Soil Colloids – Inorganic and Organic

Soil Colloids

The most active component of soil is called soil colloids, and these colloids control the chemical and physical characteristics of the soil. One thousandth of a millimetre (0.0001 mm; 0.0004 in) in size, these particles are very tiny. Some colloids are biological, whereas others are minerals, like other soil particles. Typically, fine clay particles are found in mineral colloids. They have thin, plate-like bodies that are visible under a microscope. When these particles are well combined with water, they stay suspended for a very long time and cause the water to become murky. Organic colloids are microscopic fragments of organic material that resist rotting.

Nature of Colloids:

Soil colloidal are two kinds:

(1) Inorganic (minerals) and

(2) Organic (humus).

Together, the two make up the soil’s colloidal complex. The inorganic colloidal complex makes up a sizable portion of the colloidal complex in almost all soils. On the other hand, it is virtually exclusively made up of organic colloids in peat soils. Colloidal particles float and do not often settle in a medium. Large colloidal particles could settle quite slowly in some cases. The scattered systems are called colloids.

(I) Inorganic Colloids:

Four primary constituents—silica, alumina, iron, and combined water—are identified by the chemical examination of clay. Between 90 and 98 percent of the colloidal clay is made up of them. Important plant nutrients like Mg++, Ca++, and K+ are more prevalent in the colloidal matter of soil.

The individual particles have a plate- or flake-like form (Fig. 5.1). An enormous number of positively charged ions are drawn to clay colloids because they are negatively charged (anions) (cations). The micelle-like clay colloids, also known as micro cells, often have negative charges.

Diagrammatic representation of colloidal clay crystals

(II) Organic Colloids:

The main cause of organic colloids is humus in the soil. The result of the breakdown of plant and animal remains is humus. Instead of silicon, aluminium, and oxygen as in clay colloids, humus colloids are made up of carbon, hydrogen, oxygen, and nitrogen. Compared to colloidal clay, organic soil colloids exhibit better adsorptive characteristics for water and cations (Ca++, K+, etc.) and a larger capacity for cation exchange (inorganic colloids).

Properties of Soil Colloids

1. Charge particles force colloidal particles to be constantly in motion.

2. By adding an oppositely charged ion, colloidal particles are converted from a liquid into a soft semisolid or solid mass.

3. Colloidal particles may pull gases, liquids, and solids out of suspension.

4. A semipermeable barrier can never let colloidal particles through.

5. Cohesion and adhesion are characteristics of colloidal particles.

Importance of Soil Colloids

The surfaces of soil colloids are attracted to soil nutrients that are dissolved in water as positively charged mineral ions or cations. Calcium (Ca++), magnesium (Mg++), potassium (K+), and sodium (Na+) are some of the cations that are necessary for plant development. To be accessible to plants when they are in close contact with root membranes, they must be dissolved in a soil-water solution.

The soil’s capacity to store and exchange cations—known as the cation-exchange capacity—determines the fertility of the soil-water mixture for plants. Without soil colloids, water would percolate through the soil and carry away the majority of essential nutrients.

The proportionate quantity of bases a soil holds determines the soil’s base status. When the soil has a high base status, it signifies that the base cations required for plant development are present in large quantities. The soil has a poor base status and is less fruitful if the colloids in the soil only contain a limited amount of bases. Humus colloids have high soil fertility.

Acid ions have the power to swap out the nutritional bases that adhere to the soil colloids’ surfaces. The bases are ejected into the soil solution as the bases are driven out by the acid ions and accumulate. The soil fertility is thus progressively diminished as the bases are gradually washed downward below rooting level. The acidity of the soil increases as a result. Despite not being plant nutrients, aluminium ions (Al+++) have the capacity to exhibit base cations, which lowers the base status and soil fertility.

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