Crop Rotation is a method of maintaining soil fertility and structure by planting a particular parcel of agricultural land with alternating plant species. Most crop rotation schedules require that a field contain a different crop each year, and some schemes incorporate times when the field remains uncultivated, or lies fallow. Farmers rotate crops to control erosion, promote soil fertility, contain plant diseases, prevent insect infestations, and discourage weeds. Crop rotation has been an important agricultural tool for thousands of years. Modern organic farmers depend on crop rotation to maintain soil fertility and to fight pests and weeds, functions that conventional farmers carry out with chemical fertilizers and pesticides. Crop rotation can also prevent or correct some of the problems associated with monocultures (single crop farms),including persistent weeds, insect infestations, and decreased resistance to plant diseases.