposure to near-freezing conditions corresponded with reduced S oxidation later in the evaluation period. Temperature patterns following application influenced timing of S oxidation more strongly than starting temperature alone.

Summary

Elemental S fertilizers differed in S oxidation depending on fertilizer source, soil conditions, and temperature patterns following application. Faster S oxidation occurred where soils were alkaline and well aerated and where temperatures increased after application. Matching fertilizer source and application timing with soil conditions may help improve predictability of S availability.