1. Stable Organic Matter - Humus

2. Humic substancesNon-humic substances Fulvic acidHumic acidHumin i.e. polysaccharides, proteins, lignins, etc. In their natural or transformed states soluble in acid soluble in alkali insoluble in acid soluble in alkali insoluble in acid insoluble in alkali Humus Sparks (2002) Stable Organic Matter - Humus

3. Fulvic AcidHumic AcidHumin Light yellow Yellow brown Dark brown Black Grey black decrease in degree of solubility increase in molecular weight increase in carbon content 45% 62% decrease in oxygen content 48% 30% decrease in exchange acidity 1,400 500 meq 100 g -1 meq 100 g -1 Sparks (2002) Stevenson (2002) increase in nitrogen content 0.8% 5.0% Stable Organic Matter - Humus

4. Carboxyl Alcoholic OH Carbonyl (ketone) Amine Acidic groups Phenolic OHQuinone Neutral groups Basic groups Polyfunctionality Soil Organic Matter - Composition

5. Probable structures can be estimated using the dimer concept ? Soil Organic Matter - Composition

7. Carboxyl dimer Alkyl chain Phenolic - OH Carbonyl Amine

8. Soil Organic Matter - Composition