Why do farmers change what grows in a field?

A field that grew beans one year may grow a cereal crop the next. Changing crops in a planned order is called crop rotation. The soil stays in place, but the living conditions in that soil change with the crop.

A pest that depends on one kind of plant can build up when that plant returns again and again. If the next crop is a poor host, the pest may find less food or fewer places to reproduce. Rotation does not stop every pest: some can feed on many plants or remain in soil for years. It can still interrupt an important part of a pest's life.

Plants also differ below ground. Their roots explore different spaces and leave different amounts of material after harvest. Bacteria living with legumes, such as peas and beans, can turn nitrogen gas into forms living things use. Some of that nitrogen may become available after plant material breaks down. Harvesting seeds removes nutrients too, so a legume crop does not automatically replace everything taken away.

A rotation is therefore a way to change several relationships over time. Farmers match the sequence to climate, soil, water and the crops they can grow. A field's next harvest depends partly on what lived there before.