Relationships among soil nitrate, leaf nitrate, and leaf yield of burley tobacco: Effects of nitrogen management

Burley tobacco (Nicotiana tabacum L.) requires large amounts of fertilizer N to produce high yields of cured leaf with the quality traits demanded by buyers. However, excessive N use produces air-cured leaves with undesirable le,els of NO3- is uneconomical, and is environmentally unsound if substant ial levels of residual soil NO3- remain following harvest. Effects of N fer tilizer on relationships among leaf yield, NO3- concentrations of air-cured leaves, and soil NO3- levels were investigated in 1991 and 1992 at two loc ations near Lexington, KY. Fertilizer N was broadcast at 0 to 448 kg ha(-1) (56-kg increments) before transplanting or banded at 168 kg ha(-1) about 5 wk after transplanting. Soils were a sell-drained Maury silt loam (fine, m ixed, mesic Typic Paleudalf) and a moderately well-drained Captina silt loa m (fine, silty, siliceous, mesic Typic Fragiudult), Cured leaf yield and la mina NO3- increased with increasing amounts of broadcast fertilizer N. Yiel d increased 3.7% with banded N, compared with an equivalent amount of broad cast N. Banding N also increased the NO3- level of cured leaf lamina by 37% for bottom leaves and 17% for middle leaves; top leaves were unaffected. S oil mineral N (NH4+ + NO3-) was proportional to the amount of broadcast N a pplied, and NO3- levels in the upper 30 cm of soil declined during the grow ing season. For predicted maximum leaf yields of 90%, critical soil mineral N values of 46 and 88 mg kg(-1) (for Captina and Maury soils, respectively ) were estimated from average mineral N concentrations in the upper 30 cm a t 3 and 5 wk after transplanting. Early-season soil NO3- testing to predict the NO3- level of cured leaf lamina was not useful; a nearly twofold diffe rence in lamina NO3- was observed among years when soil NO3- levels were eq uivalent. At 280 kg N ha(-1), a rate commonly recommended for burley tobacc o, as much as 37 mg NO3--N kg(-1) soil was found in the upper 30 cm of soil following harvest. Decreasing the amount of fertilizer N broadcast just be fore transplanting to 168 kg N ha(-1) caused a 10% reduction in yield, a 37 to 65% decrease in lamina NO3-, and about a 60% decrease in residual soil NO3- at harvest. Better N management can reduce both the NO3- level of cure d leaves and the amount of residual NO3- following harvest.