THE SOIL 67 



In general, light sandy soils have fewer numbers than 

 the heavier, richer, types. The amount of food (organic 

 matter) has an important bearing on the number of 

 bacteria. Also virgin soils have lower counts than 

 cultivated soils, due in part at least to the fact that 

 cultivation, especially deep plowing, increases the depth 

 of their operations by giving better access to air 

 (oxygen). The greatest numbers occur between the 

 fourth and sixth inch levels in average soils. This, of 

 course, varies greatly. Soils may be sterile at three feet 

 if impervious in nature, and are not infrequently sterile 

 at a depth of eight or ten feet. As would be expected 

 the highest numbers are found in the spring and fall. 

 Owing to the heat and lack of moisture, the numbers are 

 reduced in midsummer. In winter many of the 

 vegetative forms are killed and great numbers go into a 

 spore or resting stage to await more favorable condi- 

 tions. 



46. Functions of the Bacteria in the Soil. — The func- 

 tions of the bacteria in the soil may be classed under four 

 heads : 



1. Destruction or decay of dead matter. 



2. Preparation of available plant food for plants. 



3. Parasitism (disease). 



4. Symbiosis. 



The activity of the bacteria in bringing about the de- 

 cay of plant and animal remains has already been al- 

 luded to. In farm practice this accounts for the decay 

 and incorporation with the soil of the stubble, straws, 

 corn stalks, manure, etc., that are plowed under. These 

 substances under the action of the bacteria are decom- 

 posed, part becoming dissipated into the air as gases 



