Ca

Calcium

Content and sources of soil calcium : The calcium concentration of the earth’s crust is about 3.20 %, it may be as low as 0.015 % in humid region to as high as 5% in arid region. High calcium content in arid regions could be due to low rainfalland hence little leaching. The major sources of calcium are the weathered products of rocks and minerals containing calcium like the following sources

Mineral Chemical formula Total CaO (%)

1. Calcite CaCO3 56.0

2. Apatite Ca10(PO4)6 50.0 – 53.2

3. Dolomite CaCO3.MgCO3 33.2

4. Gypsum CaSO4.2H2O 32.2

         Besides these secondary minerals, primary minerals like Augite (18.7%), Hornblende (4.6%), Anorthite (10.0 – 20.0 %)Epidote (22.0-25.0%) etc., are the sources of calcium in the soil.

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The fate of released calcium

1. Absorbed by organisms: Legumes use much more calcium than grasses.

Prolonged cropping with crops like alfalfa on sandy soils low in Ca depletes Ca resource.

2. Losses due to leaching: The loss of calcium in drainage water due to leaching by rain or irrigation is very large. The loss of Ca2+ increases with increase in rainfall or with increase in permeability of the soil. Light textured soil suffers greater loss of calcium than heavy soils. Leaching losses range between 84 to 224 kg ha-1.

3. Adsorbed by clay colloids: If the activity of calcium in soil solution suddenly increases, there will be a shift in equilibrum with subsequent adsorption of some Ca2+ by exchange complex. Conversely, if the activity of calcium in the soil solution phase decreases due to either  leaching or crop removal, the adsorbed calcium releases into soil solution.

4. Reprecipated calcium: Calcium is re precipitated as secondary calcium compounds in arid climate. In arid region, due to low rainfall, Ca in soil solution will be precipitated as least soluble calcium compounds.

Factors affecting the availability of Ca in soils

1. Total calcium supply: Total supply of calcium in very sandy acid soils with low cation exchange capacity can be too low to provide sufficient available calcium to crops.

2. Soil pH: High H+ activity occurring at low soil pH will impede calcium uptake due to high base unsaturation.

3. Cation exchange capacity and degree of saturation of exchange complex : The absolute amount of calcium present is not that important but the relative amount of other cations or degree of calcium saturation is important. Montmorillonite requires a 70 % base saturation to release Ca, while kaolinite requires only 40 % base saturation.

4. Type of soil colloid. The type of clay influences the degree of calcium availability. A much higher degree of saturation is required for 2 : 1 clay minerals than 1:1 clay.

5. Ratio of calcium to other cations in soil solution: Calcium availability isinfluenced by the ratio between calcium and other cations. Calcium / total cation ratio of 0.10 to 0.15 is desirable. Blossom end rot, a calcium deficiency disorder in tomatoes can be prevented by maintaining a Ca soil solution salt ratio in the range of 0.16 and 0.2.

Calcium

         Calcium is absorbed by plants as Ca2+ and its concentration ranges from 0.2 to 1.0% and it is supplied through mass flow method.

Functions of calcium                                                              

1.     It is immobile in plants and hence the deficiency is observed in younger leaves.

2.     It is a constituent of cell wall and increases in stiffness of plants.

3.     Promotes root development and growth of plants, root elongation and cell division.

4.     Helps to translocate the sugar in the plants.

5.     It involves chromosome stability and that it is a constituent of chromosome structure.

6.     Affects translocation of CHO in plants.

7.     Encourages seed production.

8.     Activates enzyme phosphate and kinease.

9.   Accumulated protein during respiration by mitochondria and it increases their protein content.

10.  It binds DNA to protein molecules.

Factors affecting Ca2+  availability

1.  Total Ca supply            :    Sandy acidic soils with low CEC having less Ca.

2.  Soil pH                         :    In acid soils Ca is not readily available to plants at low cane.

3.  CEC                             :  Soils having low CEC might will supply more Ca2+.

4.  % Ca saturation           :   High Ca2+ saturation indicates favourable pH for growth and                                                       microbial activity.

5.  Type of soil colloid       :    2 : 1 type require higher Ca saturation than 1: 1 type.

6.  Ratio of Ca2+ to other catians :  Increasing the Al3+ cane in soil solution reduces

                                                         Ca uptake in pt.

Management

         Calcium deficiency can sometimes be rectified by adding agricultural lime to acid soils, aiming at a pH of 6.5, unless the subject plants specifically prefer acidic soil. Organic matter should be added to the soil to improve its moisture-retaining capacity. However, because of the nature of the disorder (i.e. poor transport of calcium to low transpiring tissues), the problem cannot generally be cured by the addition of calcium to the roots. In some species, the problem can be reduced by prophylactic spraying with calcium chloride of tissues at risk.

         Plant damage is difficult to reverse, so corrective action should be taken immediately, supplemental applications of calcium nitrate at 200 ppm nitrogen, for example. Soil pH should be tested, and corrected if needed, because calcium deficiency is often associated with low pH..