Sources of charges in soil

1. Sources of charges,
ion exchange - Positive and
Negative charge
Isomorphous substitution,
pH dependant charge
CEC. AEC & BS
K. Maheshwaran, Asst.Prof. (SS&AC)
Sethu Bhaskara Agricultural College &
Research Foundation

2. Sources of negative charges
There are generally two types of charges
 pH dependent (Exposed crystal edges)
 pH independent (Isomorphous substitution)
 Ion exchange is the exchange of ions
between soil solution and colloidal complex
 Cations and anions in soil solution
adsorbed by soil colloids such as silicate
clays, hydrous oxides of Fe, Al and Humus

3. pH dependent (Exposed crystal edges)
 Develops due to unsatisfied valences at the
broken edges of Si & Al sheets
 Flat surface also serve as negative charges
 Oxygen and hydroxyl groups exposed at the
broken edges
 pH >7.0, the H ions of these OH group
dissociates and negative charge carried by
oxygen
 These type of charge develops in 1:1 clay
minerals, organic colloids than other

4. (Exposed crystal edges)

5. pH independent (Isomorphous substitution)
 Substitution of one ion for the another ion of
similar size but lower positive valence
 Si4+replaced by Al3+ and even more
extensive replacement of Al3+ by one or
more of Fe3+, Fe2+, Mg4+or Zn2+
 Without substitution the positive and
negative energy are in balance

7. pH independent (Isomorphous substitution)
 Three positive charges of Al are satisfied with
three negative charges of oxygen or hydroxyl -
No net negative or positive charge
 When Mg2+ is replace the by Al 3+ results an
imbalance occurs
 Similar way negative charge develops due to
isomorphous substitution
 Not dependent pH, referred to as permanent
charges

8. Positive charges
 Some soil colloids posses positive charges and
attract anions
 Fe and Al oxides clay minerals and amorphous
clay minerals and layer silicates develops
positive charges
 At soil pH is greater than ZPC
(Zero Point Charge), the minerals is
negative charge and absorb cations
 At soil pH is lower than ZPC
(Zero Point Charge), the minerals is positive
charge and absorb anions

9. Positive charges
 Also occurs due to edges of clay minerals
or broken surface of octahedral sheet at
restricted condition (pH <ZPC / Isoelectric
point)
 Kaolinite and other 1:1 type clay minerals
exhibit small amount of positive charges on
crystal edges
 Kaolinite called as amphoteric because
can attract both cation and anions (Basal
surface is negative and edges are positively
charged)

10. Importance of ion exchange
 Retention and liberation of plant nutrients
 Control soil structure
 Control process of soil formation
 Reclamation of acid and alkali soils
 Influence the effect of fertilizer and fertilizer
practices

11. Factors affecting ion exchange
Nature and
amount of clay
Soil with more amount of clay
have more ion exchange
phenomena
2:1 type of clay has higher ion
exchange capacity than that of
other clay
Organic matter Soil with higher OM the ion
exchange also high because OM
has more –COOH functional
group
Application of lime Application of lime in acid soil
decease the ion exchange
capacity
Soil pH Ion exchange capacity increase

12. Ion exchange
 Cation exchange
 Anion exchange

13. Cation exchange
 Exchange of cations between the soil
solution and soil colloids
 CE is one of the most common and
important of soil reaction
 Example. In High pH soil, Ca is replaced
the Na
Na (Clay)+CaSo4  Ca (Clay)+ NaSo4

14. Cation Exchange Capacity (CEC)
 Capacity of soil to exchange the cations
between the soil colloids and soil solution
 Indicates the sum of net negative charge on
soil colloids
 The sum of cations adsorb by the soil
colloids termed as CEC
 Expressed as me/100 g soil and later it was
expressed as CEC (C mol (p+) / kg)

15. Cation Exchange Capacity (CEC)
 Clay has a CEC of 1 (1 me/100 g), capable
of exchanging 1 mg of H+ ions its equivalent
to every 100 grams.
 In certain clay 100 g of soil exchange 250
mg Ca, the CEC of clay is 12.5 me/100g
 250/20 (equivalent weight of Ca)

17. CEC of different colloids
Colloids CEC (C mol (p+) / kg)
Humus : 200-300
Vermiculite : 100-120
Montmorillonite : 80-100
Kaolinite : 3-15
Halloysite : 10-40
IIlite : 10-40
Chlorite : 10-40
17

20. Factors affecting CEC
Soil texture Fine textured soil has more
CEC than coarse textured soil
Soil organic matter
content
Soils with more amount of OM
has Higher CEC
Amount and kind of
clay
Soils with more clay has
higher CEC, 2:1 type clay has
more CEC than 1:1 type of
clay
Soil reaction CEC increase with increase in
soil pH

21. Importance of CEC in plant nutrition
 CEC is an important reaction in soil fertility in
correcting soil acidity, basicity
 Nature and amount of exchangeable bases in
soil have important bearing on its physical,
chemical and biological properties
 In normal fertile soils the total exchangeable
cations (Ca, Mg, K & Na) about 80-90% of the
total CEC
 Among the cations Ca is the dominant cation
 pH of these soil varies from 6.5 to 7.5 which is
most conductive for the availability of plant
nutrients

22. Importance of CEC in plant nutrition
 If clay is saturated with H+ cations and
forms H-Clay complex and its called
base unsaturated soil – Less fertile
 If clay is saturated with Na+ cations
and forms Na-Clay complex it gives
rise to alkaline soil– Less fertile

23. Anion exchange
 Capacity of soil to adsorb or release anions
under normal soil condition
 Exchangeable anions (H2PO4
-), (HPO4
-),
(Cl-), (SO4
2-), (NO3
-) and (MoO4
-)
 Similar to CEC Anion Exchange Capacity
will be occur

24. Factors affecting AEC
Soil reaction AEC increase with decrease in soil
pH
Valence of
ions
AEC Increase with increase in
valence of anions and decease with
increase in valence of cations
Type and
amount clay
AEC high in 1:1type of clay and
kaolinite has higher AEC than
montmorillonite clay
Salt
concentration
AEC decease with increase in
concentration of salts
(NO3
-) < (Cl-) < (SO4
2-) < (H2PO4
-)

25. Importance of AEC in plant nutrition
 AEC significance with PO4 fixation
 In acid soil the dominant anions are Fe3+, Al
3+ and Mn 2+
 Dominant minerals are limonite and
goethite
 Phosphorous fixation is more in acid soils
soluble form of ‘P’ is fixed by insoluble form
of ‘P’ is fixed by Al and Fe

26. Percentage base saturation of
soil
 Cations have opposing effects on soil
acidity and alkalinity
 H+ and Al3+ (Hydrolysis) are dominantly
found in acid soils
 Na2+ (Hydrolysis) is dominantly found in
alkali soils
 Al3++H2O  Al(OH)++ + H+
 Al(OH)+++H2O  Al(OH)2
+ + H+
 Al(OH)2
+ +H2O  Al(OH)3 + H+

27. Percentage base saturation of
soil
% BS = S / T x 100
Here
 BS = Base saturation
 S =Total exchangeable bases
 T = Total CEC

28. Base saturation and fertility of soils
 BS directly influence the fertility of the soils
 Adsorbed cations and released to plants
depends on degree of base saturation
 If soil is 80% BSP, means 80% of bases
and 20% of H+
Sl. No BS % Description
1. >80 Fertile
2. 50-80 Medium fertile
3. <50 Non fertile

29. Thank you……