Unlike the carbon cycle, the phosphorous cycle does not involve a gas phase and is therefore not considered as a true cycle. This is what distinguishes it also from such cycles as the water and nitrogen cycles. Very little phosphorous circulates in the atmosphere because at earth's normal temperatures and pressure, phosphorous together with its various compounds are not in gaseous form, though small amounts of phosphoric acid may make their way into the atmosphere, and contributing to acid rain in some cases. The largest reservoir of phosphorous is in sedimentary rock.