Sulfur Supplying Capacity of Some Missouri Soils

Confirmed S deficiencies on some Missouri soils necessitated a study to determine their capacities to supply S to plants. Surface (0–30 cm) and subsurface (30–60 cm) depths of two Alfisols (Bosket loamy fine sand, Dubbs fine loamy sand) and one Entisol (Malden fine sand) from southeastern Missouri, and surface depths (0–30 cm) of an Alfisol (Kennebec silty clay loam) and Mollisol (Mexico silt loam) from northern Missouri were used. The soil properties measured included pH, organic matter, cation exchange capacity (CEC), Bray & Kurtz P1 (BK1) and P2 (BK2), labile SO₄ (LAS), SO₄ buffer index (SBI), extractable SO₄ (500 mg P L⁻¹ in 2 M HOAc), and soil texture (sand, silt, and clay). Soils were amended with 0 and 22 mg S kg⁻¹, and corn (Zea mays L., hybrid DeKalb 1189TX) dry matter yield and S uptake were measured through five 35-d intensive cropping cycles. There was little S uptake from unamended Bosket (0–30 and 30–60 cm), Dubbs (0–30 and 30–60 cm), and Malden (0–30 and 30–60 cm) zero-S soils after the second cropping cycle. The Kennebec and Mexico soils exhibited a plant growth response during the second and fifth cropping cycles, respectively. The significant increase in yield response to S fertilization suggests the Dubbs (0–30 and 30–60 cm), Malden (0–30 and 30–60 cm), and Bosket (30–60 cm) to have inadequate capacities to supply S to plants. Of these three southeastern Missouri soils, the Bosket 30- to 60-cm depths yielded the highest plant S uptake. This suggests that soils of this region with similar surface textures may possess differences in total plant available S when subsoil available S is included in this total. The SO₄ buffering index (SBI) correlated negatively with BK1 and BK2 available P suggesting that high or excess P fertilization would decrease the capacity of these soils to adsorb SO₄. With the 22 mg S kg⁻¹ treatment, using total SO₄ (TSO₄ = extractable SO₄ + added SO₄) as the independent variable, S uptake was not dependent on this TSO₄ quantity during the first 35-d cropping period. For the 0- to 70-d cropping, S uptake was significantly dependent on the TSO₄. The relationship (r = 0.99) between the extractable SO₄ and SO₄ labile pool (LAS), and their relationships to plant S uptake during the 0 to 35 d (r = 0.98, r = 0.97) and cumulative 0 to 70 d (r = 0.95, r = 0.95) cropping periods indicates that SO₄ extracted with 500 mg P L⁻¹ in 2 M HOAc to be a good estimate of plant available SO₄-S. The relationships (r = 0.84 for 0–35 d and r = 0.90 for 0–70 d) between CEC and plants S uptake suggests the need to investigate the potential mechanism of CEC to SO₄ availability and other soil properties that may be correlated to CEC. Contributed from Missouri Agric. Exp. Stn. Series no. 10408, Univ. of Missouri. This work was conducted under Missouri Agric. Stn. Project 387 (Maximizing Plant Nutrient Utilization), with the financial support provided by the Allied Corp., Int. Minerals and Chemical Corp., and the Missouri Res. Assistance Act.