PART III 
SOIL BACTERIOLOGY 
Exercise 15. The Number and Kinds of Bacteria in 
Different Soils 
MATERIALS: 
9 tubes sodium caseinate agar 
9 sterile Petri dishes 
Case sterile pipettes 
3 9 cc. sterile water blanks 
1 99 cc. sterile water blank 
Suspension of soils, 1 gram of soil in 1000 cc. of sterile water 
There are enormous numbers of bacteria in most fertile soils, and on this 
account a very small amount of the soil must be used in determining the num¬ 
ber of bacteria. Results of determinations are always given per gram of soil, 
usually calculated on a “dry” basis. In all of the following work two repre¬ 
sentative soils will be used. One sandy loam, “soil A,” and one black garden 
soil, “soil B,” Prepare cultures from one soil. 
1 . Observe that there are 1 to 1000 dilutions of the two soils on the desk. 
2 . Prepare 4 quantitative dilutions of one soil and make duplicate plate 
cultures with 1-1000, 1-10,000, 1-100,000, and 1-1,000,000 gram of soil. 
Prepare a “blank” without the soil. Label dishes with the name of the soil 
and the dilution used. 
FIG. 16—MANNER OF MAKING THE SOIL DILUTIONS 
3. Proceed by arranging the water blanks and Petri dishes as shown in Fig. 
16. Make all transfers with sterile 1 cc. pipettes as indicated by the continu¬ 
ous arrows, and transfer 1 cc. each time. All steps marked “A” are to be 
carried out with the first pipette and in the order given; steps marked “B” 
with the second pipette, etc. 
4. Shake the 1 to 1,000 gram dilution well with a rotary motion, at least 
fifteen times. While the liquid is still in motion, transfer 1 cc. to the necessary 
