Physical separation of soil organic matter

The distribution of organic matter within physical fractions of the soil can be assessed by disruption of the soil structure, followed by the separation of physical fractions based on particle size or density. Disruption of the soil structure can be accomplished by physical or chemical methods, or some combination of the two. The most commonly used methods of physical disruption are shaking and sonication. Shaking is the more gentle method with the advantage of being able to obtain a wide range of disrupting energies relatively easily. Sonication can impart more energy to the soil in a shorter period of time. The greatest potential problem associated with the use of sonication is the redistribution of organic matter among size/density fractions. Chemical extraction methods are commonly used prior to disruption of soil for particle size analysis. Some chemical dispersants can selectively solubilize organic matter. This specificity can be used to determine the kinds and amounts of organic matter that bind particles into aggregates. Three methods of physical separation of soil have been used, sieving, sedimentation and densitometry. Sieving separates soil particles based strictly on size and is used primarily for aggregate separations of non-disrupted soil samples. Sedimentation separates particles based on an equivalent spherical diameter, which may vary in size, shape and density. It is most often used in conjunction with a disruption pretreatment to obtain fine fractions. Densitometry separates particles based on the weight per unit volume, independent of size and shape, and is used to separate lighter from heavier fractions. It is possible to combine any or all of these separation methods in order to isolate, for instance, organic matter from a particular size fraction that has a specific density.