1Unit 5

Soil Colloid

5.1 Introduction:

The soil colloids are the most active portion of the soil and largely determine the

physical and chemical properties of a soil. Inorganic colloids (clay minerals, hydrous oxides)

usually make up the bulk of soil colloids. Colloids are particles less than 0.001 mm in size

and the clay fraction includes particles less than 0.002 mm in size. Therefore, all clay

minerals are not strictly colloidal. The organic colloids include highly decomposed organic

matter generally called humus. Organic colloids are more reactive chemically and generally

have a greater influence on soil properties per unit weight than the inorganic colloids. Humus

is amorphous and its chemical and physical characteristics are not well defined. Clay

minerals are usually crystalline (although some are amorphous) and usually have a

characteristic chemical and physical configuration. Both inorganic and organic colloids are

intimately mixed with other soil solids. Thus, the bulk of the soil solids are essentially inert

and the majority of the soil's physical and chemical character is a result of the colloids

present.

5.2. General Properties of Soil Colloids

The general properties of soil colloids are described below.

i. Size: The most important common property of inorganic and organic colloids is their

extremely small size. They are too small to be not seen with an ordinary light microscope.

Only with an electron microscope they can be seen. Most are smaller than 1 micrometer in

diameter.

ii. Surface area: Because of their small size, all soil colloids expose a large external surface

per unit mass. The external surface area of 1 g of colloidal clay is at least 1000 times that of 1

g of coarse sand. Some colloids, especially certain silicate clays have extensive internal

surfaces as well. These internal surfaces occur between plate like crystal units that make up

each particle and often greatly exceed the external surface area. The total surface area of soil

colloids ranges from 10 m2/g for clays with only external surfaces to more than 800 m2/g for

clays with extensive internal surfaces. The colloid surface area in the upper 15 cm of a

hectare of a clay soil could be as high 700,000 km2/g.

2iii. Surface charges: Soil colloidal surfaces, both external and internal characteristically

carry negative and/or positive charges. For most soil colloids, electro negative charges

predominate. Soil colloids both organic and inorganic when suspended in water, carry a

negative electric charge. When an electric current is passed through a suspension of soil

colloidal particles they migrate to anode, the positive electrode indicating that they carry a

negative charge. The magnitude of the charge is known as zeta potential. The presence and

intensity of the particle charge influence the attraction and repulsion of the particles towards

each other, there by influencing both physical and chemical properties.

iv. Adsorption of cations: As soil colloids possess negative charge they attract the ions of an

opposite charge to the colloidal surfaces. They attract hundreds of positively charged ions or

cation such as H+, A13+ Ca2+ and Mg2+.

v. Adsorption of water: In addition to the adsorbed cations, a large number of water

molecules are associated with soil colloidal particles. Some are attracted to the adsorbed

cations, each of which is hydrated; others are held in the internal surfaces of the colloidal

particles. These water molecules play a critical role in determining both the physical and

chemical properties of soil.

vi. Cohesion: Cohesion is the phenomenon of sticking together of colloidal particles that are

of similar nature. Cohesion indicates the tendency of clay particles to stick together. This

tendency is primarily due to the attraction of the clay particles for the water molecules held

between them. When colloidal substances are wetted, water first adheres to the particles and

then brings about cohesion between two or more adjacent colloidal particles.

vii. Adhesion: Adhesion refers to the phenomenon of colloidal particles sticking to other

substances. It is the sticking of colloida1 materials to the surface of any other body or

substance with which it comes in contact.

viii. Swelling and shrinkage: Some clay (soil colloids) swell when wet and shrink when dry.

After a prolonged dry spell, soils high in smectites (e.g. Vertisols) often are crises-crossed by

wide, deep cracks, which at first allow rain to penetrate rapidly. Later, because of swelling,

such soil is likely to close up and become much more impervious than one dominated by

kaolinite, chlorite, or fine grained micas. Vermiculite is intermediate in its swelling and

shrinking characteristics.

ix. Dispersion and flocculation: As long as the colloidal particles remain charged, they repel

each other and the suspension remains stable. If on any account they loose their charge, or if

3the magnitude of the charge is reduced, the particles coalesce, form flocs or loose aggregates,

and settle out. This phenomenon of coalescence and formation of flocs is known as

flocculation. The reverse process of the breaking up of flocs into individual particles is

known as deflocculation or dispersion.

x. Brownian movement: When a suspension of colloidal particles is examined under a

microscope the particles seem to oscillate. The oscillation is due to the collision of colloidal

particles or molecules with those of the liquid in which they are suspended. Soil colloidal

particles with those of water in which they are suspended are always in a constant state of

motion. The smaller the particle, the more rapid is its movement.

xi. Non permeability: Colloids, as opposed to crystalloid, are unable to pass through a semi-

permeable membrane. Even though the colloidal particles are extremely small, they are

bigger than molecules of crystalloid dissolved in water. The membrane allows the passage of

water and of the dissolved substance through its pores, but retains the colloidal particles.

