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Through both laboratory and field experiments, we studied the effects of soil moisture content on ammonia (NH3) volatilization from urea, surface-applied to bare Hagerstown silt loam (fine, mixed, mesic Typic Hapludalf). In the laboratory trial, urea fertilizer treatments were applied to cylindrical cores of soil in a flow-through apparatus. Volatilized NH3 was measured by passing humid air through the head space and into boric acid traps for 12 d. Total loss from urea on soil with 0.15 kg kg−1 moisture content was 30.7% and on soil with 0.22 kg kg−1 moisture content, 18.3%. Nitrogen-15-enriched urea was applied to the surface of soils at 0.15 and 0.22 kg kg−1 moisture content in confined cylindrical microplots in a field site in May and July 1985. Soil in the microplots dried during the 14-d trials, because the site was covered during periods of rainfall. Samples were analyzed 0, 3, 7, and 14 d after urea after urea application. The mean air temperature was 15.0°C in May and 20.5°C in July. After 14 d, 15N recovery from 0.22 kg kg−1 moisture soil was 71.9% in May and 60.0% in July. Recovery from 0.15 kg kg−1 moisture soil was 95.5% in May and 84.9% in July. We concluded that under drying conditions greater volatilization occurs in soil with a greater initial moisture content, but when soil moisture conditions remain essentially constant, as occurred in the laboratory experiment, there is less volatilization from the soil with the greater moisture content. Also, there are greater NH3 volatilization losses at higher temperatures, given adequate moisture to ensure complete and rapid urea hydrolysis.
Burch et al. (Mon,) studied this question.