The phosphorus cycle is the way that compounds of the element phosphorus, an essential nutrient, are recycled in the environment. Plants absorb phosphorus compounds from the soil, and decaying organic matter, including both plant matter and animal droppings, returns it to the soil. Phosphorus is a vital part of ATP, the energy currency of all cells, and is vital to all forms of life.
The phosphorus cycle is not perfect. Losses occur, mainly through the action of rain eventually washing phosphorus into rivers and out to sea. Fortunately, in a stable ecosystem such losses are minimal, and other natural processes renew the supply. In particular, the weathering of rocks releases new phosphorus compounds into the environment. Phosphorus is a major limiting agent on the growth of both plants and algae. Where not enough phosphorus is present, plant growth is stunted.
This loss of phosphorus is even more of a problem on farms, where many elements of the plants are moved away from the soil where they grew, thus denying a crucial part of the phosphorus cycle from occurring. Thus, farmers using conventional farming methods must use fertilizers to enhance their soil. Some types of small-scale farming, however, are much slower to deplete soils, and return phosphorus-bearing wastes to the soil.