Mo and N Effects on Growth, Yield, and Mo Composition of Burley Tobacco

Nicotiana tabacum L., cv. ‘Ky-14’, was grown in field experiments on Maury silt loam soil (clayey, mixed, mesic; Typic, paleudult) to determine the influence of Mo and N nutrition on growth, yield, and Mo concentration of hurley tobacco. Previous studies have shown that high rates of commercial fertilizers commonly applied in hurley tobacco production decreased soil pH, concentration of soil and plant Mo, and depressed dry matter accumulation during early growth stages. The present studies were designed to evaluate the use of Mo and NO⁻₃ sources of N fertilizer to increase early growth and final cured leaf yields. In one experiment conducted during 2 years, N as KNO₃, NH₄NO₃, or urea was applied at rates of O, 224, and 448 kg N/ha. Potassium in the form of K₂SO₄, and in amounts equivalent to the K in KNO₃ was added to plots receiving NH₄NO₃ and urea. In a second experiment conducted during 1 year N as Ca(NO₃)₂ or urea was applied at rates of 280 kg N/ha. No K₂S0₄ was applied. Rates of Mo assodium molybdate were O, 0.22, and 0.44 kg Mo/ha in Experiment 1 and O, 0.22, 0.44, and 0.88 kg Mo/ha in Experiment 2. In Experiment 1, dry matter accumulation during early growth was inversely related to rate of N and K fertilizer applied. Mo fertilization did not overcome the depressing effects of fertilizer on early growth, but a trend existed for both Mo and NO₃⁻ sources of N to improve early growth. Mo significantly increased yields of cured leaf. Significant N rate × Mo rate and N and K fertilizer treatment × Mo rate interactions occurred for cured leaf yields. Response to Mo occurred only at the 224 kg N rate, and Mo applied at the rate of 0.44 kg/ha increased yields about 900 kg/ha over plots unfertilized with Mo in the presence of KNO₃. Average plant Mo concentration during early growth was doubled by adding Mo at the 0.44 kg/ha rate and halved by adding N at the 448 kg/ha rate. The critical concentration of Mo for dry matter accumulation during early growth appeared to be about 0.38 μ/g dry weight. Results of Experiment 2 confirmed those of Experiment 1 regarding effects of NO₃⁻ sources and Mo fertilization. Additionally, Mn concentration in plants treated with Ca(NO₃)₂ was about half that of urea-treated plants, and in urea-treated plants Mn concentration was reduced by one-half by Mo fertilization. Since Mn toxicity of tobacco is common in hurley production, these findings are of particular significance.