HUMUS                                               

 Formation and properties

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

Humus Formation

The humus compounds have resulted from two general types of biochemical reactions: Decomposition and Synthesis.

1. Decomposition:

a)Chemicals in the plant residues are broken down by soil microbes including lignin.

b)Other simpler organic compounds that result from the breakdown take part immediately in the biochemical synthesis.

c) These simpler chemicals are metabolized into new compounds in the body tissue of soil microbes.

d)The new compounds are subject to further modification and synthesis as the microbial tissue is subsequently attacked by other soil microbes.

2. Synthesis:

Involves such breakdown products of lignins, phenols and quinones.

a)These monomers undergo polymerization by which polyphenols and polyquinones are formed.

b)These high molecular weight compounds interact with N-containing amino compounds and forms a significant component of resistant humus.

c)Colloidal clays encourage formation of these polymers.

d)Generally two groups of compounds that collectively make up humus are the humic group and the nonhumic group. 

Properties of Humus

1. The tiny colloidal particles are composed of C, H, and O.

2. The colloidal particles are negatively charged (-OH, -COOH or phenolic groups), has very high surface area, higher CEC (150 – 300 cmol(p+) kg-1), 4 - 5 times higher water holding capacity than that of silicate clays.

3. Humus has a very favorable effect on aggregate formation and stability.

4. Impart black colour to soils.

5. Cation exchange reactions are similar to those occurring with silicate clays.

Clay – Humus Complex

1. Humus, the organic amorphous colloid supplies both basic and acidic ions which is transitory and ultimately disappears from soil.

2. Clay, the inorganic crystalline colloid supplies chiefly the basic nutrient ions is more or less stable.

3. Both these colloids form the soil colloidal complex and are extremely active and form important sources of plant nutrients.

4. It is believed that humus and clay exist in the soil as clay – humus complex, the two being held together by cations like Ca, Fe, etc.

5. Depending upon the nature of binding cation, two types of Clay – humus complex have been recognized.

6. The colloidal complex bound by Ca ions is more stable and is responsible for the favorable physical condition of the soil, particularly its structure.