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TOBACCO PRODUCTION

Relationships among Soil Nitrate, Leaf Nitrate, and Leaf Yield of Burley Tobacco

Effects of Nitrogen Management

^a USDA-ARS, Grazinglands Research, 7207 W. Cheyenne St., El Reno, OK, 73036 USA
^b USDA-ARS, Western Cotton Research Lab, Phoenix, AZ 85040-8830 USA
^c Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091 USA

cmackown{at}grl.ars.usda.gov

Received for publication August 21, 1998.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
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 levels of NO^-₃, is uneconomical, and is environmentally unsound if substantial levels of residual soil NO^-₃ remain following harvest. Effects of N fertilizer on relationships among leaf yield, NO^-₃ concentrations of air-cured leaves, and soil NO^-₃ levels were investigated in 1991 and 1992 at two locations 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 well-drained Maury silt loam (fine, mixed, mesic Typic Paleudalf) and a moderately well-drained Captina silt loam (fine, silty, siliceous, mesic Typic Fragiudult). Cured leaf yield and lamina NO^-₃ increased with increasing amounts of broadcast fertilizer N. Yield increased 3.7% with banded N, compared with an equivalent amount of broadcast N. Banding N also increased the NO^-₃ level of cured leaf lamina by 37% for bottom leaves and 17% for middle leaves; top leaves were unaffected. Soil mineral N (NH⁺₄ + NO^-₃) was proportional to the amount of broadcast N applied, and NO^-₃ levels in the upper 30 cm of soil declined during the growing 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 at 3 and 5 wk after transplanting. Early-season soil NO^-₃ testing to predict the NO^-₃ level of cured leaf lamina was not useful; a nearly twofold difference in lamina NO^-₃ was observed among years when soil NO^-₃ levels were equivalent. At 280 kg N ha^-1, a rate commonly recommended for burley tobacco, as much as 37 mg NO^-₃–N kg^-1 soil was found in the upper 30 cm of soil following harvest. Decreasing the amount of fertilizer N broadcast just before transplanting to 168 kg N ha^-1 caused a 10% reduction in yield, a 37 to 65% decrease in lamina NO^-₃, and about a 60% decrease in residual soil NO^-₃ at harvest. Better N management can reduce both the NO^-₃ level of cured leaves and the amount of residual NO^-₃ following harvest.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
INSUFFICIENT N substantially reduces the yield and quality of burley tobacco (Miner and Sims, 1983). Because burley tobacco is a high-value cash crop, producers are more likely to overfertilize their crop with N. Such practices lead to the production of cured leaves with undesirable levels of NO^-₃ (Broaddus et al., 1965; Brunneman and Hoffmann, 1982; MacKown et al., 1984). Excessive use of N fertilizer is economically unfavorable, because incremental increases in yield diminish with increasing amounts of N applied (Miner and Sims, 1983), and it could lead to detrimental effects on the quality of soil and water resources. Burley tobacco fields that are heavily fertilized with N often have soil acidity-induced Mn toxicity problems (Hiatt and Ragland, 1963; Sims and Atkinson, 1973; Miner and Sims, 1983), and could have high concentrations of soil NO^-₃, which can lower the quality of ground and surface waters. Even when recommended fertilizer practices are used, substantial residual soil NO^-₃ has been observed at harvest (Zartman et al., 1976; MacKown et al., 1990).

The amounts of N fertilizer recommended for tobacco fields following grass sod range from 224 to 336 kg N ha^-1, with an additional 56 kg N ha^-1 recommended for continuous tobacco (Anonymous, 1998). Typically, with well-drained soils, the entire amount of N fertilizer is broadcast up to 4 wk before transplanting. Sometimes an additional N application is banded 4 to 5 wk after transplanting. For coarse-textured or poorly drained soils, split applications of one-third of the total N before transplanting and two-thirds after transplanting are recommended, to minimize fertilizer N losses (Sims and Wells, 1985; Nesmith et al., 1993). Banding the entire amount of N at 5 wk after transplanting, when the last cultivation occurs, improves fertilizer N recovery on well-drained and moderately well-drained soils without adversely affecting yield or quality (MacKown and Sutton, 1997). Split N applications are probably more appropriate, however, because they are likely to reduce the risk of low yield if rainfall delays or prevents applying the entire amount of N at 5 wk.

