ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 41 
SECTION 14: THE QUANTITATIVE ESTIMATION OF 
MICROORGANISMS IN SOIL 
EXERCISE 56 
THE NUMBER OF BACTERIA IN SOIL 
Two samples of soil are furnished. Sample No. 1 is a rich garden soil, while 
No. 2 is a sandy loam. In determining the number of bacteria in a solid sub- 
stance, such as soil, the number is calculated per gram. 
Weigh out samples of the soils furnished and make dilutions of 1/10,000; 
1/100,000; and 1/1,000,000. Make duplicate plates from each of these dilutions 
with each soil. Melt twelve tubes of agar medium; when cooled to 42-45° C., 
pour the agar into the plates and mix with the dilution water. Incubate the 
plates at 30° C. for 4 days. 
Count the number of colonies on your plates and calculate the number per 
gram in the original soil. Record in your note book the dilutions used, the number 
of colonies per plate, and the number of bacteria per gram of the original soil. 
EXERCISE 57 
THE NUMBER OF MOLDS IN SOIL 
Plate the same samples of soil used in Exercise 56 in dilutions of 1/100; 
1/1,000; and 1/10,000. 
Use a special agar which is favorable for the growth of molds but which 
inhibits the development of bacteria. This may be accomplished by adjusting the 
reaction of the medium to a point too acid for the growth of the bacteria. The 
following medium is a good one for this purpose: 
Glucose 1.0 per cent 
Peptone 0.5 per cent 
KH,PO, 0.1 per cent 
MgSO, 0.05 per cent 
Agar 2.0 per cent 
After sterilizing the above medium adjust to pH 4.0 by adding H,80,. 
Incubate the plates at 30° C. After incubation, count the colonies and cal- 
culate the number of molds per gram of soil. 
