INFLUENCE OF SOIL CONDITIONS 



59 



the corresponding drained soils, although the number of anaerobic 

 bacteria may be greater in the former. Acid soils, especially acid 

 forest or raw-humus soils, are very rich in fungi and poor in bac- 

 teria. The fungus development is so extensive that the soil is 

 profusely permeated with fungus mycelium (Fig. 42). As soon 

 as an acid soil is limed, there is a rapid increase in the abundance 

 of bacteria. 



In general, the numbers of bacteria, as determined by the 

 common plate method, range from 1,000,000 to 20,000,000 per 

 gram of soil. These figures refer only to the aerobic bacteria that 

 are capable of developing upon certain solid media. They are 



Fig. 42. — Fungus development on surface litter of forest soil (from Rayner). 



only relative figures and should not be interpreted as representing a 

 constant number of organisms in a particular soil; they refer only 

 to the relative abundance of certain types of cells at the time that 

 the determination was made. It should be kept in mind that 

 these numbers change greatly from time to time, even without any 

 apparent alteration in the conditions of the soil. Much greater 

 changes occur when the environment is considerably modified. 



Injudicious cropping without fertilization will in time greatly 

 deplete the soil organic matter and the potential sources of nutri- 

 ent substances for plants, and will lead to depressions in the abun- 

 dance of microorganisms. Certain fertilizer practices, as con- 