With appropriate N management practices, it may be possible to limit the accumulation of NO^-₃ in leaves, optimize fertilizer N use, and reduce the potential degradation of soil and water resources. Leaf yield, market grading, and value responses of burley tobacco to fertilizer N have been well documented. However, information generated from a comprehensive experiment designed to determine the effect of N additions on the relationships between leaf yield, cured leaf NO^-₃ levels, and amounts of available and residual soil NO^-₃ is lacking. Our primary objective was to evaluate these relationships. Emphasis was placed on the changes in soil NO^-₃ following pre-transplant broadcast applications of fertilizer N. Secondarily, we sought to test our hypothesis that banding all N at the time of last cultivation and before the onset of rapid crop growth would not alter cured leaf NO^-₃ concentration or have a negative effect on leaf yield.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Experiments were conducted in 1991 and 1992 near Lexington, KY, on two farms belonging to the University of Kentucky. At Spindletop Research Farm, plots were located on a well-drained Maury silt loam. At Eden Shale Research Farm the soil was a moderately well-drained Captina silt loam. Monthly climate data for Spindletop Farm were obtained from the University of Kentucky Agricultural Weather Center (1998) and from daily observations recorded at Eden Shale Farm.

During the fall of 1990 at Spindletop Farm, tall fescue (Festuca arundinacea Schreb.) was incorporated by moldboard plow, agricultural lime was applied to raise the soil pH to 6.6, and a winter wheat (Triticum aestivum L.) cover crop was planted. In the spring of 1991, the wheat cover crop was plowed to prepare the site for planting tobacco. A two-year-old stand of red clover (Trifolium pratense L.) hay was plowed in the spring of 1991 to prepare the Eden Shale site for tobacco. For the year preceding the 1992 experiments, the sites were planted with tobacco that was fertilized with 290 kg N ha^-1 at Spindletop and 250 kg N ha^-1 at Eden Shale. Winter wheat cover crops were planted on these sites in the fall of 1991 and plowed in the spring of 1992 to prepare the sites for tobacco. Soil tests for available P (Bray 1) were very high at both sites and K was low to medium at Spindletop and very high at Eden Shale (data not shown). Before transplanting tobacco, K₂SO₄ fertilizer (325 kg K ha^-1 in 1991; 260 kg K ha^-1 in 1992) was broadcast and disked into the soil at Spindletop. Burley tobacco `TN86' transplants were set in mid to late May in rows spaced 1.02 m apart, with about 0.49 m between plants. Recommended cultural and management practices were followed (Nesmith et al., 1993), except as regards N levels and timing, which were treatments in the experiment. The tobacco crop was harvested mid-August to early September (87 to 107 d after transplanting). Plots were disked and winter cereal cover crops were planted before mid-October, as recommended for erosion control and residual N recovery from tobacco fields (Nesmith et al., 1993).

Nitrogen Treatments
Ten N treatments were arranged in a randomized complete block design, with six replications. Nine amounts of NH₄NO₃ (0 to 448 kg N ha^-1 in 56-kg increments) were broadcast by hand and lightly disked into the soil surface 1 or 2 d before transplanting. The 10th N treatment was of 168 kg N ha^-1 of NH₄NO₃ hand-applied in bands about 10 cm deep and 30 cm to each side of a row at about 5 wk after transplanting. Plots were 15.2 m long and five rows wide, except in 1991, when they were six rows wide at Eden Shale.

Soil and Plant Sampling
The top 30 cm of soil was sampled from 0 kg N ha^-1 treated plots at transplanting and from a subset of the broadcast N treatments (0, 56, 112, 168, 280, and 392 kg N ha^-1) at 3 and 5 wk after transplanting and at harvest. From each plot, about 30 cores (1 cm diam.) were collected and combined for analysis. Six soil profile cores (3 cm diam., 90 cm deep) from each plot were collected from the 0, 112, 280, and 392 kg N ha^-1 treatments about 2 wk after planting fall cover crops and during mid to late April the following spring. These cores were subdivided into 30-cm increments and combined by depth for each plot. Soil samples were sieved in the field to pass a 2-mm screen, placed in ice chests, and returned to the lab for immediate extraction.

Cured leaf yields were based on a sample of 40 to 50 plants harvested from the interior rows of each plot and air-cured in tobacco barns located at each farm. A subsample of air-cured leaves from each plot were grouped into upper, middle, and bottom stalk positions and analyzed for NO^-₃.