5.3. Chemical Composition of Soils:

When soils weather and the mineralogical composition changes over time, there is a

corresponding change in chemical composition. During soil formation, there is a preferential

loss of silicon relative to aluminum and iron. The major elements in soils are those with

concentrations that exceed 100 mg kg1, all others being termed trace elements. The major

elements include O, Si, Al, Fe, C, K, Ca, Na, Mg, Ti, N, S, Ba, Mn, P, and perhaps Sr and Zr,

in decreasing order of concentration.

5.4. Ion Exchange

When soils weather and the mineralogical composition changes over time, there is a

corresponding change in chemical composition. Ion exchange involves cations and anions

that are adsorbed from solution onto negatively and positively charged surfaces, respectively.

Such ions are readily replaced or exchanged by other ions in the soil solution of similar

charge, and thus, are described by the term, ion exchange. Cation exchange is of greater

abundance in soils than anion exchange.

5.5. Nature of Cation Exchange

Cation exchange is the interchange between a cation in solution and another cation on

the surface of any negatively charged material, such as clay colloid or organic colloid. The

negative charge or cation exchange capacity of most soils is dominated by the secondary clay

4minerals and organic matter. Therefore, cation exchange reactions in soils occur mainly near

the surface of clay and humus particles, called micelles. Each micelle may have thousands of

negative charges that are neutralized by the adsorbed or exchangeable cations. The negatively

charged micellar surfaces form a boundary along which the negative charge is localized. The

cations concentrate near this boundary and neutralize the negative charge of the micelle. The

exchangeable cations are hydrated and drag along the hydration water molecules as they

constantly move and oscillate around negatively charged sites. The concentration of cations is

greatest near the micellar surfaces, where the negative charge is the strongest. The charge

strength decreases rapidly with increasing distance away from the micelle, and this is

associated with a reduction in cation concentration away from the micelle. Conversely, the

negatively charged micellar surface repels anions. This results in a decreasing concentration

of cations and an increasing concentration of anions with distance away from the micellar

surface. At some distance from the micellar surface, the concentration of cations and anions

is equal. An equilibrium tends to be established between the number of cations adsorbed and

the number of cations in solution. The number of cations in solution is much smaller than the

number adsorbed (generally 1 percent or less) unless the content of soluble salts is high.

Roots absorb cations from the soil solution and upset the equilibrium. The uptake of a cation

is accompanied by the excretion of H+ from the root and this restores the charge equilibrium

in both the plant and soil.

5.6. Cation Exchange Capacity of Soils

The cation exchange capacity of soils (CEC) is defined as the sum of positive (+)

charges of the adsorbed cations that a soil can adsorb at a specific pH. Each adsorbed K+

contributes one + charge, and each adsorbed Ca2+ contributes two + charges to the CEC. The

CEC is the sum of the + charges of all of the adsorbed cations (Conversely, the CEC is

equivalent to the sum of the - charges of the cation exchange sites). The CEC is commonly

expressed as centimoles of positive charge per kilogram [cmol(+)/kg], also written as

cmol/kg, of oven dry soil.

5.7. Cation Exchange Capacity versus pH of Soils

The CEC of a soil is equal to the CEC of both the mineral and organic fractions. The

definition of CEC specifies that the CEC applies to a specific pH because the CEC is pH

dependent. To make valid comparisons of CEC between soils and various materials, it is

necessary to make the determination of CEC at a common pH. The CEC is positively

5correlated with pH; therefore, acid soils have a CEC less than the maximum potential CEC.

Changes in CEC with changes in soil pH are important in the management of intensively

weathered and acid soils in tropical regions because of the generally low CEC and the highly

pH-dependent nature of the CEC.

5.8. Fertility of soil versus pH

Fertility of soil leads the Plant growth and it is the function of nutrients available in

soil. Perhaps the greatest general influence of pH on plant growth is its effect on the

availability of nutrients for plants. Nitrogen availability is found maximum between pH 6 and

8, because this is the most favorable range for the soil microbes that mineralize the nitrogen

in organic matter and those organisms that fix nitrogen symbiotically. High phosphorus

availability at high pH-above 8.5-is due to sodium phosphates that have high solubility.

The pH requirement of some disease organisms is used as a management practice to

control disease. One of the best known examples is that of the maintenance of acid soil to

control potato scab. Potato varieties have now been developed that resist scab organisms in

neutral and alkaline soils. Damping-off disease in nurseries is controlled by maintaining soil

pH at 5.5 or less. Also, Earthworms are inhibited by high soil acidity.