1. What
factors contribute to lower and higher cation exchange?
(a) Types of clay.
A sandy soil with little organic matter
will have a very low CEC while a clay soil with a lot of organic matter (as
humus) will have a high CEC.
(b)
Amount of
clay
As the clay increases
the cation exchange will also increase. Higher quantities of clay and organic matter be get higher cation
exchange. Amounts of negative and positive charges are both expressed in
milliequivalents. One milliequivalent of negative charge on a clay particle is
neutralized by one milliequivalent of cation. A milliequivalent takes into
account both the weight and the charge of the cation.
(c)
pH value
Many
soil cation exchange is dependent upon the pH of the soil. This is due mostly
to the lyotrophic series, which describes the relative strength of various
cation absorption, and is generally as follows:Al+3 > H+ > Ca+2 > Mg+2 > K+ = NH4+ > Na+.As soil acidity
increases (pH decreases), more H+ ions
are included in the cation exchange removing other cations. Inversely, when
soils become more basic (pH increases), the available cation exchange increases
because there are fewer H+ ions
in the soil.
(d)
Hydrous
oxides of Fe and Al
Many heavily weathered or oxidized
soils, especially in the wet tropics, have a high concentration of Al+3.
Since aluminum is toxic in high quantities for most plants, there are certain
advantages and disadvantages to this. For one, due to the relatively high
absorption rate of aluminum, it is taken out of the soil and the plant cannot
use it. On the other hand, because it has 3 positive charges, it takes up a
large amount of charge in a CEC. For example, Al+3 fills the same space as three NH4+ ions. This makes many aluminum
heavy soils relatively infertile. There is no easy way to remove Al ions from
the CEC.
References:
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