Agriculture for Engineers 4 (3+1)

Lesson 7. Soil Inorganic Colloids

7.2 PROPERTIES AND IMPORTANCE OF SOIL COLLOIDS

(1) Brownian movement: colloidal particles are found to be in continual motion. The oscillation is due to the collision of colloidal particles or molecules with those of liquid in which they are suspended. This movement is mainly responsible for the coagulation or flocculation of colloidal particles. When the particles in suspension collide with each other and form a loose aggregate or floc.

(2) Flocculation: The colloidal particles are coagulated by adding an oppositely charged ion. Formation of flocs is known as flocculation. If the cations are held close to the negatively charged particles, then negative charge would be neutralized and the colloidal particles flocculate and settle down. Addition on any electrolyte brings all such dispersed particles flocculate and settle down. If the particles are deflocculated, the aggregates get dispersed, the soil gets water-logged, and the movement of air and water is impeded.

(3) Electrical charge: Colloidal particles often have an electrical charge, some positive and some negative. When clay colloids suspended in water, it carries a negative electric charge. Colloidal clay develop negative electric charge due to dissociation of hydroxyl groups attached to silicon in silica sheets of the clay mineral leaves residual oxygen (O^--) carrying a negative charge.

(4) Adsorption: Colloidal particles posses the power of adsorption gases, liquid and even solids from their suspension. The phenomenon of adsorption is confined to the surface of colloidal particles, larger the surface area grater the adsorption for water, nutrients etc.

 The adsorption of ions is governed by the type and nature of ion and the type of colloidal particle. In the case of cations, the higher the valence of the ion, the more strongly it is absorbed.

Exchange or replacement of cation would be difficult from colloidal particle. That is why divalent ions (Ca⁺² & Mg⁺²) are held more strongly than monovalent ion (Na⁺ & K⁺). A trivalent cation (Al⁺³) is most readily absorbed. Hydrogen ions (H⁺) behave as polyvalent ions so are adsorbed more strongly than even Ca^+2. Adsorption of anions (H₂PO₄, HPO₄^-2 etc.) increases with the lowering or increasing of pH. The adsoption of phosphate ions is the lowest when the medium is neutral: it increases when the pH either falls or rises, due to fixation by iron and aluminum hydroxides in acid range and by calcium in alkaline range. Among the clay minerals, kaolinitic clay has a greater anion adsorbing capacity than montmorillonitic or illitic clay.

The property of adsorption plays an important role in soil fertility. Due to his property soils is able to held water and nutrients and keep them available to plant.

(5) Non-permeability: Colloids are unable to pass through a semi permeable membrane. The membrane allows the passage of water and of the dissolved substance through its pores, but remains the colloidal particle.

(6) Cohesion and adhesion: Unlike sand, clay particles possess the properties of cohesion. While forming aggregates, the colloidal clay particles unite with each other by virtue of the property of cohesion. Clay particles envelop sand particles under the force and adhesion. The force of cohesion and adhesion are developed in the presence of water. When colloidal substances are wetted, water adheres to the particles and then brings about cohesion between two or more adjacent colloidal particles. Soil when dried. The particles remain united because of the force of molecular cohesion. These two forces help in the retention of water in the soil and thus used by plants and microorganism.

(7) Swelling: A soil colloid when brought in contact with water they imbibe a certain quantity of water and swell and increases in volume.

(8) Plasticity: Soil colloidal particles may present in gel condition possess the property of plasticity. Due to this property clay-colloids can be moulded in any shape.  

7.3 NATURE OF COLLOIDS

Soil colloids are of two kinds: (1) inorganic (minerals) and (2) organic (humus).

(1) Inorganic colloids : (a) Silicate clays (dominant in temperate regions) and    (b)  Iron and aluminum hydrous oxide clays (occurs in tropical and sub- tropical region soils)

(2) Organic colloids :  Humus (dominant in temperate region soils)

The two together form the colloidal complex of the soil. In almost all soils the inorganic colloids form a major portion of the colloidal complex. On the other hand, in peat soils, it consists almost entirely of organic colloids. Colloidal particles float in a medium and do not tend to settle. Colloids are referred as the dispersed system. The substance in solution is termed as the dispersed phase while the medium in which the particles are dispersed is called the dispersion medium. Soils formed in temperature regions usually contain more organic colloids than those formed in tropical and sub-tropical regions. In a broad way, two groups of clay are recognized silicate clay so characteristic of temperature regions and the iron and aluminium hydrous oxide clays found in tropical and semi-tropical.

