To understand how calcium gets into our bodies we have to look at calcium on and in the land. In humid, high rainfall regions like New England, most of the calcium cations have been leached out of the soil root zone by water and the addition of some calcium, often in the form of ground limestone, is necessary for good growth of most food plants. Although the amount of calcium needed by plants is small (calcium is 0.2 to 3.5 of the dry weight of plants and living plants are usually more than 60% water), calcium performs other important functions in the soil that make it useful and necessary.
Calcium cations are used to decrease the acidity and raise the pH of the soil. The pH is a measure of the hydrogen ion concentration. The pH scale runs from 0 to 14 with the greater the H
concentration, the lower the p. Hydrogen ions are given off by plant roots as they grow. Among the variously sized rock particles, organisms of all sorts and organic matter in the soil are colloidal particles of clay and humus which have negative charges on their surfaces. Cations are adsorbed and held on these negatively charged sites. An acid soil has most of these sites filled with hydrogen ions. In a good agricultural soil about 60-70% of these sites should be filled with calcium cations, 10-20% should be filled with magnesium cations, 10-15% with hydrogen cations, 3-5% with potassium cations and the remainder with micronutrients.
Besides adjusting the pH and being available for plants, proper calcium levels improve soil structure, make phosphorus and micronutrients more available, and improve the environment for microorganisms. Calcium is said to aid the growth of symbiotic and non-symbiotic nitrogen fixing bacteria which is why liming is important for the growth of legumes, whose roots host nitrogen fixing bacteria.
Limestone is relatively insoluble. How does spreading ground limestone on the soil and mixing it in make calcium ions available? Carbon dioxide is given off whenever living things respire. Plant roots and soil organisms of all sizes give off carbon dioxide which combines with water in the soil to produce carbonic acid which is able to dissolve the limestone, freeing the calcium as a cation to find an alternate negatively charged site to attach to. One important aspect of the limestone applied to the soil is particle size. The smaller the particles the larger the total surface area of limestone for the chemical activity to take place on, making for faster dissolution.
Often limestone contains magnesium as well as calcium in the form of carbonates and oxides. This limestone is called dolomite, is the kind that is found in northwestern Connecticut and is the kind often needed in Connecticut soils to reach the approximate relationships indicated earlier. (Magnesium is the central element in the chlorophyll molecule and is found in every green cell of a plant.) The analysis of limestone from Canaan, CT is magnesium oxide 18%, calcium oxide 28% and carbonates of calcium and magnesium 90%.
On the colloidal particles one cation can be replaced by another. If all ions are present in equal concentration potassium ions will replace sodium ions, magnesium ions will replace potassium, calcium will replace magnesium and hydrogen will replace calcium ions. Since hydrogen is not only given off by plant roots, but is produced in the forming of carbonic acid it is necessary to have a good supply of calcium and magnesium ions in the soil to keep their levels above that of the hydrogen on the colloidal sites.
Calcium is taken up by the roots of plants either directly from the particles or after it has moved into the soil solution. Ca
moves into roots because of its greater concentrations outside the roots and because of membrane potential. Both active and passive transport are involved in getting Ca
into the plant.
As noted before the calcium content (need) of plants varies. In general monocots, grasses such as corn and other grains, need less calcium than dicots, most other food plants. For example, calcium content is given as 1.3% in alfalfa and 0.82% in current year leaf and twig growth of white oak, both dicots, and only 0.40% in corn. In the plants the Ca
is said to be phloem-immobile, meaning that once it reaches the leaves via the xylem which carries materials up from the roots, it is not readily exported from the leaves via the food conducting tissues. This implies that most of the calcium taken up by trees and other perenials is returned to the ground with the leaf fall to be recycled as microorganisms decompose the leaves. Kormondy
reports a study of the nutrient budget of Scots pine plantation in England. For over the 55 years from planting, the total uptake by trees and ground flora was 3043 kilograms per hectare and the total return to litter and soil was 2565 kilograms per hectare. (Deborah Barnes has a good diagram of the Ca
cycle in plants in her unit.)
In the plant, calcium as well as magnesium, form salts of pectic acid which make up most of the middle lamella that binds adjacent plant cells. This makes calcium an important part of the physical structure of plants. Calcium in plants also functions as an enzyme cofactor and has a direct effect on the physical properties of the cellular membranes. If there is a deficiency of calcium the membranes seem to lose their integrity. The solutes within the membranes or the cells then leak out.
The calcium in plants can be returned to the soil with the falling leaves, as mentioned, or can be stored in a woody part until it falls and rots or is burned, or the calcium can be ingested by an animal which eats the plant. If wood is burned the ash content is typically 0.1-3.0%. Of this, 30-60% contains calcium in the form of calcium oxide. This is why wood ashes are another good source of calcium and a way to raise the pH of the soil. However since they also contain 10-30% potassium oxide and other elements, often in soluble forms, ashes have to be used sparingly to avoid excess potassium and salinity.