Fractions and properties of humus

Fractions and properties of humus
    I. Humic group: The humic substances make up about 60-80 % of the soil organic matter. On the basis of resistance to degradation and of solubility in acids and alkalis, humic substances have been classified into five chemical groupings.
    1. Fulvic acid: lowest in molecular weight, lightest in colour, soluble in both acid and alkali and most susceptible to microbial attack. Contain uronides, simple carbohydrates and their sugars, phenolic glycosides, tannin and other organics, also rich in N and P. 
    2. Humic acid : Medium in molecular weight and colour, soluble in alkali but insoluble in acid and intermediate in resistance to degradation forms largest bulk, responsible for importing its characteristics properties to humus. 
    3. Humin: Highest in molecular weight, darkest in colour, insoluble in both acid and alkali and most resistant to microbial attack. Polymerized product of a part of the FA and HA fractions. 4. Apocremic acid 5. Hematomelanic acid. All the five fractions are amorphous and show no signs of crystallization.
    II. Non humic group: It comprises about 20-30% of the organic matter in soils. They are less complex and less resistant to microbial attack than those of humic group. They are comprised of specific organic compounds with definite physical and chemical properties.
    E.g.: a. Polysaccharides- Polymers with sugar like structures. They are effective in enhancing soil aggregates stability.
             b. Polyuronoids- They are not found in plants but have been synthesized by the soil microbes and held as part of the organisms body tissues.
            c. Organic acids and protein like materials.
    Properties of humus
  1. It is a light bulky amorphous material of dark brown to black colour. The black colour of surface soil is usually due to the presence of humus.
  2. It has a great water absorbing and water holding capacity. 100 part humus-181 part of water. 
  3. It possesses the power of adhesion and cohesion (but less than clay) so it acts as a cementing agent in crumb formation. In Sandy soils- adhesive capacity and in clay soils – cohesive capacity increases 
  4. It has a high ion adsorbing capacity (4-6 times that of clay) and CEC is very high (300-350 m.eq./100gm) 
  5. It is insoluble in water.
  6. It behaves like a weak acid and forms salts with bases.
  7. It acts as buffering agent and also as an oxidation reduction buffer.
  8. It serves as a source of energy and food for the development of various microorganisms.
  9. An important source of nutrients for higher plants.
Last modified: Wednesday, 7 December 2011, 6:19 AM