Correlation and Calibration of Soil-Test Sulfur Concentrations from Different Soil Depths with Soybean Yield

Sulfur (S) fertilization in soybean (Glycine max (L.) Merr.) production was investigated across 50 research sites in northeastern Louisiana during the 2023 and 2024 growing seasons. The objective of the study was to assess soybean yield response to six rates of S fertilizer (0, 11, 22, 34, 45, and 67 kg S ha^-1), particularly in the context of declining atmospheric sulfur dioxide (SO₂) deposition following the implementation of the U.S. Clean Air Act (US-EPA, 2024). This work represents one of the most comprehensive datasets ever compiled in the United States for developing soil-test-based S fertilizer recommendations for soybean production.

Of the 50 trials, 21 were conducted at the Macon Ridge Research Station (MRRS) in Franklin Parish, which features Gigger-Gilbert silt loam soil, and 29 were conducted at the Northeast Research Station (NERS) in Tensas Parish, characterized by Commerce silt loam soils (NRCS, 2024). Trials were laid out in a randomized complete block design with 4-5 replications per treatment. In 2023, fertilizer-S treatments included Sul4r-Plus (23% Ca, 17% S) and K-Mag (22% K₂O, 11% Mg, 21% S), while in 2024, Sul4r-Plus and gypsum (23% Ca, 17% S) were used. Each plot consisted of four rows, 10.67 m in length, with row spacing of 1.02 m at MRRS and 0.97 m at NERS.

Soil samples were collected before planting from untreated control plots at two depths: 0–15 cm (10–12 cores) and 0–30 cm (8–10 cores) using a 2.2 cm diameter AMS soil probe from the top of the middle two seedbeds. Samples were air-dried for five days at 45°C, ground to pass through a 2 mm sieve, and analyzed by Waters Agricultural Laboratories in Vicksburg, Mississippi. Nutrient analysis was performed using Mehlich-3 extractant (Helmke & Sparks, 1996), while soil pH and organic matter content were determined following Sikora and Kissel (2014) and Schulte and Hopkins (1996), respectively. Cation exchange capacity (CEC) was estimated using methods outlined by Maguire and Heckendorn (2015).

Fertilizer treatments were applied on the seedbed surface at or before planting. Soybeans were seeded between mid- to late May at a rate of 321,000 seeds ha^-1, following corn (Zea mays L.), cotton (Gossypium hirsutum L.), or soybean. All sites were conventionally tilled and furrow irrigated. Lime and additional fertilizers were applied based on LSU AgCenter soil-test-based recommendations (Parvej, 2021, 2024), with lime incorporated only at sites with soil pH below 6.0. Standard irrigation, pest, and weed management practices were followed in accordance with LSU AgCenter Extension guidelines (Padgett et al., 2024; Stephenson et al., 2024; Villegas & Towles, 2023). Soybean was harvested at physiological maturity (R8 stage, as per Fehr & Caviness, 1977), and yield was calculated based on a uniform grain moisture content of 13% (130 g H₂O kg^-1).

This 50-site-year dataset offers a unique and robust foundation for establishing science-based, soil-test-calibrated S fertilizer recommendations for soybean production. It serves as a valuable resource for producers, agronomists, and consultants aiming to fine-tune S fertilization strategies, enhance productivity, and reduce unnecessary input costs. This dataset is part of the article submitted to the Soil Science Society of America Journal (Moni et al., 2025).