Soil Organic Matter: Its Fractionation and Role in Soil Structure

An understanding of the form and function of SOM in soil is essential to sustainable crop production. Various methods of SOM fractionation are used to study SOM forms and to gain an understanding of interaction between SOM and aggregate formation. Physical SOM fractionation techniques are based on the concept that SOM associated with particles of various sizes. differs in structure and function. This distribution of SOM has been studied by disrupting soil structure, commonly by sonication and shaking, followed by the separation of physical fractions on the basis of size or density. Problems associated with the use of these techniques include the redistribution of SOM into size separates in which it is not normally located, incomplete dispersion, fragmentation of macro-OM, and arbitrary size and density limits. Physical SOM fractionation techniques have been useful in studying SOM structure and function, but their use for identifying organic binding agents, responsible for maintenance of soil structure, is limited. Most soils rely on aggregation of particles to maintain favourable conditions for soil microbial and faunal activity and plant growth. Certain components of SOM such as polysacchaJides, soil humic substances, root material and fungal hyphae have an important role in soil structural stabilisation. The measurement of labile carbon by oxidation with potassium permanganate and total carbon has shown a concentration of carbon in water stable aggregates. The stability of macroaggregates increased at higher levels of carbon, presumably due to organic bonding mechanisms. The effect of management upon SOM, specifically upon these labile components, and changes in soil structure are essential for the development of sustainable agricultural sys-