<P> On the land, phosphorus (chemical symbol, P) gradually becomes less available to plants over thousands of years, because it is slowly lost in runoff . Low concentration of P in soils reduces plant growth, and slows soil microbial growth - as shown in studies of soil microbial biomass . Soil microorganisms act as both sinks and sources of available P in the biogeochemical cycle . Locally, transformations of P are chemical, biological and microbiological: the major long - term transfers in the global cycle, however, are driven by tectonic movements in geologic time . </P> <P> Humans have caused major changes to the global P cycle through shipping of P minerals, and use of P fertilizer, and also the shipping of food from farms to cities, where it is lost as effluent . </P> <P> Phosphorus is an essential nutrient for plants and animals . Phosphorus is a limiting nutrient for aquatic organisms . Phosphorus forms parts of important life - sustaining molecules that are very common in the biosphere . Phosphorus does not enter the atmosphere, remaining mostly on land and in rock and soil minerals . Eighty percent of the mined phosphorus is used to make fertilizers . Phosphates from fertilizers, sewage and detergents can cause pollution in lakes and streams . Overenrichment of phosphate in both fresh and inshore marine waters can lead to massive algae blooms which, when they die and decay, leads to eutrophication of fresh waters only . An example of this is the Canadian Experimental Lakes Area . These freshwater algal blooms should not be confused with those in saltwater environments . Recent research suggests that the predominant pollutant responsible for algal blooms in salt water estuaries and coastal marine habitats is Nitrogen . </P> <P> Phosphorus occurs most abundantly in nature as part of the orthophosphate ion (PO), consisting of a P atom and 4 oxygen atoms . On land most phosphorus is found in rocks and minerals . Phosphorus rich deposits have generally formed in the ocean or from guano, and over time, geologic processes bring ocean sediments to land . Weathering of rocks and minerals release phosphorus in a soluble form where it is taken up by plants, and it is transformed into organic compounds . The plants may then be consumed by herbivores and the phosphorus is either incorporated into their tissues or excreted . After death, the animal or plant decays, and phosphorus is returned to the soil where a large part of the phosphorus is transformed into insoluble compounds . Runoff may carry a small part of the phosphorus back to the ocean . Generally with time (thousands of years) soils become deficient in phosphorus leading to ecosystem retrogression . </P>

Phosphorus which is often a limiting nutrient in ecosystems is important because it