The Soil Microbiological Population 341 



till' ri>lati\o ahundance of the various bacteria, fungi, actinomycetes, 

 and protozoa depends upon the nature of the soil and its condition. 

 In the treated soil, however, this equilibrium is often disturbed, 

 and certain organisms develop in great abundance, out of all propor- 

 tion to the others. The specific nature of these organisms depends 

 either upon the chemical nature of the material added or upon the 

 nature of the changes produced in the soil by the treatment. The 

 disturbance thus brought about in the microbiological equilibrium 

 may be of a lasting nature, whereby one group of organisms may 

 become predominant, to be followed later by the rapid development 

 of other groups; or it may be only temporary, that is, after a short 

 time the interrupted equilibrium may become re-established on the 

 same quantitative basis or in a modified form. 



The changing activities brought about by the microbiological popu- 

 lation of the soil can be best illustrated by following the course of 

 decomposition in the soil of fresh plant and animal residues. Pro- 

 tein-rich materials lead to an extensive development of bacteria and 

 actinomycetes; cellulose-rich materials bring about extensive devel- 

 opment of fungi and certain bacteria. Among the fungi, the Phyco- 

 mycetes may come first when fresh plant residues are added; they 

 are followed by Ascomycetes and Fungi Imperfecti, and finally by 

 Basidiomycetes. A large part of the synthesized fungus mycelium 

 will be gradually destroyed by bacteria. The bacteria may be fol- 

 lowed by protozoa. This sequence of forms does not follow under 

 all conditions. Many of the microorganisms are specific and are 

 adapted to one process; others are omnivorous and are capable of 

 performing a number of functions. The nature of the material under- 

 going decomposition, environmental conditions, and incidental oc- 

 currence of specific microbial types will influence the predominance 

 of certain forms over others. 



The changing numbers and types of organisms in the complex soil 

 population, particularly when influenced by a number of soil and 

 environmental factors, do not lend themselves readily to ordinary 

 statistical treatment. Under these conditions, one is likely to over- 

 look the forest because attention is focused upon single trees. Sta- 

 tistics alone, when not properly interpreted, may tend to overempha- 

 size certain members of the population, frequently of very little sig- 

 nificance in soil processes, and to overlook others of much greater 

 importance. 



