Tobacco

W. E. Colwell.

Tobacco is unusually sensitive to soil conditions. Practices known to be effective for other crops often must be modified because of its sensitivity to the nitrogen supply, soilborne diseases, excess water, and physical properties of the soil.

Flue-cured or bright tobacco generally is grown on light-colored, sandy loams and fine sandy loams in the Piedmont and Coastal Plain of the South Atlantic States. Fields best suited for this type of tobacco have relatively low levels of organic matter and calcium and a friable, sandy clay subsoil.

The heavier soils used for bright tobacco are usually in the Piedmont. The subsoil has a vital bearing on the suitability of the land for high-quality tobacco. It should be heavy enough to retain moisture yet permit root penetration and air exchange.

The nitrogen supply has to be controlled within narrow limits. Legumes cannot be used in the rotation except on the very sandy soils. Even then they should not immediately precede tobacco, because normal ripening requires a declining nitrogen supply. Otherwise, the leaves are green, coarse, and, when cured, dark and without a desirable flavor or aroma. They may also contain unusually high levels of protein nitrogen or nicotine.

To avoid those troubles, nonleguminous crops are used in rotation with tobacco, and the nitrogen supply is regulated with commercial fertilizer. It is common to apply 35 to 50 pounds of nitrogen an acre or more or less, depending on the kind of soil.

About 65 pounds of phosphoric oxide (P₂O₅) an acre are required to maintain the high level of soil phosphate that characterizes most tobacco soils that have been heavily fertilized for many years. Potash is applied in amounts ranging between 90 and 130 pounds of K₂O.

The choice of cropping practice is dictated largely by the soilborne diseases and the need for protection against loss of soil and water.

Varieties resistant to three common diseases bacterial wilt, black shank, and fusarium wilt are available, but the widespread occurrence of nematodes has created a serious problem. None of the commercial varieties available in 1957 carried resistance to nematodes, and the grower has had to depend on soil fumigation and crop rotation.

Three groups of nematodes attack tobacco root knot, meadow, and stunt. Among them are species and strains that differ in their preference for crops. For example, corn rotated with tobacco generally gives good control of root knot, but it is ineffective against some strains of root knot nematode. Corn is a favored host of the meadow and the stunt nematodes, and therefore has little or no value in this control. Oats followed by weeds generally may give good control of all kinds of nematodes.

Careful study of the disease problem, proper selection of varieties, and proper methods of soil fumigation against nematodes are needed to assure the production of tobacco of good quality.

Often the combination of practices required for control of disease brings new problems of soil management.

1. Rowan lespedeza and peanuts are susceptible to Meloidogyne hapla species of the root knot nematode, which is commonest in peanut-growing areas.

For example, the use of soil fumigation may require adjustment in the nitrogen composition of the fertilizer because of reduced nitrification rates. The addition of chlorides through the fumigant or irrigation water may call for a reduction of fertilizer chlorine. The grower who irrigates is tempted to use the fields nearest the source of water more intensively in shorter rotations.

Tobacco is highly sensitive to a waterlogged soil. Flooding injury may be apparent within an hour after a heavy rain on a hot day. If drainage is impeded so that the root zone remains saturated for several days, permanent, injury or death usually results.

Layout of rows, especially on slightly rolling fields, is important from the standpoint of proper drainage and conservation of topsoil.

In experiments near Raleigh, N. C., control of row grade was an effective management practice to cope with rains of heavy intensity during the critical summer period. A contour row system was set up between terraces so that each row had a slight but continuous grade and carried its own water, as a miniature terrace. Flat middles between rows ridged in the conventional way gave better control of water than did the V-shaped middles. This method provides a broad water channel. Undecomposed residue left on the surface helped to reduce loss of soil and water during the summer. Winter cover crops and other residues incorporated in the soil decayed too quickly to be of benefit during the summer.

BURLEY TOBACCO, like the fire-cured and dark air-cured types, requires highly fertile soils, quite different from bright tobacco. Entirely different systems of management therefore must be followed.

