Forms of Calcium in Soil

Calcium is the main exchangeable base of clay minerals and, as such, is a major component of soils. One of the most important natural sources of calcium is underlying limestone or chalk, where it occurs as calcium carbonate (calcite). Calcium in rocks also occurs as a mixture of calcium and magnesium carbonates (dolomite). Soils over such rocks often contain large amounts of calcium carbonate, although not invariably so. The soils may not have been derived from the rock, but have come from elsewhere and been deposited by glaciers, and furthermore, although calcium carbonate is sparingly water soluble, it can be removed by leaching so that the overlying soil may be depleted of calcium carbonate and be acidic.

Some soils contain calcium sulfate (gypsum), but mostly only in arid regions. A further source of calcium in soils is apatite [Ca(OH₂).3Ca(PO₄)₂] or fluorapatite [Ca₅(PO₄)₃F]. Chlorapatite [Ca₅(PO₄)₃Cl] and hydroxyapatite [Ca₅(PO₄)₃OH] also exist in soils (98). Calcium is also present in the primary minerals augite [Ca(Mg,Fe,Al)(Al,Si)₂O₆], hornblende [NaCa₂(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂], and the feldspar plagioclase (any intermediate between CaAl₂Si₂O₈ and NaAlSi₃O₈) (98).

Within the fraction of soils where particles are as small as clay particles, calcium occurs in gypsum, calcite, hornblende, and plagioclase. Sherman and Jackson (99) arranged the minerals in the clay fractions of the A horizons of soils in a series according to the time taken for them to weather away to a different mineral. These calcium-containing minerals are all early in this sequence, meaning that calcium is lost from the minerals (and becomes available to plants) early in the weathering process, but has been entirely lost as a structural component in more mature soils (98). Any calcium present in these more mature soils will be present attached to cation-exchange sites, where it usually constitutes a high proportion of total exchangeable cations, so the amounts present depend on the CEC of the soil.

Concentrations of Ca²⁺ in soils may be affected by ecological disturbance. Acid depositions are known to decrease Ca²⁺ concentrations in soils, which while not necessarily affecting plant yields directly may have a big impact on ecosystem dynamics. Acid deposition on the coniferous forests of the Netherlands has been shown to give rise to fewer snails, and the birds that feed on the snails have fewer surviving offspring due to defects in their eggs (100). This effect seems to be related largely to the abundance of snails being depressed by low calcium concentrations in the plant litter. In terms of how serious this problem might prove to be, it should be noted that changes in soil Ca²⁺ concentration caused by acid rain are less than 1 g Ca²⁺ m^-2 year^-1. This change is small compared with a transfer of 3.3 to 4.7 g Ca²⁺ m^-2 year^-1 from mineral soil to young forest stands (101).

Experiments on the Hubbard Brook Experimental Forest in New Hampshire, USA, have shown that calcium is lost from ecosystems following deforestation. This loss is true for other cations and also for nitrate. In the Hubbard Brook experiment, during the 4 years following deforestation, the watershed lost 74.9 kg Ca²⁺ ha^-1 year^-1 as dissolved substances in the streams, compared with 9.7 kg Ca²⁺ ha^-1 year^-1 in a watershed where the vegetation had not been cut down (102). This increased loss was attributed partly to increased water flows due to decreased water loss by transpiration, but more importantly through the breakdown of the plant material enhancing the turnover of the nitrogen cycle and the consequent generation of H⁺ ions, thereby releasing cations from the cation-exchange sites of the soil (102). Recent studies have shown that calcium loss continues for at least 30 years, with the longer-term loss possibly occurring because of the breakdown of calcium oxalate in the forest soil after removal of the trees (103).