Crop rotation is the practice of growing every year (or every season) a different crop on the same field. It is a practice which is generally recommended to reduce the impact of pests and diseases and to manage soil health.
If the same crop is planted year after year on the same plot, populations of certain pests and diseases can gradually increase. Growing the same crop repeatedly will also cause the fertility of the soil to diminish rapidly. Crop rotation is recommended to avoid these problems.
Without crop rotation, nematodes can become a serious problem. In a field cultivated with tomato, root knot nematodes (Meloidogyne sp.) will start to develop. They will survive for some time in the soil after the harvest. If in the next season the farmer plants again tomatoes, these nematodes can multiply rapidly and become a serious problem. The same problem may arise if another crop that is sensitive to these nematodes (for example Okra) is planted.
However, if a crop is planted that is not attacked by Meloidogyne, the population of nematodes in the soil will diminish. A tomato crop should therefore always be followed by a crop which is is not sensitive to these nematodes (for Maize or other cereals).
Also to avoid problems with certain soil borne diseases, crop rotation should be practiced.
Rotation of cereal crops with legumes is often very useful. These crops are very different so that insects and diseases which attack the first one will not attack the other. The legumes in the rotation will also help to improve the soil fertility through fixation of nitrogen.
Also fallow (especially when the fields are inundated) will help to reduce pest populations (e.g. nematodes) and reduce the danger of soil-borne diseases.