Soil Colloidal System, Humus its formation and role in the nature

Soil Colloidal system

Usually, the soil colloid is a very tiny particle, smaller than 1 mm in diameter. Its surface area plays the important role than the weight. Since the nutrients are absorbed on the soil colloidal particles, the constituents of the soil are the state of the reaction of the soil. The soil colloids have the strong influence on the chemical properties of the soil. The soil colloidal particles are 0.001 mm or even less in the size. There are two types of the colloids as inorganic and the organic soil colloids.

Inorganic Soil colloids (Colloidal Clay)

These inorganic soil colloids are derived from the weathering of the rocks and the other parent materials and are crystalline clay colloids. Further, these are subdivided into the following types as:

• Silicate Clay

These silicate clay are found in the temperate region and are made up of layers of the plates having ordered internal arrangement. The minute silicate colloidal particles are called micelle and itself carries the negative charge. Therefore, many cations are absorbed to the micelle to form a double layer.

• Iron- Aluminium Clay

These Iron- Aluminium clay is the Fe and Al oxides clay such as the Geothite, Al₂O₃.3H₂o, and Gibbsite i.e. Fe₂O₃.H₂O at the low pH and the Al Oxide clay acquires the positive charge and attract anions and the silicate clays.

Organic colloids

These organic colloids are the amorphous humus material derived from the decomposition of the plants and the animal remains. The humus particles in the organic colloids are extremely small and the negative charge on the humus is the pH dependent. In the acidic condition, the hydrogen is tightly bound and is not easily replaced by the other cation. But in the high pH, the hydrogen of the oxalic group, phenolic group, and the enolic group are replaced by the OH group to form the water. Due to such negative charge, the positive cations are absorbed by the surface of the humus colloids.

Humus and its formation and role in nature

During the process of the decomposition of the organic matter, it is turned into the dark brown or the black stable matter, which is known as humus. The humus consists of the various chains and the loop of the linked carbon atoms. Humus is considered as an amorphous material, which is nearly insoluble in the water, and the fraction of it is soluble in the dilute alkali solution.

The humus-forming process is a complicated biological process. When the organic tissue of the living forms is incorporated into the moist warm soil, it is immediately attacked by the host of the soil organisms. Initially, the microbial resistance compounds of the higher plant origin that have been modified by the biochemical and even more resistance to the microbial attack.

In this, the modified lignin compounds are the most abundant, although the minor amount of the oils, fats, and the waxes may be present. The compounds like polysaccharides and the polyuronides are synthesized by the microorganism and held as the part of their tissue. The modified lignins are the similar materials that are partially oxidized during the decomposition, thereby increasing the reactivity of the material. The aromatic groups are associated with the lignin bounded with the protein substances, thereby protecting the protein from the further attack.

The humic substances form the side reactions then, bind the protein and the other nitrogenous compounds as an integral part of the humus complex, protecting the nitrogen from degrading to the simple inorganic forms. The organic materials including lignins, tannins, melanins and the humic acid are known to react with the protein. And protect them from the microbial attack. Such nitrogenous compounds react with the aromatic and the quinone group as well as the polysaccharide.

The humus is the complex and the rather resistant mixture of the brown or the dark amorphous and colloidal substances that are modified from the original tissue or synthesized by the various soil organisms. The tiny colloidal humus particles have more adsorption capacities than those of the layered silicate minerals. Since the surface area of the humus, the colloid is very high, exceeding that of the silicate clay the cation exchange capacity of the humus is negatively charged. The extent of the negative charge is the pH-dependent which becomes high at the high pH value. The water holding capacity of the humus is also high. The humus is varied rich in the nutrient so the productivity is also high.

Significant properties of the humus

During the decomposition process, the fresh organic matter is turned into the dark brown stable organic matter which is also rich in the nutrients is known as the humus. Since it is an amorphous material, it is nearly insoluble in water and is nearly soluble in the dilute alkali solution.

The significant characteristics of the humus are:

1. The tiny humus particles are composed of C, H, and O.
2. The surface area of the humus colloids very high, generally exceeding that of the silicate clay.
3. The colloidal surface of humus are negatively charged, the source of charge being carboxylic or the phenolic group. The extent of the negative charge is pH dependent which becomes high in the high pH.
4. The cation exchange capacity of humus far exceeds than that of the silicate clay.
5. The water holding capacity of the humus on the weight basis is 4-5 times than that of the silicate clay.
6. The humus has the low plasticity and the cohesion which helps for its very favourable effect on the aggregate formation and the stability.
7. The black color of the humus tends to distinguish its form and most of the other colloidal systems in the soil.
8. The cation exchange reactions with the humus are qualitatively similar to those occurring with the silicate clay.
9. The humus is very rich in nutrient. So the productivity of humus rich soil is high.

References:

Santra, S. (2004). Environmental Science . India: New Central Book Agency (p) Ltd.

T., R. (2008). Towards a Sustainable Future . India: PHI (p)Limited.

Keller, E.A.Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company,Bell,and Howell Company, 1985.