Total Organic Compounds
This test is applicable to all sample types and will measure all organic carbon species including fixed carbon species, i.e. coal, coke, electrode carbon. However, it will not include volatile organic compounds, the majority of which are lost during digestion and drying.
Total Organic Carbon (TOC) is quite simply the organic carbon within a sample. However it will not include the organic carbon from volatile compounds as these are lost during digestion and drying.
Fraction Organic Carbon (FOC) is TOC expressed as a fraction although this is commonly reported as a percentage and therefore the same as TOC.
Soil Organic Matter (SOM) The term SOM is generally used to represent the organic constituents in the soil, including undecayed plant and animal tissues, their partial decomposition products, and the soil biomass.
This includes other elements present in the organic matter such as hydrogen, oxygen and nitrogen. Soil Organic Matter is a calculation based on the organic carbon content.
A weighed portion of the air dried and ground sample is treated with sulphurous acid at 40°C for 16 hours in order to remove any inorganic carbon species which react to form carbon dioxide. The excess sulphurous acid is then evaporated off. The dried residue is analysed for carbon content using an induction furnace fitted with a NDIR cell.
In this instrument the sample is combusted at 1600°C in an oxygen atmosphere, the combustion gases then pass through an infra red cell which measures the carbon dioxide concentration. The total quantity of carbon, which is now purely organic carbon, liberated is calculated and reported as a percentage of the original mass of sample.
The equation used to convert the organic carbon to Soil Organic Matter is: SOM = foc x 1.72
This is a different test method which measures of the organic matter present in the sample and is based upon the oxidation of the organic carbon present in the soil. The method is only suitable for soils containing natural organic matter. It will not include some volatile organic compounds that may be lost during digestion and drying. In addition fixed carbon species i.e. coals, coke, electrode carbon, are not oxidised by dichromate under normal test conditions and will not be included in the result.
The test is performed at room temperature so only easily oxidised organic matter is measured. Studies have shown that the method recovers between 60% and 80% of the carbon, incomplete recovery is corrected for by applying a recognised factor that assumes 77% recovery.
The method assumes that on average 58% of soil organic matter is carbon and applies this factor when calculating organic matter result. The method does not measure a specific determinant; no specific definition of organic matter is available. Because of the assumptions made re recovery and % carbon results are semi empirical. In addition these corrections only allow comparison of organic matter results for soils of a similar matrix containing similar organic materials. All species present that are readily oxidised by dichromate give positive interference.