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Soil – may be defined as the unconsolidated mineral on the top layer of the earth’s crust that serves as a natural medium for the growth of land plants.


Definition: Soil Colloids – are very small organic and inorganic parties present in the soil which are responsible for potential fertility of the soil determine the physical and chemical properties of the soil.

1.      How soil colloids are formed

·         As soil is formed during weathering processes, some minerals and organic matter are broken down to extremely small particles.

·         Chemicals changes further reduce these particles until they cannot be seen with the naked eyes.

·         The very smallest particles are what we call soil colloids.

   Types of soil colloids

Soil collards may be

a)     Inorganic colloids

–          Clay minerals (layer silicate clay).

–          Iron and Aluminium oxide clays.

–          Allophane and amorphous clays.

b)     Organic colloids

–          Includes highly decomposed organize matter called humus.

–          Organic colloids are more reactive chemically and generally have greater influence on soil properties.

Therefore the four major colloids present in the soil are,

i)                  Layer silicate clay

-These are most important silicate known as phyllosilicates (life-like).

– They comprised of two kinds of horizontal sheets, one dominated by silicon and other by aluminium magnesium.


ii)  Iron and Aluminium

These are remnant material which remain after extensive teaching due to its low solubility these are sesquioxides

Sesquioxides are either Al oxide or iron (iii) oxide contaminated with Al(OH)3 or Fe(OH)3

iii)  Allophane and other armophous minerals

– Mainly these silicates are mixture of silica and alumina.

– They are amorphous in nature.

iv) Humus

-Humus is armophous, dark-brown to black nearly insoluble in water but more soluble in dil. Alkali e.g NaOH, KOH solution.

– It consists of various chain loops of linked carbon atoms.

– Humus is a temporary intermediate product left after considerable decomposition of plant and animal remains.

-Humus contains partially dissociated enolic, carboxyl and phenolic groups.




i.  Surface area

ii.  Electric change (surface charge )

iii. Ion exchange (adsorption of cation)


i)  Surface area.

Because of their small size, all soil colloids expose a large external surface area per unit mass.

The external surface area of 1g of colloidal clay is at least 1000 times of 1 g of colloidal clay is at least 1000 times that of 1g of coarse sand.

ii)  Electric charge

·    Soil colloids surface, both external and internal characteristically carry +ve and or -ve charge.

·  Most soil colloids the +ve charge predominate.

·  Both organic and inorganic soil colloids when suspended in water carry a negative charge.

Where the negative charge on colloidal particles comes from?

·  Negative charge on clays comes from

i)    Ionizable hydrogen ions.

ii)   Isomorphous substitution.

i) Ionizable hydrogen ions are hydrogen ions from hydroxyl group on clay surfaces

·   The O-H bond from Al –OH or Si – O –H portion of clay heterolytically breaks and ionizes to give H+ leaving unneutralized negative charge on oxygen.

·  Presence of strong alkaline solution activates the clearage of O-H bond yielding H+ which will combine with OH from strong alkaline solution in the neutralization reaction.


Hence soil colloids becomes more negative in more alkaline solutions.The same applies to organic colloids which also contain OH in enolic, phenolic or enol.

ii) Isomorphous substitution.

This is due to the substitution of one ion for another of similar size often with lower change

iii)    Ion exchange (Adsorption of cations)

As soil colloids posses negative charge, they attract the ions of an opposite charge i.e positively charge ions to the colloidal surfaces. The attraction of cations such as H+, Ca2+, and Mg2+to colloid surface leads to formations of an ionic double layer. The outer layer is made up of a swarm of rather loosely held cations attracted to the negative charged colloidal surface.



Colloids are primarily responsible for chemical reactivity in soil

Each colloid has net negative charge thus making possible for the colloid to attract and hold positively charged particles.(cations) like Na+, H+, Ca2+ and Mg2+

– Thus there are cations attached to colloids and in the soil.

– When one of the cations in the soil solution replaces one of the cations on the soil colloids cation exchange is said to take place.

– This exchange only take place when the cations in the soil solution are not in equilibrium to the cations on the soil colloid.


