BACTERIA OF THE SOIL 



27 



reducing, or hydrolyzing the numerous substances which serve as 

 sources of energy and as materials for cell construction. 



Bacteria of the Soil. — The most abundant group of organ- 

 isms in the soil are the bacteria, exceeding both in numbers and 

 in the variety of their activities all the other soil organisms, but 

 not, however, in the bulk of the organic cell substance found in the 

 soil in the form of living and dead microbes. The fact that 

 bacteria are so abundant and that they participate in numerous 

 soil processes is largely responsible for the frequent reference to 

 the total microscopic population of the soil by* the term bacteria 

 of the soil, and reference to the whole subject of soil microbiology 

 as soil bacteriology. These bacteria are so small that if 500 to 1000 

 of them were placed in one hne, end to end, they would only 

 extend for a length of one millimeter. They are in general about 

 the size of colloidal particles, the maximum size of which is about 

 5 to 8 microns — one micron being 1/10,000 of a centimeter. It 

 takes about 1,000,000 to 1,000,000,000 of them to weigh one 

 milligram, or one-thousandth part of one gram; even then this 

 mass of cell material 

 will be found to consist 

 of 80 to 85 per cent of 

 water. 



It is not exceptional 

 to find hundreds of 

 millions of bacterial 

 cells per gram of soil, 

 particularly where or- 

 ganic substances have 

 been added. Let us 

 assume that there are 

 one hundred million 

 bacteria in one cubic 

 centimeter of soil and 

 that each cell occupies 

 only one cubic micron 

 of space. Since there 

 are 1,000,000,000,000 cubic microns in one cubic centimeter, 

 and since the one hundred million bacteria would occupy only 

 100,000,000 cubic microns, the cells would occupy only one-ten 

 thousandth of the total volume of the soil. It is not surprising 



Fig. 14. — Nitrate-forming bacterium, Nitro- 

 bader sp. (from Fred and Davenport). 



