22 Historical 



It was at first believed that the bacteria are the all-important agents 

 concerned in soil fertility. Information began to accumulate, how- 

 ever, indicating that other groups may also have important func- 

 tions, Ehrenburg had shown in 1839 that the soil contains numerous 

 protozoa. Darwin directed attention in 1881 to the role that earth- 

 worms play in certain soil processes. Adametz found in 1886 that 

 fungi occur abundantly in the soil, and the abundance of actino- 

 mycetes was studied by Hiltner and Stormer. Algae and other 

 organisms had also received their share of attention as important 

 groups of soil-inhabiting microorganisms. It thus gradually be- 

 came established that the soil harbors an extensive population repre- 

 senting all the foregoing groups in varying degrees of abundance. 

 The activities of these organisms were found to result in products 

 which are essential for plant growth. Some are important agents of 

 decomposition, whereas others may exert injurious effects, such as 

 causing plant and animal diseases. 



This complex soil population can be considerably modified by 

 soil treatment, such as liming, cultivation, addition of organic mat- 

 ter, and partial sterilization. Some of the soil processes, for example 

 nitrification, nitrogen fixation, and sulfur oxidation, are carried out 

 by specific groups of organisms, whereas other processes, such as 

 decomposition of proteins, cellulose, and complex plant residues, 

 are carried out by a variety of organisms or by large groups of 

 organisms. 



The complexity of the relationships between various microorgan- 

 isms inhabiting the soil can best be illustrated by the theory of 

 Russell and Hutchinson. According to this theory, protozoa are 

 able to consume bacteria and thus exert a controlling effect on the 

 bacterial processes in the soil and thereby on soil fertility. Any 

 treatment that leads to elimination of the protozoa would, theo- 

 retically, result in an improvement of soil fertility. Although sub- 

 sequent studies did not confirm this theory, the abundance of proto- 

 zoa in the soil was definitely established. 



Other important soil microbiological processes received much con- 

 sideration. Among these was the decomposition of cellulose by 

 microorganisms. In spite of the fact that cellulose makes up 20-50 

 per cent of most plant residues, little was previously known about 

 the nature of the organisms concerned in decomposition of this 

 important material. As late as 1902, when Omeliansky published his 

 studies on the anaerobic cellulose bacteria, no consideration was 

 given to the role of fungi or other microorganisms in this process. 