   Ion exchange :

Is reversible reaction which involves an interchange of ions between ion in soil solution and another ion or surface of soil colloid.

It can be anion exchange or cation exchange.

Cation exchange:

Is the interchange between a cation in a solution and another cation on surface of any negatively charged material such as day or organic matter.

   In the soil:Cation exchange is the interchange between cation in a soil solution and another cation on the surface of soil colloid.



   Anion exchange

·  Soil colloids being negatively charged cannot attract and hold negatively charged particles (like charged repels) like SO4 and NO3

Why? Nitrate is more leached from the soil than ammonium.

·  This is because nitrate (NO3) has negative charge like soil colloids. So NO3 is not held by the soil solution to be leached under rainfall conditions.

·  NH4 being positively charged is attracted and held by soil colloids and hence it become difficult for NH4 to be leached.

Mechanism of ion exchange in the soil.

·   Ion exchange in the soil is well explained by electron –kinetic theory of ion exchange.

According to the theory:

The negative and positive charge associated with soil colloids (clay minerals and organic matter) are balanced by electrostatic attraction of cations and anions respectively.

The balancing ions are turned as EXCHANGEABLE CATIONS OR ANIONS.

·   Exchangeable cations and anions form outer sphere complexes with charged surfaces in which water of hydration exist between the charged ion and the oppositely charged colloids.

·   Thus the adsorbed cations and anions are said to being state of oscillation forming a diffuse double layer.

·   Due to these oscillations, some of the ions move away from the surface of the colloid micelles.

·   In presence of the solution of an electrolyte an ion of the soil solution slips in between the inner charged layer and the outer oscillating ion.

·   The ion in the soil solution is now adsorbed on the colloid micelles and the surface in remains in solution as an exchange ion and hence the ion exchange occurs.

Factors affecting composition of exchangeable ions in the ion exchange

Explain the factors affecting composition of exchangeable ions in the soil.

i.   Strength of adsorption

ii.   Relative concentration of the ion in the soil solution.



– Is the maximum quantity of total cations of any class, that a soil is capable of holding at a given PH value, available for exchange with soil solution.

– It is a measure of the quantity of the negatively charged sites on soil surfaces that can retain positively charged ions by electrostatic force.

Significance of CEC

CEC is useful in the following ways:

 It is a measure of soil fertility

-The more cation exchange capacity a soil has the more likely the soil will have fertility.

It is a measure of nutrient retention capacity

-Soil with large value of CEC has large nutrient retention capacity.

It is a measure of the capacity of soil to protect ground water from cation contamination.

-Soil with large CEC has good ability to protect ground water from cation contamination as it exerts more resistance for its cations to be leached away.

Expression of CEC value

It is expressed in terms of number of equivalents (or more specifically as number of millequivalents) of cations per 100 grams of dry soil written as:

e.g 100g (meq /100g)

No of equivalents= ————- (i)

Equivalent means the mass of cation that will replace (exchange) 1g (1mole) of H+.


23g of Na+ (i.e 1mole of Na+) exchange 1g of  1+ and hence Na+ has equivalent wt of 23g.

4og of Ca2t (I mole of Ca2+) exchange 2g (2moles of H+ which means 20g of Ca2+ exchange 1g of H+

Hence Ca2+ have equivalent wt of 20.

equivalent t= ——— (iii)


= no of moles of the cation


number of equivalents = x Amount of ionic charge



Where no of mill moles = No of moles x 1000

No of mill moles = x 1000





Factors affecting value of cation exchange capacity.

Cation exchange capacity is affected by the following factors:

–    Amount of clay (soil texture)

–   Type of clay

–   Soil organic matter

–   PH of the soil.

i)Amount of clay in soil

A high silicate- clay soil hold more exchangeable cations than a low silicate clay soil and hence have greater CEC value.

Silicate clay (Si – (OH) easily lose H+ off Hydroxy group in basic medium resulting into net negative charge i.e the greater the –ve charge the greater the CEC

(ii)Clay types

Different clay types have different CEC value due to difference in surface area.

(iii)Soil Organic Matter

Organic matter in the Soil, also are negative charged. Therefore

– High – Organic Soil have greater CEC value than a low organic Soil.

