Effect of Urea on Yield and Quality of Flue-cured Tobacco

Flue-cured tobacco (Nicotiana tabacum L.) can be sensitive to NH⁺₄-N in the soil, especially in the presence of CI⁻. Nitrogen sources such as (NH₄)₂SO₄ and ammonium phosphates have depressed growth in the presence of strong, general-purpose fumigants containing CI⁻ that are sometimes used in tobacco culture, but little information is available for urea under similar conditions. Thus, urea was compared to NaNO₃ in eight field experiments to evaluate the interactive effects of N sources and fumigants on the growth and chemical composition of flue-cured tobacco. The various soils are listed in the Materials & Methods Section. Urea and NaNO₃ were applied singly or combined to provide four combinations of NO⁻₃-N and NH⁺₄-N in 1976 and 1977 and three combinations in 1978. Nitrogen was applied at 67 kg/ha except for an extra level of 112 kg/ha from urea in 1978. Fertilizers were banded in nonfumigated soil and soil treated with recommended rates of MoCap [O-Ethyl S, S-dipropyl phosphorodithioate] and Telone C-17 [dichloropropylene, trichloronitromethane] in 1976 or Terr-0-Cide 30 [ethylene dibromide, trichloronitromethanel and Telone C-17 in 1977 and 1978. Yield and grade index were increased 26 and 20%, respectively, by urea compared to NaNO₃ when excess rainfall occurred shortly after planting. Apparently, N from urea was still present as NH⁺₄-N and remained within the root zones while N from NaNO₃ moved below the root zone. In a dry year, urea resulted in a 7% Under conditions of adequate and well-distributed rainfall, both N sources had similar effects on tobacco. From 4 to 6 weeks were required to complete nitrification, depending on year and location, and the extent of nitrification at 2 and 4 weeks was reduced by both Terr-0-Cide 30 and Telone C-17 compared to nonfumigated soil. Chloride concentrations in leaves increased until the 6th week and then abruptly declined, coinciding with the disappearance of NH⁺₄-N. Leaf CI⁻ levels as high as 3.5% at 6 weeks were obtained for urea in the presence of Telone C-17 but they were less than 0.5% in cured leaves at harvest, irrespective of N source or soil chemical treatment.