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The effect of subsoiling on tobacco (Nicotiana tabacumL.) yield is variable across soils, and the reasons for this are not completely understood. As a result, it is difficult to identify soils where subsoiling will increase yields. This study evaluated relationships among relative yield increases due to subsoiling (RYI's), soil properties, and rainfall. Single‐season tillage experiments were conducted at 32 locations from 1981 through 1984. Soils were Paleudults and Hapludults. Tobacco yields were measured for subsoiled and nonsubsoiled (moldboard plow/disk) tillage treatments at all sites. Rainfall was measured daily. Soil properties evaluated included bulk density, cone index, texture, water retention, exchangeable cations, pH, and base saturation. Relative yield increases due to subsoiling ranged from −9.0 to 31.8% across locations. Twelve sites had yields significantly (0.05 level) increased by subsoiling. Stepwise regression analysis showed that selected weekly rainfalls and drought duration were major factors related to RYI variations, and with water retention of Ap and E‐B horizons, they accounted for 60% of the variation in RYI across all sites. Relative yield increases were greatest during dry years on sandy soils with low water retention values. Sites where relatively large RYI's (> 13%) occurred were characterized as having both bulk densities ≥ 1.63 Mg m−3in the lower Ap or E‐B horizon and sand contents ≥ 73% in the Ap horizon.

Leaching in coarse‐textured soils contributes to inefficient N utilization by flue‐cured tobacco (Nicotiana tabacumL.). Slow‐release N materials reduce N leaching but those with rate of release dependent on biological degraduation have not been successful in furnishing the high proportion of total N required by tobacco during early growth. The objectives of this study were to investigate the release characteristics of isobutylidene diurea (IBDU), with N‐release characteristics dependent primarily upon the effects of particle size on chemical dissolution rather than the effects of environmental variation on biological activity, in laboratory and greenhouse studies and to evaluate it as a source of N for flue‐cured tobacco in the field. Urea, NaNO3, and four particle sizes of IBDU (1.05, 0.75, 0.55, and 0.40 mm) were applied at rates of 200 and 400 mg N per kg of soil to a Norfolk sandy loam (Typic paleudult) in lysimeters and subjected to leaching at 10 intervals over a 109‐day period. All applied N from NaNO3and 60% from urea were removed by the first leaching (0 days) at both rates of application. Only 5% of the applied N from IBDU was removed in the first leaching. At the 200 mg N application rate, 50 to 60% of applied N from the 1.05 to 055 mm IBDU particle sizes were removed after 10 leachings (109 days) but only four leachings (32 days) were required to remove equal amounts from the 0.40 mm particle size. At the 400 mg N application rate, 10 leachings were required for 55 to 60% removal of applied N from all particle sizes. The effects of particle size on the ability of IBDU to release N fast enough for maximum growth, but not so fast as to be subject to leaching, were evaluated in a greenhouse with two successive. crops of corn (Zea maysL.) grown under leached and non‐leached conditions. IBDU particle sizes of 1.05, 0.55, and 0.40 mm were applied to Norfolk soil in pots at rates of 214, 321, and 428 mg per 5 kg of soil. Dry matter for the first crop was not affected by particle size, but it was greatest for the 1.05 mm IBDU in the second crop. Within a particle size, dry matter was not altered by leaching but total N uptake was greatest from 0.40 mm IBDU. This suggests that N release from 0.40 mm IBDU was rapid enough to support maximum growth but was not so rapid as to be subject to leaching under imposed conditions. In a field experiment on Norfolk soil, I BDU particle sizes of 1.05 and 0.40 mm were compared with NaNO3and urea as N sources for flue‐cured tobacco. Materials were banded at 67 kg N/ha: (a) days after planting, (b) as a split application of a single source 12 and 26 days after planting, and (c) as a split application of one source at 12 days after planting and NaNO3at 26 days after planting. NaNO~ gave the highest yield of cured leaves when applied 12 days after planting, followed by urea, 0.4 mm IBDU, and 1.05 mm IBDU, respectively. Yields from both IBDU materials were improved when one‐half of the total N was applied 2 weeks later as NaNO3. Chemical composition of cured leaf varied little with treatment but urea gave higher levels of N and total alkaloids and lower sugars than the other materials. IBDU was effective in preventing loss of N by leaching in laboratory and greenhouse studies. As with other slow‐release N sources, however, it was inferior to water‐soluble N sources for flue‐cured tobacco in the field.

