THE MICRO-ORGANIC POPULATION OF THE SOIL 265 



*' Nitrogen-fixing power " is measured by inoculating soil 

 into Beijerinck's or some similar solution (p. 197) and in- 

 cubating for a definite time. This reaction proceeds only 

 slowly. The results usually show a general resemblance to 

 those given by nitrification experiments.^ 



Bacterial Counts. — ^The method of counting the number of 

 colonies that develop on gelatin or agar plates is admittedly 

 faulty, but it has the advantage of showing whether the 

 numbers are high or low and whether they are increasing or 

 decreasing. It has, unfortunately, three serious defects. No 

 medium is known that brings out all the soil organisms, so 

 that the results are invariably low,^ and their quantitative 

 appearance is wholly illusory. No medium even distantly 

 resembles the soil in composition or in structure, so that the 

 flora developing on the plates does not necessarily reflect the 

 flora active in the soil ; in particular it is impossible to tell 

 which of the forms developing on the plate are active and 

 which are spores in the soil. Account is seldom taken of the 

 kinds of bacteria on the plates; in practice it proves far too 

 laborious to attempt any but the simplest identifications. 



This disregard of the nature of the bacteria constitutes a 

 fundamental distinction from the method dependent on physi- 

 ological grouping, and the two methods do not always give 

 similar results. The counts show fairly correctly whether 

 any given treatment of the soil has raised or has lowered the 

 number of bacteria, but unless the change has been drastic 

 they do not show whether all varieties have been equally 

 affected. Thus they have always to be combined with deter- 

 minations of the amounts of ammonia and nitrate in the soil. 



Chemical Analysis in Conjunction with Bacterial Counts. — 

 The third method of making bacteriological counts in con- 

 junction with chemical analysis, has been largely used in the 

 Rothamsted laboratories. Increases in bacterial numbers 

 are so often associated with increased production of nitrate as 



1 E.g. see papers by P. E. Brown ; also P. S. Burgess. 



' Comparisons of various media have been made by Cook {Soil Sci., 1916, 

 I, 153-161). The more uniform the results the better the medium. 



