Rotation helps to maintain or increase the plant food in the surface soil. When crops like cowpeas or clover which take mineral food from the subsoil and nitrogen from the air, are plowed under, they give up the plant food in their leaves, stems and upper roots to the surface soil, and thus help to maintain or increase fertility.
Rotation tends to protect crops from injurious insects and diseases. If one kind of crop is grown continuously on one piece of land the soil becomes infested with the insects and diseases which injure that particular crop. If the crop is changed, the insects and diseases find difficulty in adapting themselves to the change and consequently diminish in numbers.
Rotation helps to keep the soil free from weeds. "If the same kind of crop were grown year after year on the same field, the weeds which grow most readily along with that crop would soon take possession of the soil." For example, chick weed, dock, thistle, weeds peculiar to grain and grain crops tend to increase if the land is long occupied by these crops.
Rotation helps the farmer to make a more even distribution of labor throughout the year. This is because crops differ as to the time of year at which they are planted and harvested.
Rotation of crops enables the farmer to provide for his stock more economically. Live stock fares better on a variety of food, which is more cheaply secured by a system of rotation than otherwise.
THE TYPICAL ROTATION
A typical rotation for general farming should contain at least:
One money crop which is necessarily an exhaustive crop.
One manurial crop which is a soil enricher.
One feeding crop which diminishes fertility only a little.