calcium and magnesium in soil

1. Calcium & Magnesium
Dynamics in Soil
Dr. B. Balaganesh, M.Sc. (Ag.), Ph.D.,
Assistant Professor (Soil Science)
School ofAgricultural Sciences,
Karunya Institute of Technology and Sciences,
Coimbatore – 641 114

2. Calcium
• Important plant nutrient and also an element
deciding the conducive physical and chemical
properties of soil.
• Calcium pectate, a constituent of cell wall
• Important for cell wall formation, cell elongation
and division

3. Sources:
• Originated in rocks and minerals, from which soil was formed.
• Most important primary source - plagioclase mineral Anorthite
(CaAl2Si2O3)
• Basic rocks- epidote, gabbro, basalt and dibase.
• Cacite(CaCO3),Dolomite(CaMg(CO3)),
Gypsum (CaSO4.2H2O)
• Biotite, apatite, Boro silicates and other simple salts also contribute.
• Fertilizers and manures also add Ca to the soil

4. Simple representation of Ca & Mg Equilibrium &
transformations in soil
Soil Solution
(Ca2 / Mg2)
Exchangeable Ca/Mg Mineral Ca / Mg
Uptake
Lime Manures /
Residues
leaching
• Exchangeable and solution forms are in dynamic equilibrium
• Solution Ca2+ subject to crop removal and leaching

5. Factors influencing Calcium availability
1. Total calcium supply:
• Soils that have a low cation exchange capacity are
typically low in calcium.
• Calcareous and black soils are rich
• Alluvial and acid soils are deficient.
2.Soil pH:
• Acidic soils tend to be low in calcium due to high
aluminum saturation.
• High H+ activity impede Ca uptake
• High in neutral and alkaline pH (7-8.5)

6. 3.Type of soil:
• Moderately weathered soils typically have greater amounts of
available calcium as compared to highly weathered soil.
• Type of clay influences the availability
• 2:1 require high Ca saturation (>70%), but 1:1 require low (40-50%)
4.CEC:
• If the cation exchange capacity contains less than 25% calcium, it is
recommended that calcium should be applied to the soil.
• Ca availability depends on CEC
• Black soil high CEC with high Ca availability
• Red soil low CEC with low availability

7. 5.Per cent Calcium Saturation:
• Directly proportional to Ca availability
• Poor base saturation exhibit deficiency
• Below 40-60% reduce the yield of cotton
• 20% or less reduce the yield of soy bean
6.Ratio of Ca2+ to other cation in solution:
• A Ca/total cation ratio of 0.10 to 0.15 is desirable
• Ca2+ uptake depressed by NH4
+,K+,Mg2+, Mn2+ and Al3+
• Absorption increased with supply of NO
3
-N

8. Losses of Ca
• Normally leaching is high
• Tail end soil having salinity is due to Ca and Mg with
SO4 and Cl
• 200-500 lb of Ca2+ is leached through drainage water per
hectare per year

9. Magnesium
• Primary constituent of chlorophyll (15-20% of total Mg)
• ATP requires Mg2+ and promotes synthesis of fat and oils.
Sources:
• originates from weathering of rocks.
• Primary minerals are biotite, dolomite, hornblende, olivine and
serpentine.
• Secondary clay minerals chlorite, illite, montmorillonite, and
vermiculite contain Mg
• Substantial amounts supplied through epsomite (Mg SO4.7H2O) and
bloedite (Na2Mg (SO4)2.4H2O
• Manures and fertilizers also add Mg

10. Fixation
• Mg2+ replaces Al3+ in 2:1 type clay through isomorphic
replacement.
• Chlorite and vermiculite have greater amount of Mg in
minerals
• Amount of Mg in minerals
chlorite – upto 23%
vermiculite – 12-15%
Biotite – 7%
Muscovite and montmorillonite – 6%
Illite – 1%

11. Leaching
• Like Ca2+,Mg 2+ also leached
• Leaching accounts 12.5 to 150 lb/ha/yr.
• Severe in sandy soil particularly following the addition of
KCl/K2SO4.
• Leaching is next to Ca, but greater than k.

12. Factors influencing availability of Mg
1.Soil type:
• Inherently low in magnesium-containing minerals
• Acidic condition, Light textured soils rich in K – less available
2.Amount of exchangeable Mg:
• Exchangeable Mg2+ below 60 ppm cause deficiency
• Highly leached – less availability
3.Liming:
• Limed with non-magnesium-containing material – low availability
4.Ratio with other ion:
• Excessive amounts of other cations compete with magnesium and
reduces at cation exchange sites
• With a Ca: Mg ratio greater than 10:1 to 15:1, magnesium will likely be
deficient.
• K:Mg ratios < 5/1 for field crops, 3/1 for vegetables, 2/1 for fruit crops.
• NH4+ induced Mg stress also reported in soil of low Mg.