210 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



grams, peptone in 1,000 cc. distilled water. He shakes 100 grams, 

 soil with 200 cc. water, niters, and inoculates 100 cc. of the 

 sterilized culture solution with 20 cc. of the nitrate. Ammonia 

 is determined in the culture after incubation for six or seven 

 days. When soil is to be used as a culture medium, 100 grams. 

 air-dried soil are mixed with 5 grams, dried blood or 5 grams, 

 cottonseed meal and 5 grams, water and inoculated with 20 cc. 

 soil infusion, and incubated as before. 



Various conditions which affect the process of ammonification 

 can be studied in this way, such as the temperature, character of 

 medium, time of incubation, kind of bacteria, etc. If pure cul- 

 tures of different bacteria are compared, it is of course necessary 

 to estimate the number of bacteria in the liquid used for inocula- 

 tion, according to the method already outlined. Two important 

 ammonifying bacteria are Bacillus mycoides, and Proteus vul- 

 garis, but a large number of bacteria take part in this process. 



The ammonifying power of the soil has been defined by some 

 workers as the quantity of ammonia produced on inoculating a 

 definite quantity of a suitable culture medium with a definite 

 quantity of soil, and incubating under definite conditions. Am- 

 monifying power measured in this way depends upon the number 

 and activity of the bacteria in the soil at the time of inoculation, 

 and will be affected by anything which affects them, such as the 

 soil temperature, its moisture, kind and quantity of food present, 

 character of soil, etc. In carrying out such tests, it is exceedingly 

 important that the soils studied be kept under comparable condi- 

 tions as regards these varying factors, so that only one factor, the 

 one being studied, is variable. Ammonification in soils is, how- 

 ever, quite different from ammonification in solution. 



Nitrification. Nitrification takes place in two stages: nitrates 

 are first formed from ammonia, and then changed to nitrites, 

 Two kinds of bacteria have been isolated, namely, the nitrous and 

 the nitric organisms. 



The bacteria which oxidize nitrites to nitrates may be isolated 

 from the soil without any great difficulty. 1 A solution is pre- 

 pared as follows: 



1 Wiley's Principles and Practice of Agricultural Analysis, Vol. i. 



