Relation of Bacteria to Fertility. 175 



It is impossible to determine in any way the total 

 number of bacteria in the soil. What can be done is to 

 determine the number that will grow on such substances 

 as are used in the laboratory as culture media. Exam- 

 ined in this way a sandy soil will be found to contain a 

 few hundred thousands in each gram (1-30 of an ounce) 

 while a rich loamy soil may contain several million per 

 gram. A garden soil may show ten, twenty-five, or even 

 fifty millions of these minute plants so essential to the 

 fertility of the soil. The greatest number of bacteria is 

 found in the first few inches of the soil, in what is known 

 as the soil proper. In the sub-soil smaller numbers are 

 found and as still lower depths are examined the bac- 

 teria decrease rapidly in numbers. At a depth of a few 

 feet they completely disappear. The reason for this 

 rapid decrease in numbers is due to the lack of food and 

 air, and to the filtering effect of the soil. In the upper 

 layers of the soil food is abundant and other growth con- 

 ditions favorable. 



The soil contains a large number of different kinds of 

 bacteria of the most varied nature and appearance. It 

 is the home of some of those that produce diseases in man 

 and animals, as well as the most of those forms that cause 

 the spoiling of food substances, and the various fermen- 

 tations, many of which are so important. The number 

 of bacteria in the soil is influenced by a number of fac- 

 tors, chief of which is the amount of food present. The 

 addition of organic matter in the form of stable manure, 

 or plowing under a green crop, increases the amount of 

 food material, thus stimulating the growth of bacteria. 



The effect of temperature is also of importance. As 

 the soil becomes warm in the spring, the conditions be- 

 come more favorable and the bacteria increase rapidly in 



