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.