120 SOIL CONDITIONS AND PLANT GROWTH 



but to confine ourselves to the activities of the organisms producing 

 humus and nitrates. 



Several methods have been adopted to ascertain whether any 

 relationship can be traced between the activity of these organisms and 

 the growth of plants. The direct method consists in picking out 

 definite organisms and studying them in conditions calculated to throw 

 light on their action in the soil. This has proved very difficult, and 

 has been successfully achieved only by a few of the best bacterio- 

 logists; instances are afforded by the work of Winogradsky (pp. 15, 

 89, 92), Beijerinck (p. 93) and others. 



Three indirect methods have therefore been used : 



(1) Soil is inoculated into various media each arranged to bring 

 out one group of organisms, and the amount of decomposition 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) Counts are made of the numbers of colonies developing on 

 gelatin or agar plates, and these are expressed as millions of bacteria 

 per gram of soil. 



(3) Chemical analyses are made at stated intervals to determine 

 the rate of progress of the various changes going on notably the 

 production of nitrates. 



The difficulty with the first or direct method to imitate the soil 

 conditions, and the history of the subject affords many instances of the 

 danger of getting away from them : in example Krzemieniewski's work 

 on the nitrogen-fixing organisms may be quoted (p. 94). It is imprac- 

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

 work, and most investigators have therefore used the indirect method. 



Physiological Grouping. This method was introduced by Remy 

 (2370) and has become very popular. Four distinct media are in use, 

 arranged respectively to favour nitrification, ammonia production, 

 nitrogen fixation and denitrification. The experiments are easy to 

 carry out, but they require skilful interpretation and the results may 

 prove treacherous unless carefully handled. The fundamental objec- 

 tion 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 relation- 

 ships 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 either that suggested by 

 Omelianski or Ashby's modification (p. 89) ; it is inoculated with a 

 definite weight of soil and incubated : the nitrates produced after a 



