Soil profile sulfate in irrigated southern high plains cotton fields and Ogallala groundwater

Abstract

Sulfate (SO₄) is one of the most important anions in soils and groundwater in semiarid regions, including west Texas. Crops' sulfur (S) requirement is about 10% to 20% of the nitrogen requirements. However, there is far less information on soil test SO₄-S (S in the SO₄ form [sulfate-S]) for the western United States, compared to the humid eastern United States. The deposition of S from the atmosphere has declined nationwide, as coal-fired power plants have been scrubbing their emissions of S. This, combined with greater cotton yields and the lack of S fertilization on the Texas Southern High Plains (SHP) point to the need for a soil profile SO₄-S assessment. The first objective of this study was to assess residual soil profile SO₄-S content in irrigated cotton (Gossypium hirsutum) fields across a range of soil types and counties of the Texas SHP. The second objective was to compile regional SO₄-S concentration data from the Texas Water Development Board irrigation well water quality database and analyze Ogallala groundwater SO₄-S levels by county, soil map unit, and well depth. Soil profile (0 to 0.9 m [0 to 36 in]) SO₄-S content ranged from 120 to 475 kg ha⁻¹ (107 to 424 lb ac⁻¹) among seven center-pivot fields sampled. About half of this S was in the 0.6 to 0.9 m (24 to 36 in) soil layer, which typically was sandy clay loam or clay loam in texture. Well water SO₄-S concentration ranged from 9 to 220 mg L⁻¹ (ppm) in clayey soils with deep groundwater to sandy soils with shallow groundwater, respectively. Groundwater SO₄-S concentration was greater than the USEPA drinking water standard of 83 mg L⁻¹ in 32% of the wells, especially in the southeastern corner of the study area. There was no consistent relationship between soil map unit and SO₄-S content in the soil profile (0 to 0.9 m). A significant negative relationship between well depth and well water SO₄-S concentration was observed in all the eleven-county study area of the SHP of West Texas. In conclusion, soil profile SO₄ content to 0.9 m is high in SHP irrigated cotton fields and in most cases should be adequate for crop S nutritional requirements.