Semester of Graduation

Spring 2025

Degree

Master of Science (MS)

Department

School of Plant, Environmental, and Soil Sciences

Document Type

Thesis

Abstract

Sulfur (S) deficiency has become a significant concern for producers due to over 90% reduction in atmospheric-S deposition since the “Clean Air Act” was established. Soil organic matter, another key source of S, is steadily declining due to continuous tillage. So, we evaluated soybean [Glycine max (L.) Merr.] yield and leaf-S concentration responses to six fertilizer-S rates (0, 11, 22, 34, 45, and 67 kg S ha⁻¹) across 50 site-years, and six fertilizer-S sources (no-S check, Ammonium Sulfate (AMS), Sul4r-Plus/Gypsum, K-Mag, Tiger90CR, and Poly4) across 9 site-years in Louisiana from 2023-2024. In S rate study, soybean yield responded positively to fertilizer-S at 11 out of 50 sites. Mehlich-3 soil-test S (STS) concentration at 0-15 cm depth correlated better with relative soybean yield, explaining 46% of the yield variability, compared to 41% at 0-30 cm depth. Critical STS concentrations were 9-11 mg kg⁻¹ at 0-15 cm and 7-9 mg kg⁻¹ at 0-30 cm depths, with 86% and 100% accuracy, respectively, in identifying S-deficient sites. Critical STS at 0-30 cm depth resulted in more errors (8%) in predicting S-sufficient sites than 0-15 cm depth (6%). Leaflet-S concentration at full-flowering (R2-3) stage explained 38% of the relative yield variation, with a critical concentration of 2.65-3.00 g kg⁻¹. Sites having concentration below this range also has STS concentrations either below or within the critical level at 0-15 cm depth. In S source study, positive yield responses were measured at 2 out of 9 sites. At both responsive sites, Tiger90CR consistently increased yields compared to no-S check when applied before planting only. Gypsum and K-Mag also consistently increased yields when applied at full-flowering (R2-3) stage, with the potential for similar results if applied at or before planting. Overall, positive yield responses were observed in both fertilizer-S rate and source studies when STS and leaflet-S concentrations were below or near the critical ranges. Based on our data, a 22 kg S ha-1 is recommended when STS concentrations are below or near the critical limit at the 0-15 cm depth. Limited site responses made it difficult to recommend a specific fertilizer-S source, and further research is warranted.

Date

2-7-2025

Committee Chair

Parvej, Md Rasel

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