Amounts and Functions of Calcium in Plants
Calcium is taken up as a divalent cation, Ca2+, by plants. Plant Ca varies between 0.1 and 0.5% whilst the critical concentration level of Ca in plants is around 0.20%.
A critical concentration level is the concentration of an element in a plant below which yield is reduced. The critical level of an element depends on plant species.
Generally, dicotyledonous plants, especially legumes, require more Ca than monocotyledons. Potatoes and cereals do not require high amounts of Ca.
Groundnut responds easily to Ca, especially in the Northern part of Nigeria; other crops may not easily show a response to Ca application. Specific functions of Ca in plants are enumerated as follows:
Calcium together with other cations (Magnesium, sodium, potassium) is important in maintaining ionic balance in plant cells.
Calcium is carried in xylem vessels to growing points and to the cell walls where it is immobilized in the formation of the middle lamella. Calcium is present as calcium pectate in the cell wall and for this reason, it tends to accumulate in the leaf. Just a small fraction of plant Ca is used to maintain ionic balance.
Calcium is involved in the structure and permeability properties of membranes thus influencing the selectivity of cell membranes in allowing nutrients to pass through. Ca oxalate crystals may be formed in the cells of plants probably to get rid of oxalate which may be toxic to the plant.
The blossom-end rot of tomatoes might be caused by Ca- deficiency as a result of a breakdown in the cell wall, but this is still very controversial. The disease has been described as a physiological disease of the tomato plant.
Calcium is known to favor the formation of mitochondria by increasing its protein content. Also by its enhancement of NO–3 – N uptake, Ca is related to protein synthesis and the activity of certain enzyme systems.
Calcium is also involved in cell elongation and the development of meristematic tissue.
Calcium is an immobile element that does not easily translocate from one part of the plant to another because it forms the structural part of the middle lamella of the cell wall; and when formed, Ca remains there.
Most deficiency symptoms of Ca therefore appear on new vegetative growth such as die-back from growing tips. The meristematic tissue such as hair roots fails to develop as a result of Ca- deficiency. General stunting of the whole plant may occur. In maize, new leaves fail to unroll.
It is usually very difficult to differentiate between Ca-deficiency and acidity. However, Ca, deficiency symptom, is common in acid soils, soils with low inorganic matter, and sandy soils.
Excess Ca occurs in calcareous soils where it is found to depress uptake of other nutrient elements, especially magnesium, manganese, and boron when taken up as anion borate.
Calcium is the predominant cation at exchange sites because, as a divalent cation, it is held more tightly than monovalent cations. Compared with other divalent cations such as Mg2+, Ca2+ has a low charge and small water hydration surrounding it.
It, therefore, has higher strength with which the cation is adsorbed to exchange sites than magnesium. Exchangeable Ca+ is between 125-200mg g-1 with the highest values in clayey soils and lowest in acid sandy soils. Major sources of Ca in soils are listed below:
- Primary minerals such as apatite (Ca10 (PO4) X2, amphiboles, pyroxene, dolomite, calcium-feldspar, and calcite all form the primary sources of Ca in most soils.
- Secondary minerals such as Ca CO3, Ca SO4, and Ca- phosphate. The peculiarity of Ca is that it can be the dominant element in some soils, for example, CaCO3 can be up to 20-30% in certain soils.
- Seawater also contains Ca, Na, K, and Mg. The effect of sea spray decreases as one moves inland.
- Rainwater supplies little Ca.
- Farmyard manure and organic manure contain a lot of Ca.
- Lime is the most important source of Ca and it is added to acid soils which are larger than alkaline soils worldwide. Lime contains Ca CO3 (calcite), Ca (OH)2 and CaO but the most important is Ca CO3 and CaO. In burning, K- Na,– Mg– and CaCO3 are formed to make the soil slightly alkaline.
- The important role of Ca in maintaining soil pH implies that loss of Ca automatically leads to low pH. Calcium could be lost from the soil through leaching and the application of acidifying fertilizers such as ammonium.
- Rainfall can lead to loss of Ca in the soil in the sense that carbon dioxide, CO2, in rain forms Ca (HCO3)2 in soil which can be leached leading to decreased pH.
Read Also: Forms and Functions of Sulphur in Plants
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