About ion exchange phenomenon in soil

1. Experimental methods to study ion exchange
phenomena and practical implications in plant
nutrition

2. What is Ion exchange Phenomena?
 Ion exchange Is defined as a reversible process by
which cations and anions are exchanged between
solid and liquid phases, and between solid phases
it in close contact with each other.
 For exchange of cations and anions term is used
as cation and anion exchange respectively.

3. Site for Ion exchange
 The Ion exchange property of soil is almost entirely to the clay
and silt fractions and the organic matter.
 So it is well known that the colloidal material of the soil with
effective particle diameter of less than 20 microns being the
most important site for causing the ion exchange phenomena.
 The charge density and potential are higher on edges and
corners and in furrows and cavities as compared to flat surface
consequently ion exchange phenomena are probably
concentrated more in those sites.

4. Kind of Exchangeable Ions
 The electric charge on the soil particle is neutralized by an
equivalent amount of oppositely charged ions known as
exchangeable counter ions, which are held to surface mainly
by coulomb forces and Van-der-waals forces.
 In most of the soil Ca2+ is the dominant exchangeable ion.
 in strong acid soils Al(OH)2 may act as a dominant
exchangeable ion.
 whereas in alkali soils Na+ becomes dominant exchangeable
ion.

5. Factors Affecting Ion Exchange
 Nature and amount of clay
 Organic matter content
 mineralogical composition of soil
 Soil reaction
 liming
 Irregular shape of the clay particles
 Non uniform distribution of charges throughout the
particles

6. Cation Exchange
 The process of interchange of cations in soil solution with
those in exchangeable form is known as cation exchange.
 Example:
Na
Clay micelle Ca + 2NaCl Clay micelle + CaCl2
Na
 The most important cations in soils are calcium(Ca2+),
magnesium(Mg2+), hydrogen(H+), sodium(Na+), potassium(K+)
and aluminium(Al3+).

7. Anion Exchange
 The process of interchange of anions in soil solution with
those in exchangeable form is known as anion exchange.
 Example:
Clay micelle OH+ H2PO4
- Clay micelle H2PO4 + OH-
 The most important anions in soils are hydroxide(OH-), H2PO4
-
, SO4-, NO3-.

8. Ion Exchange Formula
(i) Empirical formula:
The Freundlich equation is one of the first used in soil studies
and it stats that,
m=ABC
m=amount adsorbed per unit weight of soil;
C=icon concentration(ions to be allowed for adsorption)in
soil solution A&B are constants which vary form soil to soil.

9. (ii) Kinetic or Statistical:
The Langmuir equation is used to characterize ion
adsorption particularly phosphorus in soils. The most
common form of the Langmuir equation is,
m = ABC
1+Bc
Where,
m=amount of ion adsorbed per unit weight of soil
c=ion concentration in soil solution to be considered
A=adsorption maximum
B=constant related to bonding energy.

10. (iii) Mass action:
In homovalent (here monovalent-monovalent) exchange,
RK + Na+ RNa + K+
The equilibrium equation may be written as,
[Na+]i (K+)0
[K+]i (Na)0

11. Practical implications in plant nutrition
 Cation Exchange:-
 Cation exchange is an important reaction in soil fertility, in
correcting soil acidity and basicity, in altering soil physical
properties, and as a mechanism in purifying or altering
percolating waters.
 Plant roots, when they come in contact with colloidal
particles, adsorb exchangeable cations directly by interchange
or contact exchange between the root hairs and colloidal
complex.

12. Cont…..
 The nature and content of exchangeable bases in soil have an
important bearing on its physical, chemical and biological
properties.
 The pH value of such soils varies from 6.5-7.5 which is most
conducive for the availability of plant nutrients.
 When proportion of exchangeable hydrogen is high it gives rise
to acid soil.
 When exchangeable sodium occupies more than 15% it gives
rise to soils of alkaline reactions.

13. • Anion Exchange:-
 Effect of anion exchange in plant nutrition have not
been studied like that of cation exchange.
 The effect of concentration, mole fraction and
complementary ion on the distribution of
exchangeable anions seems to be similar to that for
cations.
 Phenomenon of anion exchange is most important
for the release of fixed phosphate in soil and thereby
increases its availability for the plants.