230 MICROBIOLOGY OF SOIL. 



soil then becomes air-dry and contains only hygroscopic water. It is 

 estimated by Lyon and Fippin that, under average conditions of humidity, 

 light sand will contain 0.5 to i per cent of hygroscopic moisture; silt 

 loam, 2 to 4 per cent; and clay, 8 to 12 per cent. The amount of water 

 present in air-dry muck or peat may range up to 40 per cent, or even 

 more. According to Hall the film of hygroscopic moisture is about 

 0.75/4 (0.00003 in.) thick. As the soil dries out bacterial activity is sus- 

 pended and many vegetative cells undoubtedly perish. Nevertheless, it 

 will be seen that the moisture film even in air-dry material is deep enough 

 to allow the bacteria a reasonable degree of protection. This will accoun- 

 for the survival of non-spore-bearing bacteria in dry soil for a long time. 

 Indeed, instances are on record of the isolation of Azotobacter and Nitrot 

 somonas from soils that had been kept in a dry state in the laboratory 

 for several years. It may be noted, in this connection, that in the pro- 

 cess of drying the soluble salts in the soil may be sufficiently concentrated 

 in the thin films to cause plasmolysis and the destruction of individual 

 cells. 



On the other hand, excessive moisture in the soil is not only directly 

 unfavorable to aerobic species in that it limits their supply of oxygen, 

 but is objectionable because it encourages the formation of reduction 

 products that are toxic to these species. It is apparent, therefore, that 

 favorable conditions for the formation of available plant food by bacteria 

 are created when a certain relation is established between the volumes of 

 moisture and air in the soil. The shifting of this relation in one direction 

 or another is bound to react on species relationships and numbers. 



AERATION. 



MECHANICAL COMPOSITION OF SOILS. Soil ventilation is an impor- 

 tant factor in crop production. It provides for the proper supply of 

 elementary oxygen so essential to decomposition processes in normal 

 soils; for the supply of elementary nitrogen required by nitrogen-fixing 

 species; for the removal of excessive amounts of carbon dioxide; and for 

 the destruction of various toxic substances. The intimate relation 

 existing between soil ventilation and the mechanical composition of the 

 soil material is bound to react on the microbial factors involved. It is 

 well known that the rate of flow of air through soils is inversely propor- 

 tional to the fineness of the material; in other words, the fine-grained 



