The element phosphorus is part of many molecules that make
up the cells of living organisms. For example, phosphorus is needed to form bones and teeth in animals. Plants get the phos- phorus they need from soil and water, while animals get their phosphorus by eating plants or other animals that have eaten plants. The phosphorus cycle is the movement of phosphorus from the environment to organisms and then back to the environ- ment. This cycle is slow and does not normally include the atmosphere because phosphorus rarely occurs as a gas.
As shown in Figure 14, phosphorus may enter soil and water in a few ways. When rocks erode, small amounts of phosphorus dissolve as phosphate in soil and water. Plants absorb phosphates in the soil through their roots. In addition, phosphorus is added to soil and water when excess phosphorus is excreted in waste from organisms and when organisms die and decompose. Some phosphorus also washes off the land and eventually ends up in the ocean. Many phosphate salts are not soluble in water, so they sink to the bottom of the ocean and accumulate as sediment.