SOIL MICROBIOLOGICAL POPULATION 9 



types. The bacteria usually range in number from a few hundred thou- 

 sand to several hundred million per gram of soil, though many species 

 do not develop on the common plate. Fungi are found in the form of 

 mycelial filaments and as spores and may therefore constitute as large 

 a mass of living matter as do the bacteria j their actual number, as deter- 

 mined by the plate method, may vary from a few thousand to several 

 hundred thousand per gram of soil. The significance of these results is 

 not always clear, since a given colony may have originated from a 

 hyphal filament, a mass of mycelium, or a single spore. Determination 

 by the plate method of the number of actinomycetes is subject to the 

 same limitations j these organisms usually constitute from lo to 50 per 

 cent of the colonies appearing on common bacterial agar plates. 



Algae are numerous in the surface layers of soil only. Protozoa are 

 present in the soil in an active vegetative or trophic state and in the 

 form of cysts. The active cells appear when excessive water is present, 

 even for a few hours j in dry soil, the cysts predominate. Flagellates 

 are represented by the largest numbers, sometimes approaching a mil- 

 lion individuals per gram of soil j amebae are next in abundance j cili- 

 ates are usually found to the extent of a few hundred to several thou- 

 sand per gram of soil. Nematodes, rotifers, earthworms, and larvae of 

 numerous insects are also abundant, often forming a large part of the 

 bulk of the living mass of cell substance. 



By means of the selective and enrichment culture methods, several 

 physiological classifications of bacteria have been recognized. The fol- 

 lowing descriptive terms are commonly used to designate these groups : 

 autotrophic vs. heterotrophic, aerobic vs. anaerobic, motile vs. non- 

 motile, pathogenic vs. saprophytic, psychrophilic and mesophylic vs. 

 thermophilic, symbiotic vs. nonsymbiotic, and antagonistic vs. non- 

 antagonistic. 



The fungi may be classified into three types: saprophytic and free- 

 living, mycorrhiza-producing, and plant pathogenic. The most com- 

 mon groups of soil fungi are found in the genera Rhizofus, Mucor^ 

 Penkilliumy Aspergillus, Trkkoderma, Fusarium, Cladosforium, and 

 Cefhalosforium. The soil often harbors an abundant population of 

 yeasts and fleshy or mushroom fungi. The latter may produce an ex- 



