Calcium is essential for many plant functions. Some of them are
- Proper cell division and elongation
- Proper cell wall development
- Nitrate uptake and metabolism
- Enzyme activity
- Starch metabolism
Calcium is transported in the xylem via an ion exchange mechanism. It attaches to lignin molecules and exchange must occur with calcium or another similar cation (e.g. Mg++, Na+, K+, NH4+, etc.). Calcium is not very mobile in the soil, or in plant tissue, therefore a continuous supply is essential.
Factors Affecting Ca Availability
Calcium is found in many of the primary or secondary minerals in the soil. In this state it is relatively insoluble. Calcium is not considered a leachable nutrient. However, over hundreds of years, it will move deeper into the soil. Because of this, and the fact that many soils are derived from limestone bedrock, many soils have higher levels of Ca, and a higher pH in the subsoil.
- Soil pH: Acid soils have less Ca, and high pH soils normally have more. As the soil pH increases above pH 7.2, due to additional soil Ca, the additional “free” Ca is not adsorbed onto the soil. Much of the free Ca forms nearly insoluble compounds with other elements such as phosphorus (P), thus making P less available.
- Soil CEC: Lower CEC soils hold less Ca, and high CEC soils hold more.
- Cation competition: Abnormally high levels, or application rates of other cations, in the presence of low to moderate soil Ca levels tends to reduce the uptake of Ca.
- Alkaline sodic soil (high sodium content): Excess sodium (Na) in the soil competes with Ca, and other cations to reduce their availability to crops.
- Sub-soil or parent material: Soils derived from limestone, marl, or other high Ca minerals will tend to have high Ca levels, while those derived from shale or sandstone will tend to have lower levels.
- Other cations: Being a major cation, calcium availability is related to the soil CEC, and it is in competition with other major cations such as sodium (Na+), potassium (K+), magnesium (Mg++), Ammonium (NH4+), iron (Fe++), and aluminum (Al+++) for uptake by the crop. High K applications have been known to reduce the Ca uptake in apples, which are extremely susceptible to poor Ca uptake and translocation within the tree.
- Sodium (Na+): High levels of soil Na will displace Ca and lead to Ca leaching. This can result in poor soil structure and possible Na toxicity to the crop. Conversely, applications of soluble Ca, typically as gypsum, are commonly used to desalinate sodic soils through the displacement principle in reverse.
- Phosphorus (P): As the soil pH is increased above pH 7.0, free or un-combined Ca begins to accumulate in the soil. This Ca is available to interact with other nutrients. Soluble P is an anion, meaning it has a negative charge. Any free Ca reacts with P to form insoluble (or very slowly soluble) Ca-P compounds that are not readily available to plants. Since there is typically much more available Ca in the soil than P, this interactions nearly always results in less P availability.
- Iron (Fe++) and Aluminum(Al+++): As the pH of a soil decreases, more of these elements become soluble and combine with Ca to for essentially insoluble compounds.
- Boron (B-): High soil or plant calcium levels can inhibit B uptake and utilization. Calcium sprays and soil applications have been effectively used to help detoxify B over-applications.
Calcium deficiency symptoms can be rather vague since the situation often is accompanied by a low soil pH. Visible deficiency symptoms are seldom seen in agronomic crops but will typically include a failure of the new growth to develop properly. Annual grasses such as corn will have deformed emerging leaves that fail to unroll from the whorl. The new leaves are often chlorotic. Extremely acid soils can introduce an entirely new set of symptoms, often from different toxicity’s and deficiencies. Many fruits and vegetables demonstrate dramatic symptoms such as Black heart in celery and broccoli, Tipburn in lettuce and cabbage, White heart or Hollow heart in cucurbits, Blossom End Rot in tomatoes and peppers, and Pops in peanuts. Tree fruit with low calcium will exhibit increased storage problems such as bitter-pit in apples, cork-spot in apples and pears, cracking in cherries, and other degradation of the fruit while in storage. Deficiency in all crops often also impairs root growth and lead to additional symptoms as a secondary effect. Calcium deficient conifer trees will have exhibit yellowing then death and dropping of the needles on the new growth. The new growth may also be deformed.
Calcium, for all practical purposes, is not considered to have a directly toxic effect on plants. Most of the problems caused by excess soil Ca are the result of secondary effects of high soil pH. Another problem from excess Ca may be the reduced uptake of other cation nutrients. Before toxic levels are approached in the plant, crops will often suffer deficiencies of other nutrients, such as phosphorus, potassium, magnesium, boron, copper, iron, or zinc.