52 Microbiological Population of the Soil 



been brought about in the particular substance by the microorgan- 

 isms in the soil are then determined. The extent of change is used 

 as a measure of the activity of the microbiological population of the 

 soil. 



These methods, whatever their value for measming the rate of 

 certain soil processes, do not lend themselves readily to the quantita- 

 tive enumeration of soil microorganisms. 



Further details of the abundance of specific groups of bacteria 

 will be found in the respective chapters. 



MiCEOORGANISMS IN MANURE AND IN CoMPOSTS 



The microbiological population of manures and composts can 

 be studied by methods similar to those used for the study of the 

 soil microorganisms. 



By the direct microscopic method, manure was found to contain 

 37,600 millions of bacteria per gram, the greatest number occur- 

 ring in stable manure kept in heaps, and the smallest number in the 

 manure that was undergoing a "hot fermentation" process. A definite 

 correlation was found in the microbiological population of com- 

 posted manure, whether determined by the plate or by the contact 

 slide method. 



The numbers of bacteria and other microorganisms in stable manure 

 vary greatly, depending on the composition of the manure, especially 

 on the nature and amount of solid excreta, and on the degree of its 

 decomposition. During the decomposition of manure, a marked 

 change takes place in the nature and abundance of its microbial 

 population. Fresh manure is very rich in cells of E. coli and other 

 enteric bacteria. During the process of composting, these soon dis- 

 appear. At first, multiplication of various bacteria takes place, to 

 be followed later by a reduction in numbers. The temperature at 

 which the decomposition of the manure is taking place exerts a 

 marked influence upon the nature and abundance of the microbio- 

 logical population, at different stages of decomposition, as shown in 

 Table 8. 



The addition of manures and other organic materials greatly in- 

 creases the nimibers of microorganisms in the soil. There may also 

 be a shift in the relative abimdance of different groups. An increase 

 in numbers of saprophytic organisms may be accompanied by a 

 reduction in numbers of plant-disease-producing fungi and bacteria. 



