434 APPLIED BACTERIOLOGY 



The number of bacteria in undisturbed surfaces has been 

 estimated by many observers ; the results vary greatly, as 

 would be expected, in different places, the number generally 

 running to several thousand per gramme of earth . 



In an investigation by Frankel of the soil of a fruit orchard, 

 he found the superficial layers contained from 50,000 to 

 350,000 organisms per gramme of soil. The greatest number 

 was not immediately upon the surface, but at one- quarter 

 to one-half a metre below the surface. At a depth of from 

 three-quarters to one and a half metres there was a very 

 abrupt diminution in the number of bacteria. 



From 200,000 organisms at a depth of half a metre, the 

 number fell to 2,000 at a depth of one metre, from 250,000 

 at three-quarters of a metre to 200 at one metre. At a 

 depth below one and a half metres, generally speaking no 

 more bacteria were found. The most important fact 

 established by these researches is that in virgin soil there is 

 a dividing-line at a depth of from three-quarters to one and 

 a half metres, below which very few bacteria are found, 

 thus showing the ground- water region is quite free, or 

 nearly free, from micro-organisms, notwithstanding the vast 

 number upon the surface of the soil. Buchanan Young has 

 investigated the nature of the soil in graveyards, and he 

 finds, on the whole, that the bodies do not greatly influence 

 the number of micro-organisms found. According to 

 Kirchner, the freedom of the ground-water from bacteria is 

 due to the great porosity of the soil, which acts as a very 

 efficient filtering medium. 



Examination of Soil. — When the deeper layers are to be 

 examined, care must be taken to prevent contamination 

 with the other portions, particularly the upper layers. 

 Frankel has devised an ingenious instrument for taking 

 samples of earth from various depths. This takes the form 

 of a borer, which contains at its lower end a small cavity. 



