Index >> Soil, Nature Medium For Plant Growth >> Soil Structure

Several workers have found that the aggregating influence of different groups of soil microorganisms can be graded in the order, fungi > strep tomyces > gum producing bacteria > yeasts. Rapidly growing Rhizopus, Mucor, Chaetomium, Fusarium, Cladosporium, Aspergillus and Rhizoctonia are good examples of fungi known to secrete gums and provide mechanical support to bind soil particles. Examples of bacteria producing significant amounts of gum are Azotobacter, Beijerinckia, Rhizobium, Xanthomonas and Bacillus.

Different soil components have been chemically extracted from soil and their effect on soil and their effects on soil aggregation properties studied. Among the carbohydrates isolated from soil, dextran containing high amounts of uronic acid which is resistant to microbial degradation has been shown to have the best soil aggregating qualities.

Several experiments have been carried out under laboratory conditions to demonstrate the effect of single or mixed cultures of microorganisms in soil amended with various carbon sources. The results of such experiments, however fascinating they may be, must be viewed with caution. It is also interesting to note that the involvement of polysaccharides in soil aggregation has been questioned by some workers based on results of experiments in which native polysac­charides of soil samples were destroyed by chemical treatments without any detriment to the property of soil aggregation.