Role of Microorganisms in Soil Processes 345 



of microorganisms and max lunc, under certain conditions, great eco- 

 nomic significance; the composting of stable manures and plant resi- 

 dues for the production of artificial composts, which involves the 

 activities of large microbial populations; and the growth of plant and 

 animal parasites, involving fungi, actinomycetes, bacteria, nema- 

 todes and N'arious other worms, and insect larvae, resulting in condi- 

 tions which require radical modification of soil management. 



The importance of microorganisms in a number of other soil 

 processes is still a matter of dispute, if not of mere speculation. 

 Here belong the activities of nonsymbiotic nitrogen-fixing bacteria, 

 in spite of the fact that the occurrence and physiology of these organ- 

 isms have been studied extensively. The specific effect upon plant 

 growth of various substances produced by microorganisms, including 

 vitamins, hormones, and other growth-promoting substances, is also 

 still a matter for speculation. The effect of the saprophytic popu- 

 lation of the soil upon plant and animal parasites which live in or 

 find their way into the soil is still insufficiently understood. The 

 influence of microorganisms and of their metabolic products upon 

 the physical condition of the soil, especially in aggregating the finer 

 soil particles, a problem of great importance in soil conservation, is 

 becoming more and more clearly recognized. The mycorrhizal rela- 

 tionships, in spite of the progress made during the last few years, 

 are still to be unraveled, and the processes involved are yet to be 

 understood and utilized for practical purposes. 



These and numerous other processes resulting from the activities 

 of the soil-inhabiting microorganisms are frequently complicated and 

 involved. So far, only very few of them have been recognized and 

 still fewer utilized. Further progress will undoubtedly result with 

 the development of new methods and with the growing appreciation 

 of the interlocking activities of the complex microbiological popula- 

 tion of the soil. 



Soil microbiology has made only a beginning. It is still facing 

 open vistas for further investigation. A highly complex population 

 active in a most complex medium, the soil, and bringing about a 

 number of most complicated processes, fully deserves the interest 

 not only of the soil microbiologist and of the soil chemist, but also 

 of the agronomist, the botanist, the zoologist, the pedologist, and the 

 biochemist, in finding the answers to some of the riddles which 

 Mother Earth still propounds for us. 



