<P> On the land, phosphorus gradually becomes less available to plants over thousands of years, because it is slowly lost in runoff . Low concentration of phosphorus 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 phosphorus in the biogeochemical cycle . Locally, transformations of phosphorus 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 phosphorus cycle through shipping of phosphorus minerals, and use of phosphorus 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>

Why is the phosphorus cycle slower than the carbon and nitrogen cycle