Fertilizers are materials added to the soil to provided elements needed for plant nutrition, and so to enable healthy growth of crops with high yield. The elements needed in large quantities are nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium; the last three are usually adequately supplied in the soil or incidentally in other fertilizers. Small amounts of trace elements are also needed, and usually supplied in fertilizers. The choice of compounds or materials containing nitrogen, phosphorus, and potassium depends mainly on cost. Fertilization in excess may harm crops and cause eutrophication.