Crop rotation is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons to help restore plant nutrients and mitigate the build-up of pathogens and pests that often occurs when one plant species is continuously cropped. Rotation can also improve soil structure and fertility by alternating deep-rooted and shallow-rooted plants. Turnip roots, for example, can recover nutrients from deep under the soil. The Norfolk four-course system, as it is now known, rotates crops so that different crops are planted with the result that different kinds and quantities of nutrients are taken from the soil as the plants grow. An important feature of the Norfolk four-field system was that it used labour at times when demand was not at peak levels.