"Calcium is primarily present as rock, minerals or as structural calcium built into mineral crystal lattices of soil particles and is not readily available. Calcium can also be added as fertilizer, lime or by-products. Water can carry calcium into the soil through weathering and natural dissolution.
When in the soil, most of the calcium is in an insoluble form until it is ‘weathered off’ of minerals or when organic matter is broken down by microbes into soluble calcium. However, some of the calcium are held loosely or tightly on soil’s cation exchange complex (CEC) or in the soil solution and are available to plants and microorganisms.
When animals, microorganisms, or plants decay, their bodies decompose and the calcium is mineralized and released back into the soil. Roots also regularly leak minerals, sugars, and other compounds back into the soil including calcium.
Since calcium is a positively charged ion, it is adsorbed in the soil to the surface of clay and organic particles which are negatively charged. Positively charged ions (cations) adsorb to soil particles and are termed “exchangeable ions” because they can be exchanged by other ions present in the soil solution" (Jason 2014).