246 GENERAL MICROBIOLOGY 



7. Make ammonia determinations after twenty days. 



Note. Ammonia determinations from the above are made by the 

 following procedure: Remove the cotton plugs, add 200 c.c. of water, 

 10 gms. MgO and a small piece of paraffin. Distill off the ammonia 

 present, collecting it in N/10 HC1 and titrating against N/10 NH 4 OH, 

 us-ing methyl orange as indicator. 



1 8. Record your results in the form given above. 



9. How does aeration affect bacterial activity? Size 

 of sand grains? 



What interrelation have grain-size and moisture con- 

 tent of soil? 



How do microorganisms obtain their food in soil that 

 is not wet? 



What influence does humus have on aeration? On 

 bacterial activity? 



10. Draw any conclusions warranted by your results 

 and point out any practical applications. 



REFERENCE 

 RAHN, O.: Tech. Bui. No. 6, Mich. Agr'l Expt. Sta. 



EXERCISE 4. TO DEMONSTRATE THE CELLULOSE- 

 DECOMPOSING POWER OF AEROBIC ORGAN- 

 ISMS FOUND IN THE SOIL 



Apparatus. Two Petri dishes; four pieces of round filter 

 paper to fit Petri dishes; 0.05% K 2 HPO 4 ; MgNH 4 P0 4 ; 

 0.05% NH 4 NC>3; 1 c.c. pipette; soil rich in humus, or 

 well-rotted manure. 



Method. 1. Put a thin layer of MgNH 4 P0 4 between 

 two filter papers in a Petri dish. 



2. Moisten this with the solution of K2HP0 4 and in- 

 oculate with a few drops from a water solution of the soil 

 or manure. Keep at 25 to 30 C. 



3. In three to six days brown spots will occur and later 

 holes will be formed by bacteria. Thin places in the filter 



