The phosphorus cycle is a sedimentary cycle (unlike carbon, oxygen, and nitrogen), the atmosphere is not a reservoir for phosphorous nor do microorganisms fix phosphorus as they do nitrogen. Phosphorus enters the biosphere almost entirely from the soil through absorption by plant roots. Weathering of rocks containing phosphate minerals, chiefly apatite [Ca 5(PO 4) 3OH], results in the relatively small pool of inorganic phosphorus available for organismal use. In most soils the major amount of phosphorus absorbed by plants comes from organic molecules that undergo decomposition releasing phosphorus in plant‐available inorganic forms. The release of organically bound nutrients to plant‐available forms is termed mineralization, a process important in the release to the soil of sulfur and nitrogen as well as phosphorus. Phosphorus is used by organisms in energy transfers (ATP, NAPD), as a component of nucleic acids (RNA, DNA), and as a structural element of membranes (phospholipids).
The phosphorus cycle has fewer compartments than the other major nutrient cycles and also has a significant “leak” of phosphorus back to its lithosphere reservoir from which it is returned to active cycling only after long intervals of geologic time. The combination of three factors makes phosphorus a nutrient of concern in most ecosystems:
- Most soils have only small amounts from the weathering of disjunctly distributed rocks.
- Phosphorus is more insoluble than other nutrients and less mobile, hence less phosphorus travels in the soil solution; roots generally must grow into a zone of phosphorus availability.
- Phosphorus that drains from the land to the ocean is used by organisms in the surface waters, but a considerable amount is lost to the sediments in the shells and bones of marine organisms and by precipitation and settling of phosphates.
Human activities alter the phosphorus cycle chiefly by adding more available phosphorus where little was available previously. Phosphate‐containing detergents used in the 1960s were carried by sewage systems into rivers and lakes and were a boon to algae and microorganisms, which responded with exuberant flushes of growth. Widespread eutrophication resulted and detergent‐makers were obliged to remove phosphates from their products. Eutrophication—the enrichment of fresh waters with nutrients—results in blooms of plankton and algae. Death of these organisms increases the populations of aerobic bacteria of decay which, in turn, deplete the dissolved oxygen in the waters, thereby killing fish and other aerobic organisms. The anaerobic microorganisms move in, and the fresh water becomes an unpleasant, smelly soup of decay.
Agricultural use of phosphate‐containing fertilizers has increased as the acreage of farmlands has expanded over time. At first, guano (the dung of seabirds) was collected from deposits on seashore rocks and added to the fields, but demand by inland farmers for phosphate fertilizer stimulated the mining of phosphate deposits (ocean sediments of past geologic ages). These applications, too, wash out of the fields into the world's waters and also can cause eutrophication.