MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 293 



at the surface causes the film of capillary water to stretch more and ' 

 more. Finally it becomes very thin, breaks, and ceases to be con- 

 tinuous. The soil then becomes air-dry and contains only hyrgo- 

 scopic 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 hygro- 

 scopic moisture is about 0.75/i (0.00003 inch) thick. As the soil 

 dries out bacterial activity is suspended 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 account for the survival 

 of non-spore-bearing bacteria in dry soil for a long time. Indeed, in- 

 stances are on record of the isolation of Azotobacter and Nitrosomonas 

 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 process 

 of drying the soluble salts in the soil the moisture may be sufficiently 

 concentrated in the 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 Umits 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 substa,nces, The intimate relatiori 



