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Use of Soil-profile Sulfate Data for Predicting Crop Response to Sulfur¹

ABSTRACT

Sulfur deficiencies in early growth stages do not always result in final yield reduction in many crops. This suggests a greater contribution of subsoil sulfate (SO₄) and/or of S mineralization later in the season to the S requirements of plants. The subsoil SO₄ contribution should be assessed in order to develop a method that can reliably predict crop response to S. A greenhouse experiment was conducted with various combinations of topsoil (0-20 cm layer) and subsoil (20-60 cm layer) materials from a Hartsells series (Typic Hapludult) where crop response to S had been observed, and a Benndale series (Typic Paleudult) where there was no response to S in field experiments. By determining soil SO₄ depletion in each layer by crops of grain sorghum (Sorghum bicolor L. Moench.), it was extimated that plants derived approximately 0.55 of their S need from the surface 0 to 20 cm, and the remaining 0.45 from the subsoil. Subsequently, a weighted concentration of soil SO₄ for the entire profile was calculated; and values of 13.8 mg S kg^–1 for extractable SO₄ and 0.25 mM for soil-solution SO₄ were the minimum concentrations above which no crop response to S is expected. The use of weighted means of soil-profile SO₄ data correctly predicted yield responses to S of soybeans (Glycine max L. Merr.), corn (Zea Mays L.), and sorghum in a field experiment, which were previously unexplained based on SO₄ concentrations of the surface soil.

¹ Joint contribution from Dep. of Agronomy and Soils, Alabama Agric. Exp. Stn., Auburn Univ., AL 36849; and Dep. of Agronomy and Soil Science, College of Tropical Agriculture and Human Resources, Univ. of Hawaii, Honolulu, HI 96822. Journal Series no. 3088, Hawaii Institute of Tropical Agriculture and Human Resources.

² Assistant Professor, Univ. of Hawaii; and Professor Emeritus, Auburn Univ., respectively.

Received for publication August 11, 1986.