CHAPTER II 

 THE MICROBE AND ITS ACTIVITIES 



The Microbe and its Importance in the Soil. — The 

 various forms of life which develop directly at the expense of the 

 chemical constituents of the soil and the decomposing plant and 

 animal residues are extremely numerous and diverse. Included 

 among these forms are all plants, ranging from the large trees to 

 the tiniest of microbes, with an almost limitless number of tran- 

 sition forms bridging this wide gap, and besides these are various 

 representatives of the animal kingdom. Indirectly, all animal 

 forms depend for their nutrition upon the elements which are 

 liberated in the soil; the microbes of the soil keep the elements in 

 circulation. Were it not for them, all plant and animal life would 

 soon cease because of the exhaustion of the supply of available 

 elements. A knowledge of these microbes and their acti\'ities is, 

 therefore, of prime importance for the understanding of soil 

 processes and plant growth. 



The study of the nature and activities of soil microorganisms 

 would be of considerable interest even if the existence of these 

 organisms had no great influence upon the growth of other plants 

 and animals. Their identity, habits of growth, and influences 

 become of much wider interest and extreme importance with the 

 realization that to these tiny living things w^e owe the continued 

 development of higher plants. Studies of the reactions with which 

 they are associated in their strife for existence, and the variety 

 of influences which they exert directly and indirectly upon other 

 living organisms, open a fascinating chapter in biology. 



The microorganisms are the least differentiated of all forms of 

 life — they are organisms either unicellular in structure or, at the 

 most, simple aggregates of a few cells with comparatively minor 

 differentiation. They are too small to be seen as individuals 

 with the naked eye, and in some cases too small to be seen even 

 with the aid of the most powerful microscope, and are known 



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