This article provides an essay on the phosphorus cycle.
Phosphorus is a major constituent of biological membranes, energy rich compounds and nucleic acids. In addition, many animals also need large quantities of this element to make shells, bones and teeth.
Since phosphorus does not occur naturally as gas, its cycle, unlike those of carbon and nitrogen, is a sedimentary cycle. Many rocks contain phosphorus, usually in the form of phosphates (PO4-) that are bound into the mineral structure.
When rocks are weathered, minute amounts of these phosphates dissolve and become available to plants. Animals then absorb this element when they eat plants or other animals. Much of the phosphorus excreted by animals is also in the form of phosphate, which plants can reuse immediately.
Thus, on land, phosphate cycles circulate from plants to animals and back again. Land ecosystems preserve phosphorus efficiently, since both organic and inorganic soil particles absorb phosphates, providing a local reservoir of this element.
In an undisturbed ecosystem, the intake and loss of phosphorus are small compared with the amounts of phosphorus that are internally recycled in the day-to-day exchange among plants and animals. Some phosphorus is inevitably lost by leaching and erosion of the soil into stream and rivers.
When an ecosystem is disturbed, as by mining or farming, erosion can become so significant that large quantities of phosphorus and other nutrients are washed away. When phosphate reaches the ocean, it reacts with other minerals and sediment at ocean floor. This is why in some open seas, available open sea have low phosphate level. An overall scheme for phosphorus cycle is given in Fig. 5.22.