FIG. 235.—Tobacco growing on a fine sandy loam soil in southern Maryland., No cover crop is used and the tobacco is grown on the same soil each year. Note the uniformly good growth
Tendency Not in All Soils
TOBACCO Not Always Helped by Rotation
That continuous culture of any particular crop on the same soil very commonly leads, sooner or later, to decreased yields must have been observed in the earliest days of agriculture. Doubtless the primitive form of crop rotation consisted in alternately cropping and resting the land or, in other words, following a rotation of farm crop and a crop of weeds. A hundred years ago scientists who made a study of the subject stated that replacing the weeds in this rotation by useful crops, so as to produce something of value on the soil each year, was to be considered as one of the great advances in the development of agriculture. Practical experience over a long period has abundantly proved the value of systematic crop rotation when applied to many crops, especially when legumes and other soil-improving crops are included in the rotation. It appears, however, that under some conditions tobacco can not be satisfactorily grown in rotation with various other crops.
The early settlers learned that the tobacco plant grew well on virgin land and on old land which had remained idle for a time. It was found also that in many cases after a few crops of tobacco had been grown the land became tobacco tired, even though excellent yields of other crops could still be produced. Recent study of the problem indicates that as a rule these tobacco-sick soils are not improved, so far as concerns tobacco, by addition of fertilizer, manure, or lime.
The system of cropping seems to be an important factor in the development or persistence of the trouble in the soil. Although continuous culture of tobacco may lead to the tobacco-sick condition of the soil, the use of leguminous or nonleguminous cover crops or rotations with various other crops may aggravate rather than remedy this condition. In this disease the roots of the tobacco plant fail to make normal growth and are brownish in color so that this trouble is often spoken of as brown root rot. This trouble seems to be distinct from the well known black root rot which is a fungous disease. So far no plant parasite or germ has been found to be connected with the disease and its exact nature has not been determined.
It is remarkable that crops like tobacco and its relative, the tomato, which are most susceptible to this disease are less active in producing it or in intensifying its effect than are other crops like corn, timothy, and rye on which the disease has little effect. Here is a case, then, where a crop may be injured rather than benefited by use of winter cover crops or the usual sort of crop rotation system intended to improve the productiveness of the soil. To bring out this fact more clearly some results of recent cropping tests may be cited. On sandy loam soil of a tobacco farm in Massachusetts a timothy sod was turned under in the spring. A liberal application of fertilizer was made to the soil and rotations including various crops were started. Although in the first year corn yielded more than 100 bushels per acre after the timothy, the yield of tobacco was only about 500 pounds. By continuing tobacco on the same land the yield two vears later had increased to more than 1,600 pounds, while after two crops of corn the yield was 840 pounds; after two years of clover it was 480 pounds, and after two years of timothy, 280 pounds. Where the land remained idle for two years the yield of tobacco was more than 1,600 pounds.
The tendency to become tobacco-sick is not found in all soils. The above cropping tests were repeated on another tobacco soil at the Connecticut tobacco substation located at Windsor and in this instance there were no important differences in effects of the various crops on the yields of tobacco. It is well known that some lands have grown tobacco continuously for more than a half century without any loss of productiveness. There are also some lands on which tobacco has long been grown with satisfactory results in rotation with wheat, clover, timothy, and corn. On the other hand, the tobacco-sick condition has been observed on sandy, sandy loam, and heavy loam types of soil and in various tobacco-growing sections.
Another important fact about the brown root-rot disease is that seasonal conditions may greatly modify the effects of the cropping system. On a fine sandy loam soil in southern Maryland hairy vetch as a winter catch crop has practically doubled the yield of tobacco in some years, whereas in other years the yield of tobacco has been considerably less than where no catch crop has been used. Similar results have been observed with rye and timothy as winter cover crops. The greatest depressing effect of these crops on the yield of tobacco usually occurs in wet seasons.
About the only simple remedy for the brown root rot known at the present time consists in resting the land for a year or longer, thus allowing the weeds to grow. It is a surprising fact that for tobacco culture on many soils no combination of crops in the various rotations which have been tried equals an alternation of tobacco and weeds. It appears, however, that one of the essential features of this plan is to allow the soil to remain undisturbed for a time. The exact function of the weeds which ordinarily grow under these circumstances has not been determined. It seems certain that the weeds at least produce no harmful effect on the tobacco, such as is apt to occur from a number of our ordinary farm crops when grown in the rotation.