Importance of Microorganisms for Soil Aggregation

Abstract

Martin et al. (1955, quoted by Allison 1968) defined an aggregate as a " natu-rally occurring cluster or group of soil particles in which the forces holding the particles together are much stronger than the forces between adjacent aggregates " . Brewer (1964) considered an aggregate as having an identifi-able morphological boundary which distinguishes it from its neighbors. There have been many major papers describing of the ways in which these aggregates are formed. These models describe the mechanisms that form and stabilize aggregates: the orientation of the mineral particles, the at-tractions between these particles, the interactions between these particles and the soil microorganisms and the production of aggregating agents by these microorganisms (Emerson 1959; Harris et al. 1963; Dexter 1988 cited by Oades and Waters 1991). In general, these papers support a hierarchic model for the aggrega-tion process. Oades and Waters (1991) tested this model on three types of soil (Mollisol, Alfisol, Oxisol). In soils (Mollisol, Alfisol) where organic matter plays a role in the stabilization of aggregates greater than 250 µm, the hierarchic model was confirmed. However, in Oxisols, the role of iron oxides in cementing mineral particles minimizes the importance of the hi-erarchic model in aggregation. Nevertheless, it is evident that most papers describing the formation and stabilization of aggregates and the determin-ing factors controlling the process have been strongly influenced by the hierarchic model of aggregation. The most important strategies proposed for maintaining and improving soil fertility are those which target the physical properties of the soil. The abundance and stability of the aggregates are critical for several soil functions. – Plant growth (Hamblin 1985; Letey 1985), – Resistance to erosion (Le Bissonnais and Arrouays 1997; Barthès and Roose 2002)