Nitrate and Ammonium Analysis
Duplicate subsamples of the field-moist soil were extracted with 1 M KCl (w/v, 1/10) by shaking vigorously for 1 h. Another subsample was used for gravimetric determination of soil water content. Extracted NO^-₃–N was measured colorimetrically following microbial dissimilatory reduction to NO^-₂ by a manual adaptation of an automated method (Lowe and Gillespie, 1975) or by an automated flow injection analyzer with a Cd reduction column. Both procedures would include NO^-₂, if present. Extracted NH⁺₄–N was measured colorimetrically with an automated flow injection analyzer equipped with a diffusion unit to trap NH₃ in a colored pH-sensitive solution (Tecator AB, 1984).

Plant tissues were dried to constant weight in a forced-air oven at 60°C, weighed, and ground to pass a 1-mm screen. Duplicate subsamples of the ground tissue were extracted with deionized water for 1 h at 97°C. Extracted NO^-₃–N was measured using the methods described for soil NO^-₃.

Statistical Analysis
Analysis of variance and regression procedures (JMP version 3.2.2; SAS Inst., 1994) were used to analyze the data. Locations and years were initially treated as fixed effects in the analysis of variance model used to evaluate the response of cured leaf yield and NO^-₃ concentration. A multivariate-fitting platform with repeated measures in space was used to analyze lamina NO^-₃ data for the three stalk positions. When the sphericity test was significant, the Geisser–Greenhouse adjusted univariate test was used to evaluate the F-statistic determined for the contrast response design. This data analysis approach with repeated measures in time was used also for soil NO^-₃ determinations during the growth of the tobacco crop. A multivariate analysis with a compound response design (repeated measures in time and space) was used to analyze data collected for fall and spring residual NO^-₃ in the soil profile.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
From May 1991 through April 1993, nearly 2270 mm of precipitation would normally have been received at Spindletop Farm (Fig. 1) . The measured amount at Spindletop Farm (2430 mm) for this period was about 7% greater than the expected normal precipitation; at Eden Shale Farm, the measured amount of precipitation was 2000 mm, which is nearly 12% less than the normal for Spindletop Farm. Deviations from expected normal monthly precipitation totals occurred during the intervals of tobacco growth (mid-May to late August or early September) and after harvesting (Fig. 1). In all cases, the amounts of precipitation at Spindletop Farm equaled or exceeded those of Eden Shale Farm during the growth of tobacco and between harvest and collection of soil profile samples for NO^-₃ (Table 1) .

View larger version (43K): [in this window] [in a new window]   Fig. 1 Monthly precipitation from May 1991 through April 1993 at Spindletop and Eden Shale farms and the long-term (1961–1990) average for Spindletop Farm

View larger version (25K): [in this window] [in a new window]   Fig. 2 Burley tobacco cured leaf yield response to pre-transplant broadcast applications of fertilizer N on silt loam soils at Spindletop Farm (Maury, well-drained) and at Eden Shale Farm (Captina, moderately well-drained). Yields are expressed as field weights (0.2 kg moisture kg-1 dry wt. of leaf). Where greater than symbol size, error bars indicate ±1 SE of the mean

Banding N fertilizer also offers additional benefits. In an earlier study with ¹⁵N-depleted fertilizer N, banding N at about 5 wk after transplanting increased fertilizer N recovery 6 to 17% more than an equivalent amount of broadcast N applied just before transplanting (MacKown and Sutton, 1997). The last opportunity to use a tractor to cultivate tobacco fields without damaging the crop usually occurs at about 5 wk after transplanting. Producers could combine the last cultivation operation with a band application of the entire amount of N, but excessive rainfall might delay or prevent the use of farm equipment to apply the N required for good leaf yield. Using one or more early cultivations to band all or part of the fertilizer N could reduce this risk. Banding N at any time within the first 5 wk after transplanting should not adversely affect leaf yield, and banding N earlier than 5 wk after transplanting may offer some of the fertilizer N recovery benefit obtained when the entire amount of N is banded at 5 wk after transplanting (MacKown and Sutton, 1997).

