Calcium in Plants and Soil

Mobility and Uptake of Calcium by Plants

Calcium uptake by the plant is passive and does not require energy input. Calcium mobility in the plant takes places mainly in the xylem, together with water. Therefore calcium uptake is directly related to the plant transpiration rate.

Conditions of high humidity, cold and a low transpiration rates may result in calcium deficiency. Salinity buildup might also cause calcium deficiency because it decreases the water uptake by the plant.

Since calcium mobility in plants is limited, calcium deficiency will appear in younger leaves (die back or burns) and in fruits (blossom end rot, bitter pit), because they have a very low transpiration rate. Therefore, it is necessary to have a constant supply of calcium for continued growth. 

The Role of Calcium in Plants

Calcium is an essential plant nutrient. It has many roles:

• Participates in metabolic processes of other nutrients uptake. 

• Promotes proper plant cell elongation. 

• Strengthen cell wall structure - calcium is an essential part of plant cell wall. It forms calcium pectate compounds which give stability to cell walls and bind cells together. 

• Participates in enzymatic and hormonal processes.

• Helps in protecting the plant against heat stress - calcium improves stomata function and participates in induction of heat shock proteins. 

• Helps in protecting the plant against diseases - numerous fungi and bacteria secret enzymes which impair plant cell wall. Stronger Cell walls, induced by calcium, can avoid the invasion. 

• Affects fruit quality. 

• Has a role in the regulation of the stomata.

Factors Affecting The Availability of Calcium to Plants

Calcium forms insoluble compounds with other elements in soil, such as phosphorous. Calcium that is in the form of an insoluble compound is not available to plants.

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 adsorbed to soil particles are termed "exchangeable ions" because they can be exchanged by other ions present in the soil solution. Soil analysis determines the level of exchangeable calcium ions, and not the total calcium in soil, because the exchangeable calcium is the form which is available to the plant.

Several factors in the soil analysis can help in assessing the availability of calcium to plants:

• Soil pH - usually soils with a higher pH level contain more available calcium. 

• CEC - this is a soil characteristic that describes the total amount of positively charged exchangeable  ions that the soil can hold. A higher CEC indicates a higher capacity of the soil to adsorb and hold calcium, and therefore higher calcium availability. 

• Presence of competing ions - calcium competes with other positively charged ions, such as sodium (Na⁺), potassium (K⁺), and magnesium (Mg⁺²). Applying too much of these positively charged ions might decrease calcium uptake by plants. Sodium ions can replace the adsorbed calcium, damage soil structure and decreases calcium availability.

Other Reactions of Calcium in Soil

Calcium-phosphorous precipitation - when free calcium accumulates in the soil solution (e.g. when soil pH is high), calcium tend to form insoluble compounds with phosphorous. Consequently, phosphorous availability is also significantly decreased.

Calcium stabilizes soil structure - the calcium that is adsorbed to soil particles helps in stabilizing the soil structure. Adsorbed sodium might cause the soil to crack when dry and swell up when wet. Calcium replaces the adsorbed sodium and prevents damages to soil structure.

Calcium Deficiency

Calcium deficiency is usually caused due to low calcium availability or due to water stress which results in low transpiration rates. The symptoms of calcium deficiency include curling of young leaves or shoots scorching or spotting on young leaves, poor growth, leaf tip burnes, stunted roots, and damage to fruit.