Soil colloids are made up of the smallest particles of humus and clay in the soil. Only particles less than 0.001 mm are officially defined as colloids. Humus particles are tiny pieces of organic matter and are known as Organic Colloids. The clay particles are known as Inorganic Colloids.
Soil colloids are the most active portion of the soil and determine the physical and chemical properties of a soil. Clay is defined as any particle that is less than 0.002 mm in size – this is tiny, requiring an electron microscope to view the individual particles. However, as soil colloids are defined as particles less than 0.001 mm in size not all clay particles are strictly colloids. Inorganic colloids usually make up the bulk of soil colloids, but the humus particles or organic colloids are the most reactive chemically and generally have a greater influence on soil properties per unit weight than the clay particles.
Nature of Colloids:
Soil colloidal are two kinds:
(1) Inorganic (minerals) and
(2) Organic (humus).
(I) Inorganic Colloids:
The chemical analysis of clay indicates the presence of four main constituents; 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 important plant nutrients such as Mg++, Ca++ and K+.
The shape of the individual particles is plate or flake-like (Fig. 5.1). Clay colloids are negatively charged (anions) and therefore attract a large number of positively charged ions (cations). The minute clay colloids particles, referred as micelles (micro cells), ordinarily carry negative charges.
(II) Organic Colloids:
Organic colloids are chiefly due to presence of humus in soil. Humus is the product of decomposition of plant and animal residues. Humus colloids are composed of carbon, hydrogen, oxygen and nitrogen, instead of silicon, aluminium and oxygen, as in clay colloids. Organic soil colloids have higher adsorptive properties for water and cations (Ca++, K+ etc.) and higher cation exchange capacity than colloidal clay (inorganic colloids).
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 the 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 of floe.
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, the negative charge would be neutralized and the colloidal particles flocculate and settle down. Na+ (Sodium Cations) are highly hydrated and are monovalent; they are not so closely bound with the negatively charged immobile particles.
Thus, the particles continue to offer resistance to aggregation and do not flocculate. Ca++ (Calcium cations) are divalent and are not as easily displaced as sodium. Thus, calcium ions are able to neutralize the negative charge more efficiently and the colloidal system tends to flocculate. In a similar manner, trivalent ions like aluminium (Al+++) are still more efficient in flocculation of colloids. Thus, Na-clay produces de-flocculation and Ca-clay encourages aggregation.
The phenomenon of flocculation plays an important part in the cultivation of soils. When clay particles are flocculated, soil develops small clods of a crumby nature. Such a soil allows free movement of air and water. If the particles are deflocculated, the aggregates get dispersed, the soil gets water-logged, 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, they carry a negative electric charge. Collodial clay develops 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.
Colloidal particles possess the power of adsorbing gases, liquid and even solids from their suspension. The phenomenon of adsorption is confined to the surface of colloids particles. Larger the surface area (area per unit weight) greater the adsorption for water, nutrients etc. For example, take a cube of 1 cm edge.
The surface area exposed by this cube is equal to 6 sq. cm. When this cube is sub-divided into 8 cubes of edges 0.5 cm, the surface area exposed is 12 sq. cm. Thus, the smaller the size of the particle the greater the surface area exposed by them. 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 like calcium and magnesium (Ca++& Mg++) are held more strongly than monovalent ion, sodium and potassium (Na+ and K+). Aluminium (Al+++), a trivalent cation, is most easily adsorbed. Hydrogen ions (H+) behave a polyvalent ions so are adsorbed more strongly than even Ca ++.
Adsorption of anions (H2 PO4– , HP O4– etc.) increases with the lowering or increasing of pH. The adsorption 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 aluminium hydroxides in acid range and by calcium in alkaline range.
Among the clay minerals, kaolinitic clay has a greater anion adsorbing capacity than montmorillonic or illitic clay. The property of adsorption plays an important role in soil fertility. Due to this property soil is able to hold water and nutrients and keep them available to plant.
Colloids, are unable to pass through a semipermeable membrane. The membrane allows the passage of water and of the dissolved substance through its pores, but retains the colloidal particles.
6. Cohesion and adhesion:
Unlike sand, clay particles possess the properties of cohesion and adhesion. 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 of adhesion. The force of cohesion and adhesion are developed in the presence of water.
When colloidal substances are wetted, water first 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.
A soil colloid when brought in contact with water they imbibe a certain quantity of water and swell and increase in volume.
Soil colloidal particles may present in gel condition posses the property of plasticity. Due to this property clay-colloids can be moulded in any shape.