174 Agricultural Bacteriology. 



The same is true with reference to the other essential 

 -elements. It thus becomes important to know something 

 concerning the. conditions that are favorable for the 

 growth of the bacteria in the soil. The yield of the vis- 

 ible crop is dependent on the way in which the farmer 

 favors the growth of the invisible crop. This presents 

 a new phase of soil management which has not long been 

 recognized, as the soil has generally been regarded as an 

 inert mass of particles of sand, clay, or gravel, inter- 

 mixed with more or less dead organic matter which gave 

 to the upper portion its black color. 



Since there is no store of available plant food in the 

 soil and since the bacteria are necessary in order to 

 change the raw food to a fitting form, it is evident that 

 a soil that is free from bacteria cannot be a fertile one. 

 If various types of soils are examined as to the number 

 of bacteria, it will be found that those of high fertility 

 are teeming with bacteria while a poor sandy soil will 

 contain very few. 



Distribution of bacteria in the soil. The soil is one 

 of the great homes of the bacteria. In a state of nature, 

 i. e., in uncultivated soil, everything that the soil yields 

 is returned to it, either directly in the i form of the dead 

 plant, or indirectly in the body or excreta of an animal 

 that has lived on the plants. This means that organic 

 matter in abundance is supplied as food to the bacteria; 

 hence their rapid increase in numbers where requisite 

 conditions obtain. This activity renders still more raw 

 plant food available with the result that the soil con- 

 tinues to increase in fertility until there have been 

 formed from the bare rock by the aid of various physi- 

 cal agencies that help in disintegrating the rock such 

 fertile soils as are found on our western prairies. 



