SUMMARY: The physical condition of soil is improved by adding readily decomposable organic material. Microbial cells and metabolic products affect soil structure by binding loose soil particles into water-stable aggregates.

Experimentally, the relative aggregating power of pure cultures of micro-organisms was as follows: fungi>actinomycetes and a few gum-forming bacteria>many gum-producing bacteria>yeasts, proactinomycetes, and many bacteria; the last three groups did not improve aggregation. Fungal hyphae entangled soil particles into stable aggregates; weaker crumbs were formed by the frailer threads of actinomycetes. A few bacterial strains produced gums capable of glueing soil into water-stable aggregates, but the majority of bacterial slimes were almost useless because they remained water-soluble after drying. The cementing properties of these gums was not improved by treatment with H or Ca ions. Bacterial gums stabilized the aggregates produced from completely dispersed soils and kaolin, but not those formed with bentonite or ferric hydroxide. The pH value of the soil played a very minor part in influencing the aggregation produced by pure cultures of micro-organisms or even by soil inoculum.

Mixed cultures of fungi or of actinomycetes gave slightly better aggregation than pure cultures, but neither capsulated nor non-capsulated bacteria in mixtures gave better results than single strains. More complex mixtures containing fungi, actinomycetes and bacteria gave good aggregation when all micro-organisms wrere compatible, but poor results when antagonistic bacteria inhibited the growth of either fungi or actinomycetes. The fair aggregation obtained with soil inoculum was reproduced in the laboratory by inoculating sterilized soil with complex mixtures of micro-organisms.

A study was made of the relative merits of glucose, starch, blood, yeast, fungal mycelium, straw, clover and farmyard manure for encouraging aggregation by mixtures of fungi, actinomycetes and bacteria.

Aggregates bound by mycelia did not last long because the hyphae were decomposed by bacteria. The temporary improvement of soil structure after the addition of organic materials can be partly explained by the action of microbes, but the permanent crumb structure of many soils must be due mainly to other causes.


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