88 SOIL CONDITIONS AND PLANT GROWTH 



produced being I :8'9. For complete oxidation of the carbon, hy- 

 drogen, and sulphur of the albumin molecule the ratio would be 

 I : 1 0*3 ; but the change was known to be incomplete, and small quan- 

 tities of leucine, tyrosine, and fatty acids could also be detected. Free 

 oxygen, however, was not essential. When grown in a culture solution 

 containing sugar and nitrate the organism took its oxygen from the 

 nitrate, but it still produced ammonia. 



Subsequent developments have been entirely on the bacteriological 

 side. A number of organisms are now known to produce ammonia 

 from complex nitrogen compounds, but soil bacteriologists have gener- 

 ally preferred to study the group as a whole, rather than isolate and 

 study individual members. The method consists in inoculating soil 

 into various arbitrary culture media each designed to favour one 

 group only of organisms. Some of the results obtained are discussed in 

 Chapter VI. ; they show the method has some value as a bacteriological 

 test, but it has thrown little or no light on the processes going on in 

 the soil. Indeed, so dependent is bacterial activity on temperature, 

 concentration, reaction of medium (whether acid or alkaline), and other 

 conditions that it may be doubted whether any method of study, 

 except in the actual soil itself, will further our knowledge very much. 



Soil bacteria can decompose other nitrogen compounds besides 

 protein. Lohnis (187) has shown that they possess the remarkable 

 power of decomposing calcium cyanamide Ca : NC j N to form NH 3 

 and CaCO 3 , while other investigators have claimed that ferrocyanides, 

 cyanides, and cyanates are also decomposed by soil bacteria. 



Nitrification. 



The ammonia formed by the action of soil bacteria, or added in 

 manures, is changed to carbonate which is then rapidly converted by 

 Nitrosomonas into nitrite, and this by Nitrobacter into nitrate, the 

 changes proceeding so rapidly that only traces of ammonia or nitrite 

 are ever found in normal arable soils (241). We may, therefore, 

 infer that the production of nitrates is the quickest of the three re- 

 actions, the production of nitrites is slower, while the formation of 

 ammonia is the slowest of all and sets a limit to the speed at which 

 they can take place. Thus a measure of the speed at which nitrates 

 are formed in soil does not measure the rate of nitrification, as is 

 sometimes assumed, but the rate of ammonia production. 



The essential facts of nitrification are readily demonstrated by 

 putting a small quantity of soil -2 to '5 grams into 50 c.c. of a 

 dilute solution of ammonium sulphate containing nutrient inorganic salts 



