System of regularly changing the crops grown on a piece of land. The crops are grown in a particular order to utilize and add to the nutrients in the soil and to prevent the build-up of insect and fungal pests. Including a legume crop, such as peas or beans, in the rotation helps build up nitrate in the soil, because the roots contain bacteria capable of fixing nitrogen from the air. A simple seven-year rotation, for example, might include a three-year ley (grassland) followed by two years of wheat and then two years of barley, before returning the land to temporary grass once more. In this way, the cereal crops can take advantage of the build-up of soil fertility which occurs during the period under grass. In the 18th century a four-field rotation was widely adopted; over four years a field might be planted with autumn-sown cereal, followed by a root crop, then spring cereal, and finally a leguminous crop. Innovative farmers such as Charles ‘Turnip’ Townshend improved cultivation…