Yield, nutrient, and soil sulfur response to ammonium sulfate fertilization of soybean cultivars

With the reduction of sulfur levels in high-analysis nitrogen (N) and phosphorus (P) fertilizers and in atmospheric deposition, sulfur (S) fertilization may become more important, especially with intensive cropping systems. When high clay content is likely to limit root development into the subsoil, low extractable sulfate-sulfur (SO4-S) levels in the topsoil may suggest possible plant response to S fertilization. Even though ammonium sulfate [(NH4)2SO4] is widely used and readily available for plant uptake, field data are limited on the use of (NH4)2SO4 as an S source for soybeans [Glycine max (L.) Merr.]. A study was initiated to determine the effect of S fertilization as (NH4)2SO4 on: (i) the yield, seed weight, grain quality, and leaf and whole-plant nutrient concentrations of four soybean cultivars grown on soils with high clay content subsoils; and (ii) selected soil chemical characteristics. Sulfur rates were 0, 28, 56, and 84 kg/ha, and soybean cultivars were two Maturity Group IV beans, DeSoto and Douglas, and two Maturity Group V beans, Bay and Essex. The study was conducted on a Parsons silt loam soil (fine, mixed, thermic, Mollic Albaqualf) in 1986 and 1987, and on a Cherokee silt loam (fine, mixed, thermic, Typic Albaqualf) in 1987. Sulfur application did not significantly affect soybean yield or seed protein or oil concentrations. For whole plants, S concentration increased and N:S ratios decreased with increasing S fertilization. Similar trends were found in soybean leaves. Although N:S ratios of both whole plant and leaf tissue were lowered with S fertilization, the values generally were not below 20:1 which is above cited critical levels. Fertilization with (NH4)2SO4 increased the levels of extractable SO4-S in the soil, especially in the 15-30 cm depth. The first-year accumulation of soil SO4-S with increasing S fertilization appeared to be more at a site that was lower in organic matter.