EXAMINATION OF SOIL 333 



BACTERIOLOGICAL EXAMINATION OF SOIL. 



Surface-soil, particularly that which is rich in organic 

 matter, is very rich in micro-organisms. That it is only the 

 surface-soil that contains any quantity of micro-organisms 

 is shown by the fact that at as short a distance as about two 

 metres in depth the soil contains but few organisms. This 

 was shown by Koch in 1881, who showed that in soil which 

 had not been disturbed, even at a depth of only one metre, 

 but few bacteria are to be found. This fact has since been 

 confirmed by the extended researches of Frankel and others. 

 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 



