250 IMPORTANCE OF MICROBES IN SOIL FERTILITY 



fides, of phosphates to phosphites and even phosphene; the reduc- 

 tion of selenium and tellurium compounds; the reduction of amino 

 acids and various organic compounds; the numerous oxidation 

 processes, some of which have been mentioned previously; the 

 various hydrolytic and other transformations of organic and inor- 

 ganic compounds in the soil. 



Summary. — By comparing, in a very general way, the activities 

 of the microorganisms in the soil with those of a population of a 

 large modern country, certain interesting similarities are evident. 

 One gram of soil possesses as many micro-inhabitants as the total 

 human population of the United States. These inhabitants also 

 take part in as great a variety of activities as the human population. 

 Some are occupied with obtaining raw materials from the earth; 

 a part of these materials is consumed by those that labor for them, 

 while a part is turned over to the industries for further exploitation 

 and processing. The industrial workers consume a part of the 

 materials either in a raw or in an industrialized form and turn 

 back a part of the manufactured products to the original cultivators 

 of the soil. Influences responsible for the concentration of raw 

 materials in a favorable environment lead to an abnormally abun- 

 dant population in the region and an accelerated consumption of 

 these materials. Similarly, limitations of the food supply or lack 

 of favorable environmental conditions cause a quick depletion of 

 the population. Even the parasites of human society have their 

 equivalent in the society of the microscopic population below our 

 feet — these are the numerous fungi, nematodes, and bacteria that 

 can cause diseases of plants, or are so cannibalistic as to feed on 

 their own kind or on other members of the soil population of an 

 equal or lower social stratum. 



Methods used in an approach to the study of the soil organisms 

 are all quite similar to those used in a study of the human popula- 

 tion. Certain observations are confined to an enumeration of the 

 total population, to those of different races or physical types, or 

 those concerned in specific activities. An entirely different type 

 of examination might be applied to studies of the activities of the 

 population in an attempt to estimate the amounts of raw materials 

 transformed, the rates of transformation, the end products 

 formed and their fate or further utilization. 



