MICROORGANISMS AND SOIL AGGREGATION

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Abstract
A study was undertaken to determine the nature and relative effectiveness of soil-aggregating substances of microbial origin. A soil fungus, Cladosporium, and an aerobic bacillus belonging to the Bacillus subtilis-mesentericus group, brought about greater aggregation of the soil than other soil microbes studied. Up to 50% of the aggregating effect of the fungus was brought about by substances produced by the cell material and the remainder was due to the binding influence of the mycelium. In the case of the soil bacillus, the cells ac-counted for approx. 20% of the aggregating effect and sub-stances produced by the cells for 80%. A polysaccharide-synthesized by the bacillus was found to be primarily responsible for the aggregating influence. The bacillus synthesized a polysaccharide and brought about marked aggregation when supplied with either organic or inorganic N and when dextrose, maltose, xylose, arabinose, or sucrose was used as energy material. The greatest aggregation occurred when the organism was supplied with sucrose. There were indications that the polysaccharide synthesized from sucrose was different from that synthesized from other sugars.