Carbon occurs in soils in four forms of mineral and organic matter.
1. Carbonate and Mineral Forms:
Chiefly CaCO3 and MgCO3 also occur as CO2 and HCO3 and CO2 ions of more soluble salts.
2. Highly condensed, nearly elemental organic carbon (Charcoal, graphite coal, etc)
3. Altered and rather resistant organic residues of plants, animals and microorganisms, some times termed as humus or humate but not as these latter terms tend suggest a singles compounds.
4. Little a altered organic residues of plants and living and dead organisms. Subject to rapid decomposition in soil. The principle involved in the determination of organic carbon.
The organic carbon inorganic matter is oxidized by known but excess quantity of chronic acid (K2Cr2O7 + H2SO4). The excess chronic acid not reduced by organic matter is determined by back titration with standard FeSO4 solution using ferroin Indicator. The organic carbon in soil is calculated from the chromic acid utilized by it.