Flue‐cured tobacco (Nicotiana tabacumL.) produced in a close‐grown system may offer an alternative to use of synthetic smoking materials. In order to develop optimum production and handling practices for this system, information is needed on the chemical and physical contents of the harvested material. Tobacco (‘Sp C‐28’) was produced in a close‐grown system with varying plant populations (45,448, 90,896, 136,344 plants/ha), N‐rates (84, 168, 252, 336 kg/ha), top ping (topped vs. nontopped), and row spacings (equidistant vs. twin). The research was conducted at the Oxford Tobacco Research Station on a Durham loamy sand (fine loamy, siliceous, thermic, Typic Hapudult). Yield, total alkaloids, and reducing sugars of lamina, midrib, and stalk from the upper, middle, and lower thirds of cured plants were compared. One‐third of the total plant weight was in each stalk position. The upper third of the close‐grown plant contributed most to total lamina weight and total alkaloid level. The proportion of lamina in the upper stalk position was higher in plants that were topped or when plant population was increased above 45,448/ha but it deoreased when N‐rate was increased above 84 kg/ha. Total alkaloid concentration was highest in the upper stalk position and in lamina. Increased plant population resulted in decreased total alkaloids in the middle stalk position. Topping and increasing N‐rate resulted in higher total alkaloids in all stalk positions. Reducing sugar content was approximately the same for all three stalk positions but highest levels in lamina and midrib were found in the upper two‐thirds of the plant. Reducing sugar concentrations were highest in stalk. Plant population had no significant effect on reducing sugar content but increased N‐rate lowered concentrations in the lower stalk position. Results indicate that agronomic practices offer a means for altering the physical and chemical content of closegrown tobacco.

The development of synthetic smoking materials has prompted researchers to consider non‐conventional production methods as a means of producing a low cost, flue‐cured tobacco (Nicotiana tabacumL.) material which might be used in the manufacture of cigarettes. In order to evaluate yield potential, chemical content, and physical characteristics of close‐grown tobacco, plants were grown in the field at populations of 45,448, 90,896, and 136,344/ha with different planting designs (twin or equidistant), nitrogen levels (84, 168, 252, and 336 kg/ha), and topping regimes (topping or no topping). The soil was a Durham loamy sand (fine loamy, siliceous, thermic, Typic Hapudult). The entire plant was flue‐cured and separated into lamina, midrib, and stalk for yield and chemical analysis. A wide range of yield, chemical content, and physical characteristics of the harvested material was produced by manipulating various cultural practices. Topped plants averaged 26% lamina, 12% midrib, and 62% stalk as compared to 19% lamina, 10% midrib, 54% stalk, and 17% top material for plants not topped. Yields increased progressively with N‐rate through 252 kg/ha and/or increased plant population. Twin or equidistant row spacing had no effect on yield. Topping decreased whole‐plant yield but increased the concentration of alkaloids and reducing sugars and the lamina: stalk ratio. Total alkaloids ranged from 0.18% for wholeplant nontopped tobacco receiving 84 kg N/ha to 0.84% for topped plants receiving 336 kg N/ha. Total reducing sugars averaged 8.0 for nontopped and 9.6% for topped plants. Results indicate that high yields can be obtained by utilizing the close‐grown production system and harvesting the entire plant. Desirable chemical content and physical characteristics of close‐grown tobacco might be obtained by manipulating cultural practices.

Manganese deficiencies occur on sandy Atlantic Coastal Plain soils that have been limed excessively. Granular Mn materials can be blended with starter fertilizer to overcome these deficiencies but variation in their water solubility and in starter fertilizer acidity affects the availability of Mn from these materials. Seven Mn materials varying in water solubility and three starter fertilizers were band‐applied to supply 11 kg ha−1of Mn to corn (Zea maysL.) on a sandy soil (fine‐loamy, siliceous, thermic Aquic Paleudult) at pH 6.9. Starter fertilizers, in order of increasing acid‐forming potential [NaNO3+ triple superphosphate (SNT); diammonium phosphate (DAP); and (NH4),SO4+ triple superphosphate (AST)] were applied at rates that supplied 28 and 31 kg ha−1of N and P, respectively, at planting. Soils were sampled through the fertilizer band at 30 days, whole plants were sampled at 30, 60, and 90 days, and leaves opposite and below the ear were collected at silking (75 days). Soil Mn, whole plant Mn concentration and uptake, and leaf Mn concentration increased with increasing solubility of the Mn materials. The MnO in the Mn materials did not contribute to these relationships. Increased acidity of the starter fertilizer also increased plant Mn concentration and uptake. Manganese deficiency symptoms occurred with the least acid‐forming starter fertilizer (SNT) and decreased with increasing water solubility of the accompanying Mn materials. Equations were developed to predict recommended rates of Mn material, depending on water solubility, to apply with each starter fertilizer

