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A considerable amount of energy is of course necessary and is 

 derived from the oxidation of organic matter. 



Between this fixation of nitrogen and the liberation of nitrogen 

 already mentioned there is generally an equilibrium. The steps 

 in these changes are entirely unknown. 



It is impossible at present either to make soil artificially, or to 

 reconstruct the bacterial flora in a sterilised soil ; it is even impos- 

 sible to sterilise soil without profoundly altering its character. 



The culture solution used in working with the nitrogen-fixing 

 organisms is one containing sugar, potassium phosphate and calcium 

 carbonate but no nitrogen compound. The solution used in studying 

 nitrifying organisms contains an ammonium salt, phosphates, etc., 

 but no organic matter; whilst in investigating the decomposition 

 process it is customary to use a solution of a highly nitrogenous 

 organic compound such as peptone. 



The results obtained are very valuable but they throw more 

 light on the morphological und physiological characteristics of the 

 soil organisms than on the changes actually going on in the soil. 



The total chemical change may be ascertained by determining 

 ammonia and nitrates in the soil. 



The total bacterial activity has been estimated in two ways : 

 Hiltner and Stormer count the number of colonies developing on 

 gelatine or agar plates inoculated with known weights of soil and 

 reduce the results to numbers per gram of soil ; the second method 

 devised by the Author consists in measuring the rate at which 

 oxygen is absorbed by the soil. The rates for a series of compa- 

 rable soils are found to run in the same order as the relative pro- 

 ductiveness. 



A third method has been used in the Rothamsted laboratories 

 by Dr. Hutchinson and the author, which is really a combination 

 of these two methods. 



The author, after giving a brief historical notice on the experi- 

 mental work on the effect of partial sterilisation describes the 

 experiments made at Rothamsted in conjunction with Dr. H. B. 

 Hutchinson on this subject. 



From the results obtained a clear picture of the microscopic 

 life of the soil can be given. 



The micro-organism flora of an ordinary soil is very mixed, 

 including, as it does, a wide variety of organisms performing very 

 different functions. 



They may be divided roughly into two classes: saprophytes 

 which live on and effect the decomposition of organic matter, and 



