Humus – A Product of Decomposition

Humus is an amorphous organic substance resulting from physical, chemical and microbial transformation of the organic compound. Decomposition of organic matter into humus signifies co-existences of biodiversity. Successive steps of decomposition results in the formation of humus and the presence of seven different organic substance such as crenic acid, apocrenic acid, geic acid, humic acid, humin, ulmic acid and ulmin are known to exist. According to Mulder (1849) these seven organic substances are intimately related and on decomposition of organic matter in soil ulmic acid is formed first and further oxidation produces humic acid, geic acid, apocrenic acid and finally crenic acid. Berzelius (1839) assigned the name crenic and apocrenic acid. Oden in 1912 suggested the name fulvic acid replacing the name crenic and apocrenic acid. Humus consists of humic substances such as humic acid, fulvic acid and humin which is defined according to their solubility.

Humic Acid:

Humic acid is a major constituent of the soil humus. It is a mixture of different acids and is not soluble in water at an acid pH. The phenolic and carboxylic group enables humic acid to form complexes with ions such as calcium, magnesium, iron etc. Formation of metal chelate is a significant aspect of humic acid. Chelate ion manage the bioavailability of metal ions. Humic acid modifies the soil structure, increase clay plasticity. The charged humic acid bind with clay particle forming aggregate.

Fulvic Acid:

Fulvic acid a component of humic substances are soluble in water. It is a natural electrolyte and is a mixture of weak aliphatic and aromatic acid (aliphatic compound, where carbon atom forms open chain and not an aromatic ring whereas, aromatic substance contain an aromatic ring). Fulvic acid can be found in a liquid state. It is a unique colloid having an ability to diffuse through the membrane.

 Humin: 

It’s a part of humic substance and is not soluble in water at any pH range. Humin present in soil is highly resistant to decomposition.

Degradation or inactivation of toxic substances is mediated by humic substances. Humus can be used for bioremediation of polluted or contaminated site.

References:

Berzelius, J. J.  1839 Lehrbuch der Chemie 3rd edition Translated by Wohler 8:11-16 also 384-431

Mulder, G. J. 1849 The Chemistry of Vegetable and Animal Physiology. Translated from the Dutch by P. F.H. Fromberg with an introduction & notes by James F.W. Johnson, Edinburgh, 1845-1849

Oden, S. 1912 Zur Kenntnis der Huminsauren des Sphagnum Torfes Ber. dtsch. chem. Ges., 35: 651