Fertilizer is a natural or artificial substance added to soil, containing chemicals to improve plant growth by increasing fertility. Manure and compost were the first fertilizers. Other natural substances, such as bone meal, ashes, guano, and fish, have been used for centuries. Modern chemical fertilizers, composed of nitrogen, phosphorus, and potassium compounds in powdered, liquid, or gaseous forms, are now widely used. Specialized fertilizers also contain trace elements.