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Soil Nitrogen Balance in selected Row-Crop Sitesin Southern California¹

ABSTRACT

Nine row-crop sites, where data for nitrogen fertilizer use, crop yields, and amounts of irrigation water used were available for a number of years, were studied to estimate the N balance as related to NO₃^– in water in the unsaturated zone from below the zone of root influence to the water table or to the 15-m depth.

The system of predicting NO₃^– concentrations in drainage waters based on the difference between N inputs and N removal in harvested crop sand the drainage volume in which the excess N, converted to NO₃^–, is dissolved, was valid in open-porous soils containing no layers that restrict water movement within the soil profile (0- to 2-m depth). A combination of losses plus net immobilization of up to 56% had to be assumed in some soils to account for all the N loss. In two soils that had been used for disposal of feedlot manure, net mineralization of N from the organic N pool had to be assumed to explain the data obtained.

The current fertilization and irrigation practices used for some row crops in southern California leave varying amounts of NO₃^– in the drainage water. The amounts depend on the total N added, crop removal, drainage volume, net mineralization, and losses.

Key Words: N fertilization • NO3- in drainage water • ground water quality • crop removal of N • denitrification

¹ Contribution of the UCR Dept. of Soil Science & Agricultural Engineering with the Dept. of Plant Sciences, University of California, Riverside 92502, and Dept. of Vegetable Crops, University of California, Davis 95616, cooperating. Financial support from the Kearney Foundation of Soil Science and Water Quality Office of the Environmental Protection Agency through Grant No. 16060 DOE is gratefully acknowledged.

² Post doctoral Fellow (Soil Science), Plant Specialist (Plant Sciences) and Professor of Soil Science, University of California, Riverside; and Professor of Vegetable Crops, University of California, Davis, respectively.

Received for publication October 21, 1971.