A. Chemical composition and structure of colloids

The constitutions of colloids are (1) inorganic and (2) organic

1. Inorganic colloids

The chemical analysis of clay indicates the presence of silica, alumina, iron and combined water. These make up from 90 to 98 per cent of the colloidal clay. The colloidal matter of soil contains a higher proportion of the important plant nutrients such as Mg⁺², Ca⁺² and K⁺. The shape of the individual particles is plate or flake-like.

The minute colloidal clay particle is technically called micelle and it possesses negative charges. The magnitude of –ve charge is different under conditions. Normally, as the pH increases, negative charges increases. Due to the formation of –ve charge, clay particle attracts +ve charged ions and thus it forms an ionic double layer.

Composition of silicate clay minerals

          On the basis of number and arrange of silica and alumina sheets. Silicate clay may be classified into two types: (a) two layer type (1:1) and (b) three layer type (2:1)

          The silicate clay minerals are composed of two types of sheets, (1) silica sheet (tetrahedral) and (2) alumina sheet (octahedral).

Fig. 7.1: (1) Silica sheet (tetrahedral) and (2) Alumina sheet (octahedral)

In a silica sheet one silicon cations is surrounded by four oxygen anions. The four-sided configuration is called as a silica tetrahedron. An interlocking of a series of such silica tetrahedron horizontally by shared oxygen anions gives a tetrahedral sheet. Similarly in alumina sheet aluminium (or magnesium) ion is surrounded by six oxygen or hydroxyls gives an eight-sided configuration termed as alumina octahedron. Numerous octahedrons linked together horizontally give an octahedral sheet.

The tetrahedral and octahedral sheets are bound together in various combinations in different silicate clay by shared oxygen anions; such association is known as crystal units.

(a) The two layer type (1:1)

It consists of one layer of silicon and oxygen atoms (SiO₂) and the other layer of aluminum and oxygen atoms (Al₂O₃), all in definite arrangement. e.g. kaolinite clay. In this type of structure, there is non-expanding space between the sheets for the activity, thus, cation exchange capacity is low in kaolinite clay.

(b) Three layer type (2:1)

Clay crystals have two outside layers made of silicon and oxygen (SiO₂) and the middle layer of aluminum and oxygen (Al₂O₃). i.e. montmorillonite. In this type, there is expanding space between the sheets. The cation exchange capacity is therefore greater in montmorillonite than kaolinite. The plasticity of montmorillonite is also higher because water can enter between the sheets.

In three-layer type, there is another group called hydrous mica. Illite is the most important example of this group. Illite has similar structure as montmorillonite (2:1 lattice structure). The structure is non-expanding type. Little is in between of kaolinite and montmorillonite type with regards to soil properties (Table 7.4).

Table 7.4: Properties of different types of clay minerals

Sr. No.	Property	Kaolinite	Montmorillonite	Illite
1	Structure	1:1 lattice (non expanding type)	2:1 lattice (expanding type)	2:1 lattice (non expanding type)
2	Size(micron)	0.1-5.0 (course particle)	0.01-1.0 (fine particle)	0.1-2.0 (medium particle)
3	Shape	Hexagonal crystals	Irregular flakes	Irregular flakes
4	Surface area (m2/g)	5-20	700-800	11-120
5	Substitution	No substitution	Substitution in alumina sheet by Mg or Fe	Substitution in silica layer by alluminum
6	Non exchangeable cations	None	Magnesium	Potassium
7	Cation exchange capacity (me/100g)	3-15	80-100	15-40
8	Anion exchange capacity (me/100g)	High	Low	Medium
9	Cohesion	Less	High	Medium
10	Plasticity	Less	High	Medium
11	Swelling capacity	Less	High	Medium
12	Porosity	High	Low	Medium
13	Permeability	High	Low	Medium