Calcium in plant physiology and its availability from the soil

Calcium (Ca) is an abundant element in soils and an important plant nutrient driving agricultural production. Parent materials, climate and pedogenesis influence the concentration and distribution of Ca in soils. Soil Ca content affects the adsorption and release of exchangeable cations and the availability of nutrients and toxic elements to both plants and soil biota, affecting community composition and biological activities. Calcium also influences soil physical properties through its effects on soil aggregation and dispersion processes. Soils with a large Ca content are described as calcareous and their properties differ from non-calcareous soils. Calcareous soils, which often contain >15% CaCO3 and are generally alkaline, cover a significant global area but are found most often in arid regions. Non-calcareous soils do not contain free CaCO3, but can contain large amounts of Ca, particularly if Ca-fertilisers or liming materials have been added.

Calcium is an essential mineral element for plant growth and reproduction. It has unique roles in maintaining the expansion and structural integrity of cell walls and lipid membranes and as a cytosolic signal coordinating cellular responses to developmental and environmental stimuli. It also has non-specific roles in maintaining cation-anion balance and osmoregulation under particular environmental conditions. Plant species vary greatly in their tissue Ca concentration, and Ca requirement, which is largely determined by the cation exchange capacity of their cell walls. Calcium enters the root from the rhizosphere and much Ca is thought to move apoplastically (extracellularly) across the root to the stele, where it is loaded into the xylem for transport to the shoot. In the shoot, Ca follows the transpiration stream and is not re-translocated from leaves in the phloem. Thus, Ca deficiency symptoms occur in developing tissues when the immediate Ca supply from the root is insufficient for growth. Calcium deficiency symptoms reflect the roles of this element in plant physiology.

Most non-calcareous soils contain sufficient Ca for crop production, especially if liming materials have been applied to correct the pH of the soil solution. Correcting the pH of the soil solution ensures the phytoavailability of plant nutrients and allows the effective use of fertilisers. Nevertheless, Ca deficiencies can occur in crops grown on acidic, sandy soils with low Ca content, especially horticultural crops with rapid growth and high yields. Acute Ca-deficiencies in crops are often addressed by foliar application of Ca-fertilisers. The agricultural productivity of calcareous soils can also be high when sufficient water and nutrients are supplied. The management of crop production on calcareous soils raises several challenges, including increased risk of NH3 volatilisation and low phytoavailabilities of P and micronutrients. These can be addressed by applying the right source of nutrients at the right rate, at the right time, and in the right place. In addition, the application of composts and manures increases the reservoirs of phytoavailable nutrients and improves soil properties.