Symptoms of Mn toxicity in tobacco (Nicotiana tabacumL.) are often observed in the mountain region of North Carolina at soil acidity and soil test levels of Mn which do not produce toxicity in the piedmont. Although genotypic differences between tobacco types grown in the regions may contribute to the differential response, variations in temperature between the regions may also be involved. To investigate the effects of temperature on plant growth and Mn tolerance, tobacco plants of a prevalent cultivar (‘Coker 347’) for the piedmont region were grown in sand culture at day/night temperatures of 22/18, 26/22, and 30/26 C. Manganese was applied daily at rates of 10, 20, 40, 80, and 120 mg/liter in a complete nutrient solution. Plants were sampled weekly during a 6‐week growth period and evaluated for visual toxicity symptoms, dry weight. Mn concentration, relative growth rate (RGR), and relative accumulation rate (RAR) of Mn. Plant tolerance to high rates of applied Mn increased with increasing temperature despite significantly greater tissue concentrations of Mn during early growth. Tissue levels of Mn in leaves associated with the appearance of visual toxicity symptoms were 700 to 1,200 mg/kg dry wt at 22/18 C, 2,000 to 3,500 mg/kg at 26/22 C, and 5,000 to 8,000 mg/kg at 30/26 C. Mn toxicity symptoms were more severe in immature leaves and often decreased as the leaves matured even though the concentration of Mn in the leaves increased. The RGR of leaves during the exponential growth phase increased with increasing temperatures at each Mn rate, but the RAR of Mn was unaffected by either temperature or applied Mn rate. It is proposed that the increased tolerance to high levels of tissue Mn by plants grown in warm temperatures is associated with more rapid rates of leaf expansion accompanied by increased vaculor capacity for disposal of accumulated Mn.

Flue‐cured tobacco (Nicotiana tabacumL.) can be sensitive to NH+4‐N in the soil, especially in the presence of CI−. Nitrogen sources such as (NH4)2SO4and 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 NaNO3in 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 NaNO3were 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‐O‐Cide 30 [ethylene dibromide, trichloronitromethane] and Telone C‐17 in 1977 and 1978. Yield and grade index were increased 26 and 20%, respectively, by urea compared to NaNO3when 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 NaNO3moved below the root zone. In a dry year, urea resulted in a 7% yield reduction compared to NaNO3while grade indices were similar. 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‐O‐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.

Relationships between mechanical impedance and root growth have not been extensively defined for field conditions, and as a result, it is usually impossible to know with certainty when soil layers of high mechanical impedance are limiting root growth or where subsoiling is needed to improve root development. The primary objective of this investigation was to describe relationships between mechanical impedance and root concentration for both subsoiled and nonsubsoiled tillage treatments in field studies. Experiments were conducted at three locations in the North Carolina Coastal Plain on soils that were coarse‐textured Typic and Arenic Paleudults with tillage pans. At each site, flue‐cured tobacco (Nicotiana tabacumL.) was grown in subsoiled and nonsubsoiled (moldboard plow/disk) tillage treatments replicated four times. Penetrometer readings were made in the row and 0.30 m to the side of the row at 6‐ to 14‐d intervals for periods of 28 to 42 d; these data were averaged by depth and position and were expressed as mean cone index (CI). At approximately 52 and 75 days following transplanting, roots were counted by the trench‐profile method. At all locations, numbers of roots below the Ap horizon were significantly greater for the subsoiled, as compared to nonsubsoiled treatments for both root count dates. Mean CI was significantly correlated with root concentration in the Ap and E‐B horizons at all locations with rvalues ranging from −0.79** to −0.88** for both root count dates. The three tillage pans examined had mean CI's between 2.8 to 3.2 MPa. Subsoiling reduced the CI of the tillage pans for up to 47 d from transplanting and permitted significantly greater root penetration of the subsoil.

Deep tillage with subsoilers can fracture tillage pans and increase root development in some soils, but little is known about the precise soil conditions for which subsoiling works best. This study evaluated relationships of soil physical properties, chemical properties, and subsoiling to root distributions in 10 Ultisols. Five sites had dense tillage pans in their E‐B horizons with bulk densities (ρb) ≥1.66 Mg m−3. At each location flue‐cured tobacco (Nicotiana tabacumL.) was grown in subsoiled and nonsubsoiled (moldboard plow/disk) tillage treatments. At approximately 100 d from transplanting, root distributions were described by the trench‐profile method. Soil properties evaluated included bulk density, texture, ped mean weight‐diameter, cone index (CI), Ca and Al saturation ratios, base saturation, and pH. Subsoiling significantly increased the proportion of roots below the E‐B horizon (0.15 m below the Ap), in three of the five sites with E‐B ρb's ≥1.66 Mg m−3, while it had no significant effect on proportion of roots below the E‐B horizon in any of the five sites whose E‐B ρb's were <1.66 Mg−3. A regression analysis showed that ρbof the E‐B horizon, B horizon sand content, and cummulative rainfall accounted for 96% of the observed variation in subsoil root development for the nonsubsoiled treatment across the 10 sites. Soil chemical properties had little effect on variation in subsoil root development across sites.