Nature and Properties of Humus–Discussed !

After reading this article you will learn about the nature and properties of humus.

Nature of Humus:

Humus is highly colloidal and amorphous in nature. Moreover, its surface area and adsorptive capacity are much higher than that of clay colloids. The cation exchange capacity (CEC) of silicate clays ranges from 8 to 150 C mol (P⁺) kg^-1, whereas the CEC of humus ranges from 150 to 300 Cmol (P⁺) kg^-1.

Humus has higher water adsorptive capacity than that of clay. Humic acids are weakly dissociated and determine the buffering capacity of soil.

Humic acids have a complex structure and it has mainly two types of structural units i.e. compounds of phenolic or quinoid nature and nitrogen containing compounds (amino acids and peptides). The humic acid molecules are not compact but have a loose structure with a large number of internal spaces which is more important in soil processes.

Fulvic acids contain considerably more acidic functional groups (—COOH) than that of humic acids. Another difference is that practically all the oxygen in fulvic acids can be accounted for in known functional groups (COOH, OH, C = O); a high portion of the oxygen in humic acids occurs as a structural component of the nucleus (e.g. in ether or ester linkages).

A humic acid molecule consists of micelles of a polymeric nature, the basic structure of which is an aromatic ring of the di- or tri-hydroxy-phenol type bridged by —O—, —CH₂—, —NH—, —N =, —S—, and other groups and containing both free OH groups and the double linkages of quinones.

Properties of Humus:

Physical Properties:

Humus possesses significantly low plasticity and cohesion. The application or maintenance of humus (through the supply of organic matter) in fine or heavy textured soils helps alleviate unfavourable soil structure due to formation of granulation.

Another physical characteristic included by the presence of adequate amount of humus in the soil is the soil colour. The development of soil colour influenced by humus is also affected by the variation of climate. The pigmentation is less intense in humid-temperate Zones and the least colouration is found in the humus of the tropics and subtropics.

The presence of soil organic matter or humus in optimum amounts improves the water holding capacity, rate of infiltration, porosity etc. of the soil.

Colloidal Properties:

Soil humus contains a number of substances like modified lignin, polyuronides and other constituents etc. which act as complex micelles. Under ordinary conditions, these carry a large number of negative charges. The humic micelles are composed of carbon, hydrogen and oxygen with trace amounts of nitrogen, sulphur, phosphorus and other elements.

In humus negative charges arises from the exposed functional groups —COOH and —OH from which at least part of the hydrogen may be replaced by cation exchange. Humic micelles also adsorb some elements like Ca²⁺, H⁺, Mg²⁺, K⁺, and Na⁺ etc.

So humus colloid may be represented as follows:

Humus and Nutrient Availability:

The capacity of humus colloid when saturated with H⁺ ions to increase the availability of certain other basic nutrient elements like Ca²⁺, K⁺ and Mg²⁺ through the cation exchange phenomenon. Once the exchange is made, the bases so affected are held in a loosely adsorbed condition and are easily available to plants.

H-humus, like that of H-clay acts as an acid and can react with soil minerals as follows:

Such change of molecular K to adsorbed K increases its availability to plants. In addition, conversion of organic N, P and S to available mineral forms (NH₄⁺, NO₃^– PO₄^3-, SO₄^2-) occurs through the activity of micro-organisms as well as by the C: N, C: P and C: S ratios 1 of decomposing plant residues.

As a general rule, an initial net mineralization (gain in mineral forms of the nutrients) will occur at C: N ratios <20: 1, C: P ratios <200: 1 and C: S ratios <200: 1.