Soil Mineral Nitrogen
The concentration of mineral N (NH⁺₄ + NO^-₃) in the top 30 cm of soil at 3 and 5 wk after transplanting increased linearly with increasing amounts of N fertilizer broadcast and incorporated into the soil surface just before transplanting (data not shown). The contribution of NH⁺₄ to the early-season total mineral N was small when the level of broadcast N was less than 280 kg ha^-1, but NH⁺₄–N contributed up to 38% of the total soil mineral N for broadcast applications greater than or equal to 280 kg N ha^-1 (data not shown). Apparently, early-season microbial NH⁺₄ oxidation capacity of these soils approached a maximum at about 140 kg NH⁺₄–N ha^-1. Differences in soil NO^-₃ levels at 3 and 5 wk after transplanting were small compared with the declines that occurred from 5 wk after transplanting until about 8 to 10 wk later just after harvest (Fig. 3) . Zartman et al. (1976) found that N accumulated by burley tobacco grown on the well-drained Maury soil of Spindletop Farm was proportional to the soil solution concentration of NO^-₃ and the upper 30 cm of soil contributed about 90% of the total N accumulated. The onset of the decreased level of soil NO^-₃ (Fig. 3) corresponds to the period of greatest N demand required to sustain the linear phase of rapid crop growth and nutrient accumulation that begins at about 5 wk after transplanting (Atkinson et al., 1977).

View larger version (30K): [in this window] [in a new window]   Fig. 3 Changes in soil NO-3 levels within the top 30 cm of soil receiving pre-transplant broadcast applications of fertilizer N. Error bars indicate the LSD (0.05)

An unexpected discrepancy between the level of residual soil NO^-₃ just after harvest and leaf yield at Eden Shale Farm was observed for broadcast N applications of 280 and 392 kg N ha^-1 (compare Fig. 2 and Fig. 3). In 1991, residual soil NO^-₃ for these treatments exceeded those in 1992, yet 1991 leaf yields exceeded those of 1992. This apparent discrepancy may be partially resolved by considering two factors. First, the preceding legume cover crop in 1991 probably contributed more N than the winter cereal cover crop in 1992. Second, precipitation during the 1992 tobacco-growing season substantially exceeded that of 1991 (Table 1). Even when recommended N fertilizer practices are used, substantial residual soil NO^-₃ can exist at harvest (Fig. 3; Spindletop 1992 and Eden Shale 1991).

The compound multivariate analysis of postharvest residual NO^-₃ levels in the soil profile revealed that the depth x date interaction among the N fertilizer x year x location effect was significant (P = 0.001). At mid to late October of each year (about 7 to 9 wk after harvest), concentrations of residual NO^-₃–N in the top 30 cm of the soil tended to equal or exceed that at the 60- to 90-cm depth (Fig. 4) . By mid to late April 1992, levels of NO^-₃–N in the top 30 cm of soil were substantially less, but at lower depths the levels of NO^-₃–N were as great or greater than the previous fall levels (Fig. 4). In April 1993, the levels of soil NO^-₃–N were uniformly low (<3.0 mg kg^-1) throughout the profile at Spindletop Farm, and <2.6 mg kg^-1 in the top 60 cm to <13.2 mg kg^-1 at the 60- to 90-cm depth at Eden Shale.

View larger version (31K): [in this window] [in a new window]   Fig. 4 Postharvest fall and spring residual soil NO-3 in the soil profile of well-drained (Spindletop Farm) and moderately well-drained (Eden Shale Farm) silt loam soils. Soil collection dates are referenced to days after transplanting (DAT). Where greater than symbol size, error bars indicate ±1 SE of the mean

Cured Lamina Nitrate
A significant sphericity test was found when the multivariate analysis of lamina NO^-₃ using leaf stalk position as a repeated measure was transformed into the univariate model. The adjusted univariate test was significant (P = 0.015) for the contrast design interaction of leaf stalk position with the between-subject model factor of N fertilizer x year x location. Concentrations of NO^-₃ in the lamina of cured leaves increased with increasing amounts of broadcast N fertilizer incorporated into the soil surface just before transplanting (Fig. 5) . This leaf NO^-₃ response to fertilizer N level is consistent with earlier reports (Atkinson and Sims, 1973; Broaddus et al., 1965). Positive slope linear relationships between lamina NO^-₃ and broadcast N fertilizer were observed, except for 1991 at Eden Shale, where increases in lamina NO^-₃ concentrations diminished with increasing amounts of N fertilizer (Table 3) . Levels of lamina NO^-₃ decreased sequentially from the bottom to the top stalk positions at both locations and for each year. Previous reports (Bowman, 1972; Broaddus et al., 1965; Hamilton et al., 1982) support this observation and document that the concentration pattern of reduced N increases from the bottom to top stalk positions (Bowman, 1972; Broaddus et al., 1965; Hamilton et al., 1982).

