228 MICROBIOLOGY OF SOIL. 



the proportion of percolating water is relatively greater; where the soils 

 are fine-grained and more or less impervious, or the topography broken, 

 the proportion is relatively smaller. 



Bacteria and other microorganisms, as well as the higher plants, are 

 directly influenced by the amount of moisture available for their various 

 needs. Hence soil microbial activities are affected not alone by the 

 amount of rainfall, but also by its distribution. It is obvious, for instance, 

 that an annual rainfall of 762 mm. (30 in.) distributed rather uniformly 

 throughout the year would produce different soil-moisture relations than 

 the same amount of precipitation confined to only two or three months. 

 As is pointed out by Abbe, a daily precipitation of 2 mm. (.079 in.) 

 distributed throughout the three summer months would be quickly 

 changed into vapor, and would hardly wet the soil; whereas the total 

 quantity of 180 mm. (7 in.) evenly divided into ten or twelve rains 

 would penetrate the soil to a considerable depth, and would furnish very 

 favorable conditions for microbial development. In a similar manner it 

 is pointed out by Hilgard that Central Montana, and the region in the 

 vicinity of the bay of San Francisco, have each a total precipitation of 

 610 mm. (24 in.). But while in Montana the rainfall is distributed over 

 the entire year and irrigation becomes necessary, the precipitation near 

 San Francisco is limited to the portion of the year that nearly coincides 

 with the growing season, and crops are enabled to mature without 

 irrigation. 



RANGE OF SOIL MOISTURE. Any given volume of dry soil consists 

 of solid particles separated by empty spaces. The sum of these spaces 

 is known as the "pore-space." It varies from about one-third of the 

 entire volume in coarse sands to more than two-thirds in pipe clay. In 

 peat and muck it may amount to as much as 80 or 90 per cent, of the 

 entire volume. Under air-dry conditions each soil grain is surrounded 

 by a very thin film of moisture designated as hygroscopic water. When 

 air-dry soil is moistened the films around the soil particles become thicker 

 and finally cease to be isolated. A continuous liquid membrane, as it 

 were, is stretched from particle to particle, and the surface tension that 

 thus comes into play is capable of lifting large amounts of water to the 

 surface. The continuous film of soil water that can hold its own against 

 the pull of gravity is known as capillary water. Finally, when the liquid 

 films around the soil grains increase in thickness beyond a certain point, 

 the attraction between the molecules in the soil grains and the more 



