DISTRIBUTION OF MICROBES IN SOIL 45 



able for the development of specific organisms, repeated intro- 

 duction of even great numbers of these microbes will fail to estab- 

 lish them as permanent members of the soil population. The 

 development of a specific microbe in a soil depends upon the 

 chemical composition of the soil, especially the presence of nutri- 

 ents essential for its growth, and the environmental conditions, 

 such as temperature, moisture, and reaction. The soil and 

 environmental conditions thus determine to a large extent the 

 abundance and kinds of organisms that will inhabit the soil and 

 the reactions which they perform there. 



Relation of Microbes to Plants and Animals. — In their 

 relation to plants and to animals, microorganisms can be con- 

 sidered as being either parasites or saprophytes. The parasitic 

 forms are capable of growing upon the living plant or animal, 

 causing a diseased condition of the tissues, leading to an abnormal 

 condition of the host and possibly to death, unless the host is 

 able to withstand the attack of the parasite. 



The saprophytic microorganisms grow only upon dead plant 

 and animal tissues or their decomposition products. They are 

 thus of extreme importance in the transformation of complex 

 organic substances into simple forms, making the nutrient ele- 

 ments locked up in those substances again available for plant 

 nutrition. The typical soil population consists of microorganisms 

 which are largely saprophytic in nature. However, certain 

 plant and animal parasites may persist in the soil for some time, 

 and a few are even capable of leading a normal existence there. 

 Certain microbes are capable of acting parasitically upon other 

 microbes which are either beneficial or injurious to plants. The 

 saprophytic microorganisms are widely distributed in the soil 

 and occur in greater or less abundance practically everywhere. 

 There is no soil known, ranging from the Sahara Desert to the 

 polar regions, that should be free from saprophytic microorgan- 

 isms. Certain specific groups of organisms may of course be 

 absent, where conditions are unfavorable for their development. 



Qualitative and Quantitative Distribution of Microbes 

 in Soil. — In most soils we find much the same qualitative dis- 

 tribution of organisms, but there are very marked differences in 

 the quantitative relationships. A change of soil conditions, such 

 as an increase in acidity or alkalinity, a change in food supply, 

 moisture content, aeration, or other physical or chemical soil 