View larger version (26K): [in this window] [in a new window]   Fig. 5 Lamina NO-3 concentrations of cured leaves sampled from three stalk positions of burley tobacco

Relationships of Soil Mineral Nitrogen to Cured Leaf Yield and Lamina Nitrate
Increases in the cured leaf yields of burley tobacco diminished with increasing average concentrations of mineral N (NH⁺₄ + NO^-₃) measured in the top 30 cm of soil before the onset of rapid crop growth (Fig. 6) . For both locations, equations with significant quadratic components gave the best fit for the relationship between these two variables. Expressing yields relative to the predicted maximum yields derived from the regression equations of Fig. 2 did not improve the relationships between soil mineral N and yield (data not shown). Site-specific effects and broad confidence intervals could limit the value of early-season soil mineral N tests for accurately predicting fertilizer N requirements to achieve a desired yield (Fig. 6). The Cate–Nelson approach (Cate and Nelson, 1972) has often been used to identify critical soil NO^-₃ values for the nonlinear response of relative corn (Zea mays L.) yield to pre-sidedress NO^-₃ concentration of soils in the Northeast and Atlantic Coast regions (Bock and Kelley, 1992; Magdoff et al., 1990; Sims et al., 1995). Linear–plateau or quadratic–plateau models also have been used to determine the plateau–threshold value that corresponds to the critical NO^-₃ value of soils from the Midwest and Atlantic Coast (Binford et al., 1992; Meisinger et al., 1992). In some cases, N recommendations for corn were improved when yield potentials of soils were considered and the soil NO^-₃ test values were within the N responsive range (Bundy and Andraski, 1995). Using a Cate–Nelson graphical approach to minimize the number of low soil mineral N–high leaf yield and high soil mineral N–low leaf yield observations for data presented in Fig. 6, the early-season (3–5 wk after transplanting) estimated critical soil mineral N value for a 90% maximum yield (location average predicted from Fig. 2) was 88 mg kg^-1 at Spindletop Farm and 46 mg kg^-1 at Eden Shale Farm. These values may be useful in predicting whether an early-season addition of fertilizer N will increase leaf yield of burley tobacco.

View larger version (31K): [in this window] [in a new window]   Fig. 6 Relationships between yield of burley tobacco cured leaf and average soil mineral N (NH+4 + NO-3) in the top 30 cm of soil measured at about 3 and 5 wk after transplanting. Dashed lines represent 95% confidence intervals that include both the variability of regression estimates and the variability of the observations, making them suitable for prediction intervals. Horizontal and vertical lines represent Cate–Nelson graphic interpretations of soil mineral N critical values at 90% maximum yield as predicted for each location from data presented in Fig. 2

View larger version (31K): [in this window] [in a new window]   Fig. 7 Relationships between lamina NO-3 concentrations of cured leaves sampled from three stalk positions of burley tobacco and average of soil NO-3 in the top 30 cm of soil measured at about 3 and 5 wk after transplanting. Dashed lines represent upper and lower boundaries enclosing the range of 95% confidence intervals that include both the variability of regression estimates and the variability of the observations for the three leaf positions sampled

	Summary and conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

Continued refinements of diagnostic tests to improve the N management of burley tobacco are warranted. Early-season soil mineral N tests could be used to better manage the N fertilizer requirements of burley tobacco by identifying fields that are likely to respond to added fertilizer N. With better N management, decreases in the NO^-₃ level of cured leaves and the amount of residual NO^-₃ following harvest should be possible. For example, 37 to 65% decreases in lamina NO^-₃ and about a 60% decrease in residual soil NO^-₃ are associated with a 10% decrease in cured leaf yield of tobacco receiving 168 kg N ha^-1 of fertilizer broadcast just before transplanting. However, using current N management practices and tobacco cultivars, it is unlikely that growers will accept lower yields to produce low-nitrate tobacco unless leaf buyers support a leaf nitrate monitoring program and provide an economic incentive.Cate Nelson 1965; SAS Institute 1994; Tecator 1984; University of Kentucky Agricultural Weather Center 1998

	ACKNOWLEDGMENTS

 
We gratefully acknowledge the technical assistance received from Bettie Jones. Joseph W. Wyles, Farm Manager, provided precipitation data for Eden Shale Farm. Also, James Cohlmeyer and Rosalyn Williams, who were supported by the USDA-ARS Research Apprenticeship program for high school students, and José R. Graxirena-Cotto, who was supported by the University of Kentucky summer internship program for undergraduate minority students, provided assistance.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Joint contribution of the USDA-ARS and Kentucky Agric. Exp. Stn. Journal no. 98-06-131.

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