– On another hand if an organic matter continue to decay, the CEC tends to decrease with the decomposition.

– They can retain more cations.

Therefore organic matter particles have greater CEC than clay particles.

(iv) PH of the soil.

PH affects CEC of colloids that are charged based on hydroxyl group instead isomorphous substitution.

Under acidic (low PH ) condition.

H+ are in excess resulting to less ionization of the colloids, less negative charge and hence low CEC value.

Under basic (High PH ) condition.

OH are in excess resulting to more ionization of the colloids , more negative charge on the colloids surface and hence high CEC value.


Acid cations and Base cations

Acid cations are exchangeable cations (mainly (H+ and AI3+) which tend to acidify the soil

–  They are also known as exchangeable acids

–  In very acidic soil, H+ and AI3+ dominates other adsorbed cations.

–  The acidity of AI3+ is explained by its cationic hydrolysis according to the following equation:


·   Base cations (or exchangeable bases) are exchangeable cations which are capable of neutralizing soil acidity . Common exchangeable bases are Ca2+, Mg2+, K+ and Na+

Base saturation

Base saturation is the fraction of exchangeable cations that are base cations.

-It is expressed as percentage and hence the name percentage base saturation.

Percentage base saturation = x 100%

Relationship between percentage base saturation and percentage acid saturation

Percentage base saturation + percentage acid saturation


= x 100%


but number of base cations + Number of acid cations= Total number Of exchangeable cations = CEC of the soil

= x 100% =100%


Percentage base saturation + percentage acid = 100% Saturation.

Q. 1.   (a)   Define the following terms as applied to soil:

(i)  Cation Exchange Capacity

(ii)  Percentage Base saturation


(b)   A soil sample has a CEC of 25meq per 100g of 200mg of soil sample were shaken with 40c of 0.1M HCl. After filtering and washing the soil the filtrate and washing were titrated against NaOH solution, 24.0cm3 of 0.1 M NaOH were required for complete neutralization. Calculate the percentage base saturation of the soil sample.

(c)   A soil sample (20g) was analyzed and found to contain 0.0015g of calcium, what is  the  concentration of Ca in the soil sample in meq/100g of soil.

1eq of Na + 23g = 0.023meq




(b) Solution:


Remaining HCl will react with NaOH hence HCl was in excess


Used = 0.004 – 0.0024

= 0.0016moles of H+ (No of moles which attached themselves to the soil colloid)

Now we know 1eq = 1 mole of H+

0.0016 eq was in   200g of soil

?      100g

0.0016 e.g.  = 200

?         = 100g

8 x 10-4 x 1000 = 0.8meq/100g


                           P. B.S.  =  3.2%

Example 4

A Soil test shows the following:

Nutrient     meq / 100 soil
Ca2+        9.9
Mg2+          2.1
K+      2.0
Al3+      7.6
NH4+      0.6
Na+        0.1


a)   Calculate the CEC of the Soil.

b)   Calculate the percent base saturation of the Soil

c)   Calculate the percent aluminium saturation of the soil


a) CEC of the Soil

= number of exchangeable base cations + number of exchangeable acid cations.
= (9.9 + 2.1 + 2.0 + 7.6 + 0.6 + 1.0) meq = 22.3 meq

Hence CEC of the soil is 22.3 meq

b) From a given cations,      are acid cations

= (22.3 – (0.6 + 7.6)) = 14.1
(Recall: CEC = Number of exchangeable base cation + Number of exchangeable acid cations, and therefore number of exchangeable base cation = CEC – Number of exchangeable acid cations)

Using; Percent base saturation

Hence percent base saturation is 63.23


Percent aluminium saturation

Hence aluminium base saturation is 34.1%


SOIL REACTION – It is the acidity or alkalinity of the soil. The soil reaction can be acidic, neutral or alkaline due to the soil solution. It is
measured in PH using electrometric methods (PH – meter).  All soil PH range from 4 to 8. Soils with PH   4 generally contain sulphuric acids while those with PH 8 contain high percentage of Na+ and thus are alkaline.

