DIRECT MICROSCOPIC METHODS 51 



stained deep red, while the organic and inorganic soil particles are 

 either not stained at all or stained yellowish or brown. By the use 

 of a graduated slide, a calibrated microscope, and a definite volume 

 of the known soil suspension, for making the stained preparation, 

 the approximate number of bacteria in any mass of soil can be 

 determined. This method has the advantage that it stains all 

 the soil bacteria, irrespective of their nutritive peculiarities, and 

 thus permits the determination of their abundance in the soil. 

 It also permits the recognition of the relative abundance of various 



Fig. 38. — Presence of bacteria in soil, as indicated by direct staining of soil 



(from Cholodny). 



morphological groups of soil organisms, as well as the physical 

 relationship between the soil and its microbiological population. 

 On the other hand, there are certain factors which limit the use- 

 fulness of the method. Some of the bacteria occur in aggre- 

 gates, which makes their counting difficult; there is also great 

 variation in the distribution of the cells in the stained preparation, 

 thus decreasing the accuracy of the enumeration; actinomyces 

 spores can seldom be distinguished from bacteria by this procedure ; 

 many of the small bacteria are difficult to distinguish from soil 

 particles, and only a minute portion of the soil (one-millionth or 

 one-five-millionth of a gram) can be examined by the microscope 



