It must be remembered that the rotation is not confined to a single field. If a perfect system is practiced, there must be as many equal fields concerned in the rotation as there are years in the course, so that every crop is grown on some part of the farm every year. The farm is therefore laid off into shifts or blocks. It is unusual, however, that a farm is sufficiently uniform in surface and soil to allow of such a perfect arrangement, and consequently the output of the various crops varies from year to year. Of course, it is not expected that the entire farm is to be laid under a rotation system. Parts of it will be needed for gardens, orchards, woods, permanent pasture, and for special crops.
Not all the crops of the farm are adapted to rotation. The cereal and hay crops are most adaptable.
Cotton ordinarily is not a part of a rotation scheme; and this is one reason why cotton-lands so soon become "exhausted." The adopting of a short and good rotation, in which cotton would be the pivot crop, would no doubt add immeas urably to the wealth of the south ern states. Some crops occupy the land for a series of years and therefore do not often become parts in a rotation. Of such is alfalfa, now largely grown in the West and rapidly working its way into the East. But even this crop will probably tend more and more to occupy a place in rotation courses ; and in the South (and even in other regions) this may be enforced in order to overcome dis ease affecting the plant.
Usually a rotation contains at least one "money-crop," that finds a direct and ready market ; one clean-tilled crop ; one hay or straw crop ; one leguminous crop. Form erly the manure was applied mostly to one crop in the rotation, but the tendency now seems to be to distribute the application of some kind of fertilizer throughout the various years of the course. Some crops, however, may receive the coarse manure, others the fine or rotted manure, and others the chemi cal fertilizer. It is now thought that there is advantage in rotation of fertilizers. In the Norfolk system, manure is usually applied heavily with the root-course. Grass crops follow clean-tilled or "exhaustion crops." Pas
turing eliminates the weeds of tillage, compacts the land following tillage practice, and provides ma nure in the droppings of the animals.
The leguminous rota tion crops most used in North America are red clover and cowpeas. The clover is adapted to the humid North, cowpeas to the South. The use of the cowpea supplies the missing link in the rotation for the South and makes humus; it adds nitrogen, obviating the necessity of depending on chemical fertilizers alone, which has been such an undesirable practice in the South. Velvet bean and beggar-weed are special leguminous crops sometimes employed in the extreme South.
Nearly all special crops can be grown without rotation, because the market value of their products is so high that the grower can afford to resort to extra manuring and other expensive practices in order to keep the land in good heart. This is the chief reason for the excessive use of stable-manure in mar ket-gardening, a use which usually far exceeds the needs of the crops in mere plant-food. When the land is not too high-priced, it is a practice with gardeners to "rest" part of the land now and then in clover. Orchards do not lend themselves readily to rotation, although peaches generally do not follow peaches directly nor apples follow apples. In order to supply the humus to these lands and at the same time to secure the benefits of tillage, the practice of cover-cropping has lately come into practice. This is the use of some quick-growing crop that can be sown in midsummer or later, after tillage is completed ; usually this is plowed under early the following spring. Acceptable cover-crops are crimson clover, vetches, peas, rye and sometimes buckwheat, rape or cereals.
A contrast of rotations (to be compared with those on succeeding pages). Tabular view of " a regular Succession of Crops in Rotation," as proposed by Varlo in "A New System of Husbandry," Philadelphia, 1785. This is part of a farm scheme for a property of 150 acres, to be stocked with horses, cattle, hogs and sheep. Counting all labor and other outlay, Varlo estimates an annual expense for the six years of £265 16s., and an annual profit of £402 4s.