<P> Unlike other cycles, P is not normally found in the air as a gas; it only occurs under highly reducing conditions as the gas phosphine PH . </P> <P> The availability of phosphorus in an ecosystem is restricted by the rate of release of this element during weathering . The release of phosphorus from apatite dissolution is a key control on ecosystem productivity . The primary mineral with significant phosphorus content, apatite (Ca (PO) OH) undergoes carbonation . </P> <P> Little of this released phosphorus is taken up by biota (organic form), whereas a larger proportion reacts with other soil minerals . This leads to precipitation into unavailable forms in the later stage of weathering and soil development . Available phosphorus is found in a biogeochemical cycle in the upper soil profile, while phosphorus found at lower depths is primarily involved in geochemical reactions with secondary minerals . Plant growth depends on the rapid root uptake of phosphorus released from dead organic matter in the biochemical cycle . Phosphorus is limited in supply for plant growth . Phosphates move quickly through plants and animals; however, the processes that move them through the soil or ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles . </P> <P> Low - molecular - weight (LMW) organic acids are found in soils . They originate from the activities of various microorganisms in soils or may be exuded from the roots of living plants . Several of those organic acids are capable of forming stable organo - metal complexes with various metal ions found in soil solutions . As a result, these processes may lead to the release of inorganic phosphorus associated with aluminium, iron, and calcium in soil minerals . The production and release of oxalic acid by mycorrhizal fungi explain their importance in maintaining and supplying phosphorus to plants . </P>

Why does it take millions of years for phosphorus to complete its cycle