MICRO‐ORGANISMS and SOIL STRUCTURE

Summary 1. Experimental analysis of the influence of micro‐organisms on soil structure has been confined to aggregation, i.e. the formation of small clusters of soil particles which are resistant to the disintegrating action of water. 2. Aggregation is influenced by the mineral constituents of the soil, notably clay, but organic matter is considered to be most important. 3. Decomposition of organic matter regularly leads to increased aggregation in non‐sterile soils. Under sterile conditions (i.e. in the absence of micro‐organisms) no increase in aggregation is observed, so that it is evident that micro‐organisms are implicated in this process. 4. Investigations of the role of micro‐organisms have been concerned with two possible effects: (1) the action of filamentous organisms (particularly fungi) in mechanically entangling soil particles to form stable aggregates, and (2) the action of products of micro‐organisms (of all groups) which may cement soil particles together. 5. In most soils the mycelium of filamentous organisms (fungi and actinomycetes) is only present for relatively short periods so that aggregation produced by them tends to be ephemeral. In sandy soils, however, mycelium appears to be more persistent and hence of greater significance. Since fungi are commonly associated with the early stages of decomposition of plant remains, it is suggested that they may also be important in situations where plant residues are regularly being added to the soil, e.g. grassland and forest. 6. Of the many compounds produced by micro‐organisms, most attention has been given to extracellular polysaccharides. These are produced by many soil organisms and can be shown to be effective aggregating agents. Compounds of similar character have been demonstrated in natural soils but satisfactory methods for their quantitative estimation have not been developed. Consequently their contribution to aggregation cannot be accurately evaluated. Evidence so far presented, does, however, indicate that in many cultivated soils they make a considerable contribution. 7. In old grassland and forest soils, extraction of polysaccharide by periodate oxidation appears to have little or no effect on aggregation and in these cases aggregation would seem to be due to other components of the soil organic matter. Although this may well be so, it is also suggested that the distribution pattern of organic materials in the soil may be an important contributory factor. Certain materials, e.g. lipids, though not effective as aggregating agents when mixed with the soil, increase stability when sprayed on the surface of unstable aggregates. The significance of such effects has not so far been critically examined.