As crops grow, they remove nutrients from the soil; however, some crops return certain nutrients over the seasons and between species. Unlike a natural ecosystem, in which there is a balanced exchange of nutrients between old growth and new growth, a crop system is totally dependent on the farmer's techniques. Farmers have three alternatives for keeping their fields productive. They can add natural or chemical fertilizers; they can let the field remain fallow, or uncropped, for one or more growing seasons; or they can rotate their crops on a regular basis.

Crop rotation requires a knowledge of how different crops interact with the soil. One of the most fundamental and earliest known facets of crop rotation was the use of legumes, which include peas, beans, and lentils. Legumes, as well as other crops like oats, return vital nitrogen to the soil. The Greeks and Romans rotated...