1. The recycling of nutrients contained in organic matter such as livestock effluent, crop residue and other organic by-products from human activities, are an important resource for fertilization.
2. Calcium is obtained from quarries in the form of calcium carbonate (limestone rock). Crushed or pulverised, it is a basic mineral fertilizer for agriculture. The rock may also be calcined in a lime kiln to produce quicklime that is also used in agriculture.
3. Calcium evolves in the soil between fixed, adsorbed and soluble forms.
4. The leaching of soluble calcium (deep entrainment by excess soil water) must be taken into account when establishing a nutrient balance.
5. Runoff and erosion (calcium bound to solid particles) from the plot also takes place.
6. Via the roots, crops only absorb calcium as Ca2+ dissolved in the soil solution.
Calcium deficiency in plants is rather rare; it mainly occurs in acidic soils low in calcium. It causes chlorosis on young leaves and fruits. In apples, for example, it causes bitter pit on fruits, which are bitter corky blemishes.
An excess of Ca is rare, but it does occur in calcareous soils. The alkaline pH reduces the availability of phosphorus and some micronutrients. Calcium also forms insoluble calcium phosphate, reducing the availability of phosphorus.