Humic Substances 
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Humic substances (humus) are organic carbon-containing materials occuring in soils, fresh waters and in the sea. They are darkcolored, predominantly aromatic acidic, hydrophylic, molecularly flexible polyelectrolytes. Humic substances are formed from the chemical and biological degradation of plant and animal residues and from synthetic activities of microorganisms.
One of their main characteristics is their ability to interact with metal ions, oxides, hydroxides and minerals.
Humic substances enhance plant growth directly through positive physiological effects and indirectly by affecting the physical, chemical and biological properties of soils. They have also nutritional effects in that they serve as sources of nitrogen, phosphorus and sulphur for plants and microbes; biological functions in that they affect the activities of microorganisms; and physical functions in that they promote good soil structure, thereby improving tilth, aeration and retention of moisture (Stevenson,1982). All of these effects increase agricultural productivity.

Humic substances consists of three main fractions:

  1. Humic Acid (HA), which is soluble in dilute alkali but is coagulated by acidification of the alkaline extract.
  2. Fulvic Acid (FA), which is soluble in both dilute alkali and dilute acid.
  3. Humin, which is that humic fraction which cannot be extracted from the soil by dilute base or acid.

References: M.Schnitzer, Humic substances effects on soil and plants, Reda 1986

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