35 6 SCIENCE PROGRESS 



The first method has been developed along the lines of 

 selective cultures adopted by the earlier investigators in isolat- 

 ing soil organisms. A solution, made to favour one group of 

 organisms, is inoculated with a sample of soil ; the favoured 

 group develops while the rest die or form spores ; finally a fairly 

 pure culture may be obtained. Thus in working with the 

 nitrogen-fixing organisms the culture solution used is one con- 

 taining sugar, potassium phosphate and calcium carbonate but 

 no nitrogen compound ; organisms incapable of utilising gaseous 

 nitrogen are therefore unable to develop. 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 and physiological characteristics of the 

 soil organisms than on the changes actually going on in 



the soil. 



A study of the whole resultant effect gives the kind of informa- 

 tion wanted for some fertility problems without, however, the 

 detail necessary for further developments. 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 method fails to discriminate between spores and 

 active organisms in the soil and also takes no account of 

 organisms active in the soil which do not develop on gelatine 

 plates; nevertheless it gives very useful results. The second 

 method, devised by the writer, consists in measuring the rate at 

 which oxygen is absorbed by the soil. The desirable changes 

 are oxidations or are brought about by aerobic organisms, and 

 the rate at which they take place is therefore some function of the 

 rate at which oxygen is absorbed. These rates for a series 

 of comparable soils are found to run in the same order as the 

 relative productiveness. 



The third method, which has been used in the Rothamsted 

 laboratories by Dr. Hutchinson and the writer, is really a 

 combination of these two. A change is induced in the micro- 

 organic flora of the soil, usually by partial sterilisation with 



