Fertilizer, material added to soil to provide essential plant nutrients. Fertilizers increase crop yields and, when used properly, add to the efficiency of farming and gardening. They include both organic materials, such as farmyard manure, compost, and activated sewage sludge, and inorganic chemical salts, known as artificial fertilizers, which contain phosphorus, nitrogen, and potassium as needed in a particular crop. Fertilizers may be used to change the mineral, vitamin, and protein contents of the produce. There is evidence that prolonged and overintensive use of fertilizers may damage the fertility of the soil by disrupting the communities of microorganisms in the soil, and that fertilizers that enter rivers and lakes contribute to the process of eutrophication (deficiency of dissolved oxygen and overabundance of dissolved nutrients, e.g., phosphates) that has polluted many bodies of water.