Biochemical Cycles  Elmhurst College Carbon Cycle Phosphorus Cycle  Chemistry Department Nitrogen Cycle    Virtual ChemBook

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Phosphorus Cycle Phosphorus enters the environment from rocks or deposits laid down on the earth many years ago. The phosphate rock is commercially available form is called apatite. Other deposits may be from fossilized bone or bird droppings called guano. Weathering and erosion of rocks gradually releases phosphorus as phosphate ions which are soluble in water. Land plants need phosphate as a fertilizer or nutrient.   Phosphate is incorporated into many molecules essential for life such as ATP, adenosine triphosphate, which is important in the storage and use of energy. It is also in the backbone of DNA and RNA which is involved with coding for genetics.   When plant materials and waste products decay through bacterial action, the phosphate is released and returned to the environment for reuse.   Much of the phosphate eventually is washed into the water from erosion and leaching. Again water plants and algae utilize the phosphate as a nutrient. Studies have shown that phosphate is the limiting agent in the growth of plants and algae. If not enough is present, the plants are slow growing or stunted. If too much phosphate is present excess growth may occur, particularly in algae.   A large percentage of the phosphate in water is precipitated from the water as iron phosphate which is insoluble. If the phosphate is in shallow sediments, it may be readily recycled back into the water for further reuse. In deeper sediments in water, it is available for use only as part of a general uplifting of rock formations for the cycle to repeat itself.

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Human Inputs to the Phosphorus Cycle: Human influences on the phosphate cycle come mainly from the introduction and use of commercial synthetic fertilizers. The phosphate is obtained through mining of certain deposits of calcium phosphate called apatite. Huge quantities of sulfuric acid are used in the conversion of the phosphate rock into a fertilizer product called "super phosphate".   Plants may not be able to utilize all of the phosphate fertilizer applied, as a consequence, much of it is lost form the land through the water run-off. The phosphate in the water is eventually precipitated as sediments at the bottom of the body of water. In certain lakes and ponds this may be redissolved and recyled as a problem nutrient.   Animal wastes or manure may also be applied to the land as fertilizer. If misapplied on frozen ground during the winter, much of it may lost as run-off during the spring thaw. In certain area very large feed lots of animals, may result in excessive run-off of phosphate and nitrate into streams.   Other human sources of phosphate are in the out flows from municipal sewage treatment plants. Without an expensive tertiary treatment, the phosphate in sewage is not removed during various treatment operations. Again an extra amount of phosphate enters the water.