Phosphorus is a chemical element with symbol P and atomic number 15. Phosphorus is never found as a free element on Earth. As an element, phosphorus exists in two major forms—white phosphorus and red phosphorus.
Phosphorus is essential for life. Phosphates (compounds containing the phosphate ion, PO4−3) are a component of DNA, RNA, ATP, and also the phospholipids, which form all cell membranes. Phosphorus was first isolated from human urine, and bone ash was an important early phosphate source. Phosphate minerals are fossils. Low phosphate levels are an important limit to growth in some aquatic systems. In a commercial sense, the vast majority of phosphorus compounds are consumed as fertilisers. Phosphate is needed to replace the phosphorus that plants remove from the soil, and its annual demand is rising nearly twice as fast as the growth of the human population. Other applications include the role of organophosphorus compounds in detergents, pesticides, and nerve agents.