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Magnesium - Sources and Content
�C Forms of calcium in soils �C
Functions �C Deficiency symptoms �C
corrective measures
JEEVAN NAIK. B
M. Sc, (ag)
ANGRAU
SPEAKER

Magnesium -Content
?Magnesium has a wide range of key roles in many plant functions
?One of the major well-known roles is in the photosynthesis process
?Building block of the Chlorophyll, which makes leaves appear green
?Magnesium is absorbed by plants from the soil solution as Mg+2
?It constitutes 1.93 % of the earth��s crust ranging from 0.1 % in coarse
sandy soils of humid region to 4 % in fine textured soils of arid and
semi arid regions.

Mg in soil originates from weathering of
? Biotite,
? Dolomite,
? Hornblende,
? Olivine, (Mg, Fe)2SiO4
? Epsomite (MgSO4. 7 H2O) and
? Serpentine bed rock
? Also found in secondary clay minerals like chlorite, illite,
montmorillonite and vermiculite;
Sources of Mg

On decomposition of primary minerals, Mg is released
into soil which may then be
? Lost in the percolating water
? Absorbed by living organisms
? Adsorbed by surrounding clay colloids.
? Reprecipitated as secondary mineral in arid regions.
Fate of released Mg :

Forms of magnesium in soils
? In soil, magnesium is present in three fractions:
? Magnesium in soil solution -This is in equilibrium with the
exchangeable magnesium and is readily available for plants.
? Exchangeable magnesium -This is the most important fraction for
determining the magnesium that is available to plants. This
fraction consists of the magnesium held by clay particles and
organic matter.
? Non-exchangeable magnesium -Consists of the magnesium that is
a constituent of primary minerals in the soil. The break down
process of minerals in soils is very slow; therefore, this magnesium
fraction is not available to plants.

? Plants take up magnesium in its ionic form Mg+2, which
is the form of dissolved magnesium in the soil solution.
The uptake of magnesium by plants is dominated by two
main processes:
? Passive uptake, driven by transpiration stream.
? Diffusion �C magnesium ions move from zones of high
concentration to zones of lower concentration.
MAGNESIUM UPTAKE BY PLANTS

?Conditions such as, low soil pH, low temperatures, dry soil conditions
and high levels of competing elements, such as potassium and calcium,
reduce the availability of magnesium. Under such conditions,
magnesium deficiency is more likely.
?Effect of Soil pH on magnesium availability:
? Acidic soils increase the tendency of magnesium to leach, because
they have less exchangeable sites (lower CEC). In low-pH soils, the
solubility of magnesium decreases and it becomes less available.
?In addition, in acidic soils, elements such as ��Mn and Al�� become more
soluble and result in reduced magnesium uptake.
The availability of Mg is influenced by

? Excessive applications of potassium and ammonium
(positive-charged ions) compete with magnesium and reduce its uptake
and translocation from the roots to upper plant parts. They cause Mg
deficiency.
? Ca / Mg ratio Ideally it should not be greater than 7:1.
? K / Mg ratio should be less than 5:1 for field crops
? 3:1 for vegetables and sugar beet
? 2 : 1 for fruit and green house crops.
? Removal by crop : Legumes exhaust more Mg than non legumes
? Additions through fertilizers, manures and rains: FYM
and oil cakes are rich source of Mg. Basic slag contains 3 �C 4% of
magnesium. Serpentines contain 26 %.

? The concentration of Mg+2 in crops varies between 0.1 and 0.4 %.
? A large part of Mg is associated with organic anions like malate.
? Mg is the only mineral constituent of chlorophyll located at its
centre.
? Serves as a structural component of ribosomes.
? Mg activates the formation of polypeptide chains to form amino
acids. About 70 % of Mg is associated with anions such as malate
and citrate.
Functions

? Seeds contain Mg as salt of phytic acids. Mg is required for
phosphate transfer from ATP (Phosphorylation) in carbohydrate
metabolism.
? Several enzymes (eg : Ribulose carboxylose) require Mg+2 as
Cofactor.
? It promotes uptake and translocation of phosphorus and
movement of sugars within the plants

? Mg+2 is a mobile element and is readily translocated from older to
younger plant parts hence deficiency symptoms are manifested in
the older leaves.
? The Mg deficient plants usually have less than 0.1% Mg.
? Mg deficiency is common in the plants grown on coarse textured
acidic soils.
? Mg+2 deficiency, the proportion of protein nitrogen decreases and
that of non protein nitrogen increases in plants.
Deficiency symptoms

? Shortage of Mg+2 results in an interveinal chlorosis of the leaf in
which only the veins remain green and the interveinal areas turn
yellow with streaky or patchy appearance.
? In more advanced stages the leaf tissue becomes uniformly pale
yellow, then brown and necrotic.
? Affected leaves turn small in final stage and curve upwards at the
margins.

? In some vegetables, interveinal chlorosis with tints of red, orange
and purple colors is observed.
? Grass tetany : Cattle consuming forages with low Mg may suffer
from ��Hypomagnesemia�� (low level of blood Mg) commonly
known as Grass tetany. This happens due to high levels of NH4 + -
N and K application.

? Use of dolomitic lime stone -Ca Mg (CO3)2
? Magnesia -Mgo 55 % (Mg)
? Basic slag -3-4 % Mg
? At a dose of 30 �C 50 kg ha-1
Correction measures :

curve upwardsyellow, then brown and
necrotic
yellow with streaky or patchy

? In more advanced
stages the leaf
tissue becomes
uniformly pale
yellow, then brown
and necrotic.

Interveinal
chlorosis of the leaf
in which only the
veins remain green
and the interveinal
areas turn yellow
with streaky or
patchy appearance

final stage
and curve
upwards at
the margins.

Grass tetany/ Hypomagnesemia :
Calcium and magnesium deficiency
Occurs when cattle graze lush and
immature grasses

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