328 



Microorganisms and Soil Fertility 



According to Swaby, pure cultures of Absidia glauca and Aspergillus 

 nidulans growing in sterilized soil enriched with glucose produced 

 242 and 374 meters of mycelium per gram, as measured by methods 

 of Jones and Mollison. They entangled, respectively, 96.5 and 80.3 

 per cent of soil into stable aggregates. Fresh soil contained 38.8 

 meters of hyphae per gram. The presence of 38 per cent of aggre- 

 gates in such soil could thus be ascribed to the function of fungus 

 mycelium (Fig. 129). This is also brought out in Table 72. The 



Table 7'2. The Ixfluexce of Mold Species and Alfalfa on the Dispersion 

 Ratios of Three Soils (from Gilmour et al.) 



aggregating effect of bacteria was calculated to be only about 2 per 

 cent of the total. 



That microorganisms differ greatly in their soil-aggregating prop- 

 erties and that the products of some of these organisms are excellent 

 soil-binders have also been brought out by Martin, McCalla, and 

 others. Fungi and certain polysaccharide-forming bacteria were 

 found to be more effective than actinomycetes, which in turn were 

 better than yeasts; certain bacteria were least effective. 



Many of the soil-aggregating substances and even the mycelium 

 and cells produced by microorganisms are later destroyed by other 

 microorganisms, thus producing an effect of disaggregation. Micro- 

 bial associations alone could not account for the formation of per- 

 manent crumbs, especially when no fresh organic materials were 



