Soil Colloids

Soil

Soil is the unconsolidated mineral layer on the top of the Earth's crust that serves as a natural medium for the growth of land plants.

Soil colloids

Soil colloids are extremely small organic and inorganic particles in the soil, responsible for its fertility. They play a critical role in determining the physical and chemical properties of the soil.

Formation of soil colloids

Soil colloids are formed during the weathering process. As soil forms, minerals and organic matter break down into extremely small particles. These particles undergo chemical changes, making them smaller until they can no longer be seen with the naked eye. The smallest of these particles are what we refer to as soil colloids.

Types of soil colloids

1. Inorganic colloids
   1. Clay minerals (layer silicate clays): These are composed of two horizontal sheets, one dominated by silicon and the other by aluminium or magnesium.
   2. Iron and aluminium oxides: These are residual materials left after extensive weathering. They are sesquioxides, which are oxides of iron or aluminium contaminated with hydroxides.
   3. Allophane and amorphous clays: These are mixtures of silica and alumina, and are amorphous (lacking a definite structure).
2. Organic colloids
   1. Humus: Humus is highly decomposed organic matter. It is dark brown to black and nearly insoluble in water, but more soluble in dilute alkaline solutions like NaOH or KOH. Humus consists of carbon chains and is an intermediate product after plant and animal remains have undergone significant decomposition. It contains functional groups such as carboxyl, phenolic, and enolic groups.

Properties of soil colloids

1. Surface area: Soil colloids have a large external surface area due to their small size. For instance, 1g of colloidal clay exposes at least 1000 times the surface area of 1g of coarse sand. This large surface area is important for nutrient and water retention.

2. Electric charge: Soil colloids typically carry both positive and negative charges on their surfaces. In general, positive charges are predominant, but soil colloids in water tend to carry a negative charge. This negative charge comes from two main sources:

   1. Ionizable hydrogen ions: Hydrogen ions (H⁺) can ionize from hydroxyl groups (OH) on the clay surfaces, leaving behind a negative charge.

      

   2. Isomorphous substitution: This occurs when one ion is substituted by another ion of similar size but lower charge, resulting in a negative charge on the colloidal surface.

3. Ion exchange (adsorption of cations): Soil colloids possess a negative charge, which attracts positively charged ions (cations) such as H⁺, Ca²⁺, and Mg²⁺ to the colloidal surface. This forms an ionic double layer, with cations in the outer layer being loosely bound to the negative colloidal surface. This ion exchange is vital for soil fertility, as it allows the soil to retain and release essential nutrients to plants.