Burley tobacco is grown mainly on silt foams of limestone origin in the bluegrass region of Kentucky and the Central Valley of Tennessee. Farm Manure and the use of grasses and legumes of various types in rotations are beneficial. Two years or more of sod well fertilized with phosphate and potash make a desirable cropping practice ahead of burley tobacco.

Under those conditions and with manure, it is common to supply directly to the tobacco crop 20 to 40 pounds of nitrogen an acre, bringing the total to a level between 90 and 120 pounds. Flue-cured types have a total requirement of about one-third to one-half that amount, which is applied directly to the tobacco crop.

It makes but little difference which grasses or legumes are used in the rotation just so they occupy the land during most of the rotation.

Bluegrass, orchardgrass, fescue, timothy, redtop, and smooth bromegrass sods have been used successfully. Red clover, sweetclover, Korean lespedeza, and alfalfa have been used in rotations alone and in combination with grass. Combinations of these grasses and legumes over a period of years have produced tobacco of good quality.

Good production has been obtained consistently at the Kentucky Agricultural Experiment Station from sod rotations, but erratic results have been had from continuous tobacco.

Black root rot, once a major consideration in developing a program of soil management, is controlled by using varieties resistant to it. Nematodes have been less troublesome than With flue-cured tobacco.

Soil management for fire-cured and dark air-cured tobacco is essentially the same as for burley. The soils are somewhat less fertile than those used for burley, and higher nitrogen rates do not have the same undesirable effect on quality.

MARYLAND TOBACCO, a light, air-cured type, is produced on loams, sandy loams, and loamy sands of medium fertility.

Two-year and three-year rotations predominate. The latter include a grass-legume mixture. Rotations have largely replaced the continuous culture methods, which involved long rest periods after a series of years of tobacco after tobacco.

The fertilization practices are quite similar to those for flue-cured leaf. Barnyard manure in large amounts prevents normal ripening of the leaf.

CIGAR TOBACCO is grown on many soil types because requirements differ for wrapper, binder, and filler classes of leaf. In the Connecticut Valley, sandy to very fine, well-drained sandy loams predominate. Near Quincy, Fla., the soils used for cigar wrapper are like those used for flue-cured types. The soils of Wisconsin used for cigar binder are dark brown, of silt loam texture, and fairly high in organic matter.

Near Lancaster, Pa., the soils used to produce cigar filler are highly fertile, yellow to brown loams or silt loams. The soils for cigar filler in southwestern Ohio are silt loams and silty clay loams of dull brown to gray color.

Rotations of 3 to 5 years are generally practiced following legume-grass sod or corn.

Continuous tobacco is the rule for the cigar-wrapper tobacco of the Connecticut Valley. Artificial shade is required. Rotation of crops is considered impractical. It is said that tobacco has been grown continuously on the same fields for more than 50 years without deterioration in yield or quality. That has been possible because of the heavy manuring and fertilization and because no highly destructive soilborne diseases have developed.

Continuous culture of tobacco is not without its problems, however. It is common to rest shade-tobacco land after 4 or 5 years, when yields and quality begin to decline. Some fields remain in high production only 2 or 3 years. Corn or potatoes may be planted. Sometimes the tobacco fields are left in fallow.

The best management practices in the Florida-Georgia area include a 2-year rotation, with cockleburs or Texas millet planted in summer and oats in winter. Fumigation of the soil to control nematodes as a fall treatment is essential in this rotation.

It is common to apply to the tobacco crop 200 pounds of nitrogen an acre-120 pounds of P₂O₅ and 200 pounds of K₂O. A cover crop of oats or rye is commonly used. One of its major values is to conserve plant nutrients against leaching. In a lysimeter experiment at Windsor, Conn., a cover crop of oats took up 56 pounds of nitrogen, 44 pounds of calcium, 24 pounds of potassium, and 8 pounds of magnesium to the acre in a year, and so kept the nutrients from leaching away.