72 AGRICULTURAL BACTERIOLOGY 



decomposition of organic matter, nitrates, it is easily leached 

 from the soil. 



The soil as a culture medium. The amount and kind of 

 bacterial growth in the soil, and therefore the quantity of 

 food available to the green plant, will depend on many fac- 

 tors. The chief factor is the food supply. If large amounts 

 of organic matter are added to the soil, as will be the case 

 under natural conditions where the entire crop is returned 

 to it, the amount of food will be large. If the crop is re- 

 moved and sold in its entirety, as in the case in grass farm- 

 ing, the amount of organic matter returned to the soil will 

 be reduced to a minimum. Again, a farmer may purchase 

 feed, hay, and grain for the use of his animals, and by add- 

 ing the manure produced to his fields, he increases the food 

 supply of the bacteria in the soil. In market-gardening 

 the addition, in the form of manure from the cities, of many 

 tons of organic matter grown on the soil of distant farms 

 permits a large development of bacteria, and therefore an 

 increased crop of vegetables. Without a beginning there 

 can be no end ; without bacterial food in the soil there can 

 be no food crop for man or beast. 



Moisture and air. The amount of water in the soil has 

 an influence not only on the total amount of bacterial 

 growth, but also on the types of organisms that may grow 

 a factor of great importance in determining the kinds of 

 green plants that may develop in any soil. 



The soil is a porous structure made up of minerals in 

 varying states of subdivision. The interspaces in the soil 

 may be completely filled with water, as in a water-logged 

 soil. Under such conditions only facultative and anaerobic 

 organisms can grow. If the soil spaces are filled with air, 

 aerobic as well as facultative bacteria can develop. The 

 so-called hydrostatic water is removed from the soil by 

 gravity. It tends to carry with it the soluble constituents 



