The phosphorous cycle is not considered a true cycle because unlike the others such as the water, carbon, nitrogen and sulphur cycles, it does not include a gas phase, although small amounts of phosphoric acid may make their way into the atmosphere contributing , in some cases, to acid rain.
Very little phosphorous circulates in the atmosphere because at Earth's normal temperatures and pressures, phosphorous and its various compounds are not gases. The largest reservoir of phosphorous is in sedimentary rock.
