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Soil Nitrates Following Four Years Continuous Corn and as Surveyed in Irrigated Farm Fields of Central and Eastern Colorado¹

ABSTRACT

High rates of fertilizer N used in many intensive farm management systems have been cited as a potential pollution hazard to surface and ground waters. The purpose of this study was to evaluate soil NO₃^– accumulations following 4 years continuous corn (Zea mays L.) grown with different nitrogen and irrigation regimes, and to compare these results to present NO₃^– concentrations found in irrigated farm fields of central and eastern Colorado.

Soil NO₃^– content in the 300-cm sampled profile was significantly influenced by both fertilizer N and irrigation treatments; the greater accumulations were associated with the two higher fertilizer N rates and two lower irrigation rates. Nitrate increased linearly in relation to fertilizer N between 67 and 269 kg N/ha and could be described by two simple regression equations separating the irrigation treatments into two groups (l-2 and l-3, low rates; l-4 and l-5, high rates). Coefficients of determination for the two groups were 0.981 and 0.975, respectively.

Two hundred and seventy irrigated farm fields were sampled to a depth of 90 cm in central and eastern Colorado. Mean soil NO₃^–-N content was 173 kg/ha and contents were not significantly different between coarse and fine textured groupings. Fields sampled immediately following beans (Phaseolus vulgaris L.) contained similar amounts as those following corn. Mean soil NO₃^– content decreased with greater soil depth.

Observed NO₃^– concentrations appear to be the result of gradual accumulations from annual or near-annual fertilizer N and/or manure applied over a period of time. Data from the 4-year corn experiment clearly illustrate that NO₃^– can accumulate in a soil profile even when fertilizer N is applied at rates below that required for maximum production.

Key Words: nitrate pollution • N fertilizer • irrigation management

¹ Supported in part by the Phillips Petroleum Co., Bartlesville, Okla., Great Western Sugar Co., Denver, Colo., and the Colorado State Univ. Exp. Sta. and published as Scientific Series Paper No. 2046.

² Associate Professor of Agronomy (Soils), Professor of Agronomy (Soils), Colorado State University, Fort Collins, CO 80523; Assistant Professor of Agronomy, SE Colorado Research Center, Walsh, CO 81090, respectively.

Received for publication April 21, 1975.

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