Residual soil nitrate in irrigated Southern High Plains cotton fields and Ogallala groundwater nitrate

Nitrate (NO₃) is the most important contaminant of the Ogallala aquifer in West Texas, especially in counties south of Lubbock. Twenty percent of the wells in the 11-county study area of West Texas had >10 mg L⁻¹ (ppm) NO₃-N in the groundwater. The source of this NO₃ is not clear. Leaching of residual soil NO₃ from the 1.2 million ha (3 million ac) cotton land is a possibility, but it may take several decades for percolating water to reach the depth of the Ogallala groundwater. In this paper, soil profile NO₃ data from eight cotton fields from five counties across the Southern High Plains of West Texas are presented. Soils were sampled to 90 cm (36 in) in four depth increments. Nitrate-N levels in the surface 0 to 15 cm (0 to 6 in) soil were low, averaging 15 kg N ha⁻¹ (13 lb ac⁻¹). However, extractable NO₃-N content averaged 58 and 96 kg NO₃-N ha⁻¹ (52 and 86 lb ac⁻¹) in the 0 to 60 (0 to 24 in) and 0 to 90 cm (0 to 36 in) soil profiles, respectively. Soil profile NO₃-N was greatest in calcareous soils, soils with petrocalcic horizons and in Acuff loam soils. Soil profile NO₃-N content or profile distribution was not consistently related to soil type. We also compiled and analyzed Texas Water Development Board irrigation well NO₃-N concentrations by county and soil map unit. Well water NO₃-N was greatest in Lynn county (12.8 mg L⁻¹) and lowest in Hale county (2.7 mg L⁻¹). Groundwater NO₃-N and soil map unit data were not related in a consistent pattern in terms of soil texture. Well depth and well water NO₃-N concentration were negatively correlated (r = -0.56). Irrigated cotton producers should test their well water for NO₃, as this can be a significant source of N. Good N management requires N fertilizer applied at rates appropriate for yield goals for specific irrigation systems and water availability, resulting in minimal residual soil NO₃ at harvest. Deep soil sampling, i.e. 0 to 60 or 90 cm (0 to 24 or 36 in) can help maximize profits and protect NO₃ contamination of the Ogallala aquifer.