EXAMINATION OF AIR, SOIL, AND WATER 245 



matter, their influence in making the soil more useful for 

 agricultural purposes has been the subject of much research. 

 The richer the soil, the greater the number of bacteria. 

 Most bacteria are found under the surface between i and 

 2 inches. 



The rod-shaped organisms predominate. 



From an agricultural standpoint the most important 

 bacteria are those capable of liberating nitrogen and break- 

 ing up protein substance. 



Carbohydrates are added to soil by manure, by the growth 

 of grasses and crops, and these are decomposed by bacteria 

 and methane and hydrogen produced. 



Ammonia Production. Most soil bacteria can produce 

 ammonia; a few, the so-called urea bacteria, are capable of 

 rapid transformation nitrification. 



Ammonia, oxidized into nitrites or nitrates, is possible 

 through the agency of a group of micro-organisms given 

 especial prominence by Winogradski. Moisture conditions 

 and the presence of lime and mineral carbonates influence the 

 nitrifying organisms. 



The character of the growing crop affects the accumulation 

 of nitrates; legumes assimilate nitrogen more rapidly than 

 non-legumes. 



Denitrification. The reduction of nitrates to nitrites and 

 ammonia is accomplished by a number of bacteria. Nitrate 

 reduction is of little importance in the field, but under exces- 

 sive manuring it may become so. Bacteria play the impor- 

 tant part of making available to vegetation the nitrogen of 

 the air. 



Azofication. Certain bacteria can fix atmospheric nitrogen 

 and make it serve, but the energy necessary must be fur- 

 nished by carbohydrates. 



The enrichment of the soil by the growth of legumes has 

 been shown to be due to the bacteria contained in the nodules 

 or tubercles of the plant, these bacteria having the power to 

 fix nitrogen and deriving their energy from the plant juices, 



