Soil Profile Sulfate in Southern High Plains Center Pivot Fields.

Sulfate (SO₄) is one of the most important anions in soils and in groundwater in semiarid regions, including West Texas.  Crops’ S requirement is about 10 to 20 % of the N requirements.  However, there is far less information on soil test SO₄ in the western US, compared to the humid eastern US.  It is well known that deposition of S from the atmosphere has declined nationwide, as coal-fired power plants have been scrubbing their emissions of S.  The objectives of this study were to assess residual soil profile SO₄ in irrigated cotton fields across a range of soil types and counties of the Texas Southern High Plains (SHP).  The second objective was to compile regional SO₄ concentration data from the Texas Water Development Board irrigation well water quality data base and analyze trends in Ogallala groundwater SO₄ levels by county, soil map unit, and by well depth.  Soil profile (0- 0.9m) S ranged from 120 to 475 kg SO₄-S ha^-1 among seven center-pivot fields sampled.  About half of this S was in the 0.6 – 0.9 m soil layer, which was typically sandy clay loam or clay loam in texture. Well water SO₄ ranged from 9 to 220 mg SO4-S L^-1 in clayey soils with deep groundwater to sandy soils with shallow groundwater, respectively.  There were no consistent relationships between soil map unit and soil profile SO₄.  In conclusion, soil profile SO₄, sampled in 529 points among seven center pivot fields was surprisingly high.  The amounts of SO₄-S in the soil, coupled with the amounts added in irrigation suggest that S fertilization of crops like cotton is not necessary in most cases in the SHP.