The phosphorus cycle starts with rocks and deposits on Earth from a long time ago. Weathering and erosion slowly breaks down the rocks and releases phosphorus which plants use as a fertilizer. When the plants die and decay, the phosphate is released back into the soil and used in the environment once again (See chart below). The phosphorus may be "locked" in the rocks and deposits for long periods of time. It may also be precipitated into an iron phosphate ion which is insoluble.
Humans always seem to figure out a way to screw up the environment. One way humans disrupt the phosphorus cycle is from synthetic fertilizers. Plants might not be able to use all of the phosphorus and much of it is lost in water run-off. The run-off can lead into rivers and lakes and disrupt the phosphorus balance in that ecosystem.
Some interesting facts about phosphorus is that it is responsible for holding DNA together. It also moves through plants and animals very quickly. Additionally, the majority of phosphorus that is mined is used for artificial fertilizers.
