32 
Exercise 16. Ammonification by Soil Bacteria 
MATERIALS: 
3 tubes of beef broth 
3 tubes of casein solution 
3 tubes of urea solution 
Soil samples 
The protein of plant and animal tissues and urea (CO(NH 2 ) 2 ), in which 
compound a great deal of the nitrogen in urine is found, readily undergo 
ammonification. Since carbon dioxide (C0 2 ) is practically always present in 
decomposing materials, the ammonia formed unites with it to form ammonium 
carbonate (NH 4 ) 2 G0 3 ). 
1. Label each of the 3 tubes of beef broth, casein and urea medium with the 
name of medium, and the soil with which it is to be inoculated. 
2 . Inoculate one of each three tubes with a small quantity of one of the 
soils used in exercise 15, by moistening the wire loop with tap water and 
thrusting it into the soil. The amount of soil that will adhere to the moist loop 
is sufficient. Another tube of each three is to be inoculated with the other soil. 
The remaining tube is left without inoculation and is to serve as a control. 
3. After 48 hours incubation at room temperature, test the inoculated tubes 
for ammonia by placing a drop of Nessler solution in a depression on a spot 
plate and adding to it one loopful of the culture. The presence of ammonia 
is indicated by the development of a color, a faint yellow with small amounts 
of ammonia, a deep yellow or brown with larger quantities and a brownish 
precipitate with still larger quantities. 
4. After incubation for one week test both the inoculated and control tubes 
for ammonia. 
5. Record the results of both tests on the regular note paper as in Table III. 
Rank the solutions according to the amount of ammonia present, the solution 
containing the smallest amount being ranked 1. 
TABLE III.—Formation of Ammonia by Soil Bacteria 
Rank afte 
r 48 hours 
Rank aft< 
?r 1 week 
Control 
soil A 
soil B 
soil A 
soil B 
Beef Broth 
0 
Urea 
Casein 
6 . In recording the conclusions, answer the following questions: 
(a) From the above data, what is to be concluded as to the cause 
of ammonia formation in soils? Why? 
( b ) What was in the casein solution and beef broth that yielded 
ammonia upon decomposition? 
(c) Give the most common sources of such substances in farm 
practice. 
