Crop rotation has long been recognized as a necessity. Its value has been more evident, perhaps, in the production of cereals and the general farm crops than in vegetable growing, but it is scarcely less important. Numerous examples might be cited of decline in yields and quality, due primarily to the failure to rotate. In some instances entire communities have been forced practically to abandon the culture of a certain crop mainly because of its continuous or too frequent production.
Plants differ greatly in their food requirements. Some utilize large amounts of nitrogen, while others must have liberal supplies of the mineral elements. Crops which have about the same food requirements should not be planted consecutively, if it can be avoided. When cabbage follows cabbage, or lettuce follows lettuce, a heavy draft is made upon the supply of nitrogen. If, instead, these crops were followed by tomatoes the food supply would be utilized more economically, successful crop production would be more certain and the yielding power of the soil would be conserved to better advantage. Many examples might be given to illustrate this point. While this principle should always be considered, certain conditions, as the use of muck soils or the accessibility of cheap supplies of manure, may justify a system of cropping which would not be permissible under other circumstances.
Crops differ widely in their ability to supply humus to the soil, and therefore to change the physical as well as the chemical properties of the soil. Some crops, as bunch onions, spinach and lettuce, leave practically no refuse on the ground, while others, as turnips, beets and potatoes, furnish considerable quantities of vegetable matter. Another class of plants, the legumes, serve as nitrogen traps, in addition to supplying humus of the most valuable character. The leguminous plants, however, vary greatly as soil improvers. The manurial crops as the clovers, vetch and cowpeas, have already been considered, but field and garden types of peas and beans deserve greater consideration in crop rotations because of their value as soil improvers.
Experiments made by the United States Department of Agriculture indicate that the roots of plants (at least of some species) exude toxics or substances which may not be injurious to other plants, but are poisonous to themselves. While there is no proof that this is true concerning vegetables, it is probable, and the principle should be regarded when planning rotations for all types of vegetable gardening.
Numerous allusions have been made on preceding pages concerning the importance of rotation to avoid losses from the ravages of insects and plant diseases. A multitude of insects and disease spores pass the winter in the ground or are protected by refuse on the surface. If the host plants are grown annually these pests are likely to become more and more destructive. This is one of the strongest arguments for crop rotation in vegetable gardening. Ordinarily, three to five years should elapse between crops of the same species, although less time is sufficient in many instances.
Several additional factors should be taken into account, as the profits of the various crops that might be grown, crop adaptation, and the physical and chemical properties of the soil.
It is common for truck crops to be included in a three or four-year rotation with general farm crops. Sweet corn, cucumbers, squashes, early and late cabbage or cauliflower often follow clover in either a three or four-year rotation which, in the North, frequently includes wheat. In trucking districts, where the general farm crops are not grown to any considerable extent, the practice varies considerably. When climatic conditions permit, a winter cover crop should be started on all cultivated areas (unless a cash crop, as spinach or cabbage, is to be planted in the fall and marketed in the spring), and this will be valuable from the standpoint of rotation.