MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 347 



the total rainfall percolates through the soil. Where the soils are 

 open and nearly level 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 inches) 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 pre- 

 cipitation of 2 mm. (.079 inch) 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 inches) 

 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 Hil- 

 gard that Central Montana, and the region in the vicinity of the bay 

 of San Francisco, have each a total precipitation of 610 mm. (24 inches). 

 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 be- 



