Another sustainable agricultural practice is called soil enrichment. Soil enrichment is the most appreciated element of agricultural ecosystems. Insects and microbes are usually killed by the use of pesticides. Good soil helps yields and makes crops less susceptible to pests. Cropped soils usually require many more fertilization applications to produce high yields. Soil quality can be improved in many different ways, for example by leaving crop residues within the field after harvest, by plowing, or by adding animal manure or composted plants. Another sustainable farming technique is called cover crops. Cover crops are when farmers have plants growing in the ground at all times because they help suppress weeds, prevent soil erosion, reduce the need for chemical inputs such as fertilizers, herbicides and insecticides, and improve soil quality