126 LABORATORY MANUAL OF MICROBIOLOGY 



(b) Add 30 milligrams of nitrogen per 100 grams of soil from 

 (NH4)2S04, blood meal, casein, or peptone. 



Mix these substances thoroughly with the soil, add sterile 

 water to give one-half to two-thirds saturation, record weight 

 and incubate at 28°C. 



Each week, weigh and restore loss of water by evaporation. 



After 10 and 20 days determine the nitrate nitrogen. 



Express the results as milligrams of nitrate nitrogen in 100 

 grams of soil; also as percentages of the original substance 

 nitrified. 



Tabulate results. 



UREA AND PROTEIN DECOMPOSITION 



Exercise 41 

 Decomposition of Urea 



Prepare three Erlenmeyer flasks or bottles, 200-cubic centi- 

 meter capacity, with 50 cubic centimeters each of urea solution 

 (Medium 45). 



After sterilizing, arrange as follows: 



(a) Control — not inoculated. 



(h) Inoculate with 1 gram of soil. 



(c) Inoculate with 1 gram of fresh manure. 



It is not necessary to weigh accurately the soil or manure. 



Incubate the cultures at 28°C. and after 2 days remove from 

 each flask 5-cubic centimeter portions of the solution, with a 

 sterile pipette, to a small Erlenmeyer flask. Add about 50 cubic 

 centimeters of distilled water to the urea solution and a few drops 

 of methyl red. Determine the ammonia production by titrating 

 with N/14: acid. 



From the results of the titrations, calculate the amount of 

 ammonia nitrogen in 100 cubic centimeters of the different urea 

 solutions. In order to find the amount of ammonia formed by 

 bacterial action, subtract the untreated from the treated series. 



Determine the percentage of urea nitrogen transformed into 

 ammonia nitrogen. 



Similar samples may be drawn after 3 or 4 days and the 

 amount of ammonia determined. Tabulate the results. 



