EXAMINATION OF THE SOIL. 139 



3. Made soils contain large numbers of bacteria per gram 

 of many different species, the deeper layers being as rich in 

 the number and variety of organisms as the upper layers. 

 After some years the number in the deeper layers probably 

 becomes less. Pathogenic bacteria are more likely to be con- 

 tained in made soils. 



The earth may be obtained at any depth by means of 

 Fraenkel's special boring apparatus. A definite amount of 

 soil is mixed with liquefied gelatin or agar and a plate or 

 roll culture made. The colonies are counted in the same way 

 as in the examination of air or water. 



Fluegge found about 100,000 colonies in a cubic centime- 

 ter of virgin soil. Sternberg advises washing the earth with 

 sterile water, and after sedimentation a sterile medium is 

 inoculated with the water. Miquel found 900,000 colonies 

 in one gram of earth obtained from a fertilized field. One 

 investigator examined the soil of a churchyard, and found 

 1,152,000 bacteria at a depth of four meters. The specimens 

 of earth should be examined as promptly as possible, so as to 

 avoid contamination, and also because of the rapid develop- 

 ment of the contained organisms when in contact with oxygen 

 and more suitable environments. 



