EFFECT OF DEPTH ON NUMBER OF BACTERIA 



23 



2. Divide the sample of soil, taking one portion for 

 plate count, the other for moisture determination. For 

 virgin field soil the following dilutions have been found 

 satisfactory. 



3. Take soil samples and plate as follows* 



{a) Surface soil i : 100,000. 



(])) Soil I foot deep i : 10,000 and i : 100,000. 



(c) Soil 2 feet deep i : 1000 and i : 1O5O00. 



id) Soil 4 feet deep i : 100 and i : 1000. 



Follow the method given in previous exercises. 



4. Pour triplicate plates, using the medium that gave 

 the highest count. (See preceding exercise.) 



5. Tabulate results as follows: 



Table 2. — Effect of Depth on Number of Bacteria 



Position. 



Moisture. 



Bacteria per plate. 



Bacteria in i 

 gm. of dry soil. 



Surface, 

 do, 

 do. 



Per cent. 



Total. 



Average. 



Average. 



I foot, 

 do. 

 do. 





, 







2 feet, 

 do. 

 do. 











4 feet, 

 do. 

 do. 