Importance of soil PH

  •   Most crop plant prefers PH to be between 6 and 7.5 i.e slightly acidic up to slightly alkaline.
  •   The availability of nutrients to plant depends on soil PH. All plant nutrients are reasonably available between PH 6 and 7.5 which is the optimum PH. The availability of N, P, K, S,Ca, Mg decreases with the increase in soil acidity. Thus acidic soils are often deficient of these nutrients.

E.g.  Nutrient phosphorus is found as phosphate (v) in the soil. At PH   5, the soluble iron, aluminium and manganese react with         phosphates (v) to form insoluble complexes and then fix them (makes them unavailable for plants).

Fe, Mn and Cu precipitate at high PH. Thus deficiency of these nutrients often limits plants growth in alkaline soils. Manganese and Iron become plant toxic at low PH.


The problem with heavy metals toxic to humans arises near industrial area due to acid rain.

E.g.  Ladmium and lead, below PH 4 get dissolved and may enter plant and make crops unfit for human consumption


Determination of PH of agricultural soil is very important for the selection of suitable crops. Below the PH 4.8 a soil may need liming in order to avoid aluminum toxicity.

. Soil acidity is caused when heavy rains leach bases like Ca2+, Mg2+, K+ and Na+ from the soil to the ground water table leave surplus H+ in the soil.

. Also industrial regions may bring sulphuric (vi) acid and nitric (v) acid to the soil which increases its acidity.

. Acidic mineral fertilizers like ammonium sulphate (vi) and ammonium chloride make the soil more acidic due to hydrolysis.

. Also the nitrification of ammonium ions by bacteria produces H+(aq)

NOTE: Organic acids are produced during the decomposition of organic matter. Due to this, most soils in humid tropics are Acidic.

Liming & liming materials


–  Meaning and significance of liming as treatment to soil PH.

–   The efficiency of liming materials i.e. neutralizing values of carbonates, oxides, hydroxides and silicates.

–   Beneficial effects of liming

–   Detrimental effects of over-liming


It refers to the process of adding basic compounds of calcium and magnesium to acid soil (P    5) in order to raise the PH of the soil to the required level.

The compound of calcium and magnesium (oxides, hydroxides, carbonates and silicates) are called agricultural limes. Such compounds are contained in limestone, dolomite, building (slaked) lime, oyster shells and blast furnace slag which can be used for liming soil if finely grind.

Significance of Liming:

  • To raise the soil PH

Q.  1.      Explain similarities and differences between industrial fertilizers and manure

Q.  2.      To mention the advantages of straight and mixed fertilizers of N, P and K

Q.  3.      Mention examples of straight and mixed fertilizers and classify them

Q.  4.      Compare the relative advantages and disadvantages of manures as compare industrial fertilizers.

How to maintain and improve soil fertility:

A fertile soil provides all essential plant nutrients in amounts which are suitable for the growth of most plants.

How can a soil be made more fertile

1.   Good cropping system

2.  Adding manure to the soil

3.   Adding industrial fertilizers to the soil


Crop rotation should be practiced. Every season, another crop should be planted on a given field. Legumes should be rotated with cereals, shallow rooted plants with deep rooted ones .


Manures are organic materials that can be added to the soil to increase soil fertility.

E.g. Kraal manure – from cattle kraal

Farmyard manure – wastes from animals

Compost manure-mixture of soil and decomposing organic matter

Bio gas manure – from effluent of bio gas plants


–   Fertilizers are most inorganic compounds which contain one or more plant nutrients in a concentrated form.

E.g.  Nitrogen fertilizers, phosphorus fertilizers, potassium fertilizers.

Similarities between manure and industrial fertilizers.

–   They both increase humus to the soil and promote good soil structure

–   Both manure and industrial fertilizers contain minerals Nitrogen, phosphorus and potassium which are essential for plant growth.

Differences between manure and  industrial fertilizers

Manure Industrial fertilizers
1.   It is organic material 1.  It is inorganic compound
2.   It is composition varies depending on the type of manure 2.  It is composition is fixed
3.   It is less toxic 3.  It is toxic if used in large amounts
4.   It  has low contents of nutrients 4.  It has high content of nutrients


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