Soil samples to the 15‐m depth beneath asparagus (Asparagus officinalis L.) and celery (Apium graveolens L.) were taken to determine the NO3− concentration in the solution of the unsaturated zone and to estimate the soil N balance. Transit time for water to move to the 15‐m depth was calculated from drainage volumes and volumetric water contents. Nitrogen balance was calculated from data for N input, N removal in harvested crops, and water records for the calculated transit time. The NO3− concentration in the unsaturated zone increased with increase in N rate but was inversely related to the leaching volume. Denitrification was assumed to be the cause of high N losses with high irrigation levels. However, the rather high loss of 67.7% in the celery experiment, which received large amounts of chicken manure, was assumed to be a result of both net immobilization and denitrification. A combination of high losses of N, which ranged from 18.3 to 67.7%, and large amounts of excess N, in the NO3− form in the soil could have been one cause of the low efficiency of recycling N by these crops. The data suggest that high rates of N combined with high levels of water use are conducive to denitrification in relatively permeable soils.
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Adriano et al. (1972) studied this question.