CHAPTER V 



BACTERIA IN THE SOIL 



SURFACE soils and those rich in organic matter supply 

 a varied field for the bacteriologist. Indeed, it may 

 be said that the introduction of the plate method of culture 

 and the improved facilities for growing anaerobic micro- 

 organisms have opened up possibilities of research into soil 

 microbiology unknown to previous generations of workers. 

 From the nature of bacteria it will be readily understood 

 that their presence is affected by geological and physical 

 conditions of the soil, and in all soils only within a few feet 

 of the surface. As we go down below two feet, bacteria 

 become less, and below a depth of five or six feet we find 

 only a few anaerobes. At a depth of ten feet, and in the 

 *' ground water region,*' bacteria are scarce or absent. This 

 is held to be due to the porosity of the soil acting as a filter- 

 ing medium. Regarding the numbers of micro-organisms 

 present in soil, no very accurate standard can be obtained. 

 Ordinary earth may yield anything from 10,000 to 5,000,- 

 000 per gram, whilst from polluted soil even 100,000,000 per 

 gram have been estimated. These figures are obviously 

 only approximate, nor is an exact standard of any great 

 value. Nevertheless, Frankel, Beumer, Miquel, and Mag- 

 giora have, as the result of experiments, arrived at a num- 

 ber of conclusions respecting bacteria in soil which are of 

 much more practical use. From these results it appears 

 that, in addition to the ** ground water region " being free, 



137 



