28 SOIL BACTERIOLOGY 



3. Use a clean spatula to mix the soil and limestone. 



4. Add water to bring the moisture to two- thirds satura- 

 tion. Cover the tumblers with Petri dishes. 



5. Allow the soils to incubate at room temperature. 



6. Make plate counts after one and three weeks. 



7. Arrange the results in a table (see p. 27). 



Exercise 9 

 Effect of Partial Sterilization on Number of Bacteria 



1. Weigh out three portions, 200 grams each, of garden 

 or field soil into small jars (pint Mason). The field soil 

 should be mixed thoroughly before sampling. 



2. Treat the jars of soil as follows: 



(a) Control. 



(6) Heat to 100 C. for one hour. 



(c) Treat the soil with i per cent, of carbon bisulphid. Pour the CS 2 

 into small holes (use a glass rod) in the soil and cover imme- 

 diately. 



3. After the soil in jar (c) has been exposed to the action 

 of carbon bisulphid for one hour, and the soil in (b) heated 

 for one hour, determine the number of bacteria in all three 

 soils. In the case of the untreated soil, plaj:e from the 

 dilution i : 100,000, the heated soil dilution i : 100, and 

 the carbon bisulphid soil dilution i : 10,000. 



4. The carbon bisulphid jar. should be left uncovered for 

 one day. 



5. Incubate all of the samples at room temperature. 



6. The second plate count should be made one week 

 after treatment. Here again, untreated soil, use the dilu- 

 tion i : 100,000, and the treated series, dilutions i : 100,000 

 and i : 1,000,000. 



7. A third count should be made two weeks after treat- 



