CALCIUM CYCLE

There are three components of the calcium cycle: calcium precipitation, exchangeable calcium and dissolved calcium. Various rocks and minerals in the lithosphere provide natural sources of calcium) (e.g. limestone and calcite), and the weathering of these minerals slowly dissolves some of the calcium from its sequestered state. The dissolved calcium is in its ionic state in bodies of freshwater, the oceans and soils. Concentrations of calcium ions (Ca2+) in freshwater are typically 0.01–0.1 mM, whereas in seawater concentrations are 100–1,000 times greater (see seawater composition). The calcium ion remains in the seawater until it is precipitated out to form calcium carbonate (CaCO3) or more rarely calcium sulfate (CaSO4). The upper levels of the ocean are supersaturated with Ca2+ and carbonate ions (see supersaturation), although the amount varies with different locations and conditions. Microorganisms accumulate CaCO3 and are responsible for much of the calcium deposition, as when the organisms die their shells fall to the ocean floor and accumulate or dissolve depending on the depth, temperature and pressure (see carbonate[Page 123]compensation depth (CCD)). The majority of the calcium makes its way back to land by major geological processes, uplifting the calcium carbonates deposits to the surface or near surface.

Calcium is the dominant cation in most soils and it is readily available in solution for uptake by plants. Calcium is important in the physical structure of plants, and most of the calcium is returned to the soil with leaf fall, or is ingested by animals eating the plants. Microbes decompose the fallen leaves and woody material, which in turn release the calcium back to the soil so that it becomes available again to be taken up by plants. In soil, calcium is a mobile nutrient and it may be lost to leaching, retained by soil particles or precipitated as secondary minerals.

[See alsobiogeochemical cycles, decomposition]