Analytical Methods:Organic Matter

SASSA Home Page &#8658; Analytical Methods Home Page &#8658; Laboratory Techniques &#8658; Organic Matter Content

Questions
Organic matter content can become elevated in soils through the addition of organic refuse materials and manures, hence organic matter content along with phosphate and other element concentrations can become enhanced around settlement sites. Organic matter content can also be an important indicator of buried soils as organic matter would have accumulated in the old topsoil and declined in concentration through the buried soil profile. Organic matter content is also a key property determining erosion risk. Soils with low organic matter concentrations can be more at risk of wind and water erosion than more organic soils. This is because organic compounds are important in binding together individual soil particles. Hence, organic matter content can be an important property in determining site management strategies.

Example case studies in which organic matter analysis has been useful in answering archaeological questions include:

Sampling and samples
The sampling scheme used is highly dependant on the questions being asked. For identification of buried soils samples will need to be taken vertically through the deposits, including above and below the possible soil. To assess erosion risk bulk samples of the topsoil should be taken, usually using some sort of auger and a grid type sampling pattern. If you wish to try to calculate the amount of organic matter that has been added to the soil it will be necessary to take samples of known field volume.

Samples can be stored refrigerated for short periods of time (no more than a few weeks). But for long term storage freezing or air-drying the samples is recommended. If soils are air-dried they may also be sieved to remoe stones prior to storage.

Analytical methods
There are three main laboratory methods of analysing soil organic matter content. There are, loss-on-ignition, wet oxidation, and specialist carbon analysers. The latter uses specialist equipment and includes a number of different analytical methods. The first to methods are the most widely used and readily availbale techniques.


 * Loss-on-ignition
 * Wet oxidation

Data and interpretation
Whereas carbon analysers and wet oxidation produce data directly on soil organic carbon. Loss-on-ignition produces data on organic matter content of which only a proportion if actually carbon. This is because organic matter also contains hydrogen and oxygen as well as small amounts of other elements such as nitrogen. Care must be taken, therefore, when presenting results that the right units are used. When comparing results it is important that the same methods have been used and you should be careful that the different data sets are presenting the same type of results as % organic carbon is sometimes converted to % organic matter and vice versa. There are a number of different conversion factors used to do this, so before comparing results, you must know what, if any, conversion factor has been used.

Related Analyses
Field organic matter / manganese determination

 &larr; Back to Laboratory techniques