THE MICRO-ORGANIC POPULATION OF THE SOIL 261 



(i) Soil is inoculated into various media each arranged to 

 bring out one group of organisms, and the amount of decom- 

 position is taken as a measure of the number and vigour of 

 the members of the group. This is often called the method 

 of physiological grouping. 



(2) Platings on gelatin or agar media are made of soil 

 suspensions suitably diluted, and the colonies which develop 

 are counted. The results are expressed as millions of bacteria 

 per grm. of soil. 



(3) Chemical analyses are made at stated intervals to 

 determine the rate of progress of the various changes going 

 on — the absorption of oxygen, the evolution of carbon dioxide, 

 the production of nitrates, etc. 



The difficulty with the first or direct method is to imitate 

 the soil conditions, and the history of the subject affords 

 many instances of the danger of getting away from them : 

 for example, Krzemieniewski's work on the nitrogen-fixing 

 organisms may be quoted (p. 199). It is impracticable, for 

 reasons already given, to keep to the soil as the medium for 

 work, and most investigators have therefore used the indirect 

 methods. 



Physiological Grouping. — This method was introduced 

 by Remy (237(3:) and developed by Lohnis (182); it has 

 become very popular. Four distinct media are in use, 

 arranged respectively to favour nitrification, ammonia pro- 

 duction, nitrogen fixation, and denitrification. The experi- 

 ments are easy to carry out, but they require skilful 

 interpretation, and the results may prove treacherous unless 

 carefully handled. The fundamental objection to the method 

 is that the reaction goes on in a medium very different from 

 ordinary soil, so that it throws no light on the relationships 

 obtaining in the soil itself The results really only prove 

 that the bacteria from one soil will flourish better in a certain 

 artificial medium than those from another. 



The medium for studying nitrification is usually that sug- 

 gested by Omelianski or Ashby's modification (p. 188); it is 

 inoculated with a definite weight of soil and incubated : the 



