182 EXAMINING AIR, SOIL, WATER, AND FLUIDS FOR BACTERIA 



These microorganisms are most readily detected by direct animal 

 inoculations. The search for nitrifying and other bacteria, important 

 in the study of agricultural problems, requires certain special culture 

 media and the dilution of the finely divided soil with sterile distilled 

 water. 



Quantitative Examination. This is very unreliable and generally 

 not very satisfactory. In the first place it is difficult to divide the soil 

 so finely that it can be intimately mixed with melted culture media, 

 and, again, the soil bacteria (mostly bacilli) have such a wide range of 

 conditions of growth that no single cultural method can furnish even 

 an approximate picture of the species and number of germs present 

 in a specimen of soil. For example, there are aerobic and anaerobic 

 bacteria, some growing at very low and others at extremely high 

 temperatures. 



Specimens of soil from the superficial strata are generally obtained 

 with a small platinum spoon or scoop containing, when filled, about 

 5*0 or 2*5- of a cubic centimeter. The material may first be rubbed 

 up in a sterile agate mortar, or it may be mixed directly by violent 

 snaking with the melted culture medium, which is subsequently 

 poured into Petri dishes. For the examination of soil of deeper strata 

 a drill or auger has been devised by Frankel. Above the tip is a 

 metal tube with an opening on one side, with a cover arrangement 

 on hinges like a door. During the downward movement the door 

 remains closed, but as soon as the motion is reversed it opens and 

 the tube is filled with soil. In this manner it is possible to obtain soil 

 from a definite depth. 



FIG. 107 



Tip of FrankePs instrument for obtaining earth from various depths for bacteriologic study. 

 B shows the instrument with its cavity closed, as it appears during boring: A, open, as it 

 appears when twisted in the other direction to collect the earth. (McFarland.) 



Bacteriologic examination has shown that soils are generally richer 

 in bacteria the more they have been mixed or contaminated with 

 manure, sewage, and other decomposing vegetable or animal material ; 

 further, that this richness is confined to the superficial layers. At a 

 depth of 1 meter the number of bacteria is much reduced, and at 

 a depth of 1J to 2 meters very few are present; still deeper there are 

 none at all. 



