242 GENERAL MICROBIOLOGY 



Method. 1. Remove the coarser surface debris from 

 the soil. Sink the soil borer to the depth of 30 cm., remove 

 the borer and place the soil in the bag. 



2. Take six borings so that your composite sample 

 will be representative of the entire plot under considera- 

 tion. 



3. At the laboratory, carefully mix and pulverize the 

 composite sample with the spatula on the large piece of 

 paper. 



4. Weigh out 20 gms. on sterile paper and transfer 

 immediately to the flask containing 200 c.c. of sterile water. 

 A large amount of soil is used to reduce the error as much 

 as possible. The manure should be treated in a similar 

 manner. 



5. Shake thoroughly for one minute, allow the coarser 

 particles to settle and transfer 10 c.c. (equivalent to 1 gm. 

 of soil) of the supernatant liquid to 90 c.c. of .sterile water. 

 Each cubic centimeter of this dilution then contains 0.01 

 gm. soil. 



6. Make and plate from the following dilutions: 1-10, 

 1-100, 1-1000, 1-10,000, 1-100,000, 1-1,000,000. 



7. Incubate at room temperature for four to eight 

 days. 



8. Count and record the results as number of bacteria 

 per gram of soil or manure, in tabular form. 



9. Record the number of the various types of micro- 

 organisms. Note the numbers of chromogenic bacteria 

 and the streptothrix forms. 



10. Examine some of the manure in the hanging drop. 

 What forms are seen? Make drawings. Are all of these 

 forms found on the plates? Give reasons for what does 

 occur. Add sterile water to the manure in a sterile deep 

 culture dish or flask and examine every three or four days 

 during the course of the experiment. Record your obser- 

 vations. 



11. When the peat sample is obtained, at the same time 



