344 Recent Developments in Soil Microbiology 



substances; mature plant residues favor extensive development of 

 fungi, especially when available nitrogen is present in the soil or is 

 added to it. 



To illustiate further the effect of changing conditions upon the 

 development of specific microorganisms, the population of a compost 

 may be examined in further detail. If a compost is kept at 28° C, 

 the population will consist largely of bacteria, fungi, protozoa, and 

 nematodes; actinomycetes develop only to a limited degree; aerobic 

 cellulose-decomposing bacteria, especially members of the Cyto- 

 phaga group, are most active. At 50 °C, where the rate of decom- 

 position is highest, certain thermophihc fungi and actinomycetes pre- 

 dominate; bacteria are also present but they are not the most abun- 

 dant forms; and the animal population is almost completely lacking. 

 At 65 °C, the fungi are eliminated entirely; certain actinomycetes, 

 belonging to the Micromonospora type, and the thermophihc bac- 

 teria are most abundant; cellulose decomposition is brought about 

 by anaerobic, spore-forming thermophilic bacteria. At 75°C, decom- 

 position is limited and takes place largely at the expense of the 

 proteins and hemicelluloses; cellulose is not attacked at all; certain 

 bacteria of the Plectriditim type and certain species of Micromono- 

 spora make up the population. The most rapid decomposition of 

 the manure takes place first at 65 °C. At this temperature, the nitro- 

 gen is completely consumed. The inoculation of hot composts with 

 an active thermophihc population has been found to hasten the 

 process of decomposition. Animal pathogens present in the manure 

 are also destroyed at the high temperature. 



Role of Microorganisms in Soil Processes 



The role of microorganisms in the minerahzation of waste ma- 

 terials in soils, water basins, and composts no longer requires em- 

 phasis. One need not dwell upon the function of microorganisms in 

 bringing about the liberation of nitrogen in an available form, as 

 ammonia, and in the oxidation of the ammonia to nitrate. R is now 

 universally recognized that the growth of legumes and their associ- 

 ated bacteria are of tremendous economic significance to agriculture. 

 Numerous other microbiological reactions have been elucidated and 

 are at present well understood. R is sufficient merely to mention 

 the oxidation of sulfur by bacteria, a process which frequently be- 

 comes of considerable importance; the reduction of sulfates, nitrates, 

 and arsenates, processes which involve the activities of various groups 



