Humus in Soil: Synthesis and Colloidal Characteristics

After reading this article you will learn about the synthesis and colloidal characteristics of humus in soil.

Synthesis of Humus:

Humus is a heterogeneous mixture of complex organic compounds. The bulk of these compounds have resulted from decomposition and synthesis. The organic residues which undergo breakdown and transformation are complex in nature, containing carbohydrates, proteins and other nitrogenous compounds, lignins, fats etc.

Humus is a complex mixture of amorphous and colloidal organic substances which is more or less biologically stable product.

On these facts humus may be defined as follows:

Humus is a complex and rather resistant mixture of brown or dark brown amorphous and colloidal substances that results from microbial decomposition and synthesis and has chemical and physical properties of great significance to soils and plants.

—Nyle C. Brady

The humic and non-humic compounds collectively make up humus.

(A) The humic substances comprise about 60-80 per cent of the soil organic matter. Two more complex substances polyphenols and polyquinones are formed by decomposition, synthesis and polymerisation. Humic substances are dark in colour, amorphous and have high molecular weight.

Humic substances can be classified into three chemical groups:

(i) Humin, highest in molecular weight, dark in colour, insoluble in both acid and alkali and more resistant to microorganisms attack;

(ii) Humic acid, medium molecular weight, medium-dark in colour, soluble in alkali, insoluble in acid, intermediate resistant to microbial degradation ; and

(iii) Fulvic acid, lowest molecular weight, light in colour, soluble in both acid and alkali, and most susceptible to microorganism attack.

(B) The non-humic group makes up about 20-30 per cent of the organic matter in soils. The non-humic substances consist polymers, polysaccharides, polyuronides. Polysaccharides are especially effective in enhancing soil aggregate stability. Non-humic group also have some simpler compounds such as organic acids and some protein-like materials. Non-humic substances are less complex and less resistant to microbial attack than humic substances.

Humus is more resistant to decay than are most compounds found in plant residues. Humus may remain present in soils even hundreds of years. This resistant property of humus is important in maintaining organic matter level in soils and protect nitrogen and other essential nutrients that are found in humus complex.

Colloidal Characteristics of Humus:

Important colloidal properties of Humus are as under:

(i) Colloidal humus particles are composed of carbon, hydrogen and oxygen (probably in the form of polyphenols, polyquinones, polyuronides and polysaccharides).

(ii) Humus—colloids are black in colour.

(iii) Humus helps in aggregate formation and provides stability.

(iv) Humus and silicate clay are similar in cation exchange reaction. At high pH values the cation exchange capacity on a mass basis far exceeds that of most silicate clays.

(v) The water-holding capacity of humus is more than silicate clays.

(vi) The surface area of humus colloids is higher than silicate clays.

(vii) The collodial surface of humus are negatively charged.

Difference between Organic Matter and Humus. Humus is a complex and rather resistant mixture of brown (or dark brown) amorphous and colloidal substances modified from the original tissues or synthesised by the various soil organisms.

Soil organic matter may be distinguished from humus. Soil organic matter consists of any substances of organic origin (living or dead), while formed as a result of decomposition of plant and animal residues, which is formed as a result of decomposition of plant and animal residues, with no trace of the structure of the material from which it is derived.

Humus in reality is not merely a decomposition product since decomposition implies transformation of simpler compounds, whereas humus is more complex than original organic matter. Besides decomposition of organic matter, synthesis or organic substances is involved in humus formation.

Humus contains on an average 58% carbon which is somewhat greater than the carbon content of plant tissues (45-50%). Humus possesses a definite C : N ratio which varies from 10 : 1 to 12 : 1. The actual variation in C : N ratio of soil organic matter is very wide (40 : 1 to 12 :1). Carbon content of humus is less variable than nitrogen.

Another characteristic which distinguishes humus from non-humified organic matter is its behaviour with hydrogen peroxide. Three per cent hydrogen peroxide has been found to oxidize humus leaving the un-decomposed matter un-attacked.