190 Agricultural Bacteriology. 



mense amounts of nitrates are used in the manufacture 

 of explosives as gun powder, nitroglycerine, etc. Be- 

 fore the discovery of the Chilean deposits, the. nitrate 

 needed for such purposes was largely made on the salt- 

 peter plantations. A mixture of earth and organic mat- 

 ter of any kind was made and placed in a pile which was 

 protected from leaching. The air was allowed access to 

 the inside of the pile by placing brush wood in it. The 

 -nitric acid formed was neutralized by mixing lime stone 

 with the soil or by the addition of soapy water. When 

 the process was completed, the entire pile was leached 

 and the nitrate obtained by evaporating the water. The 

 deposits in Chile will soon be exhausted, but it is certain 

 that methods will be found by which the nitrogen of the 

 .air can be brought into combination as nitrates, and it 

 will not be necessary to rely on the action of bacteria for 

 the nitric acid needed in the industries, and as fertilizers. 

 The farmer must always depend upon the action of this 

 class of bacteria in order to obtain the nitrates needed by 

 the various crops grown by him. 



Denitrification. In the broadest sense denitrification 

 may include any process due to micro-organisms by 

 which the nitrogen available to the plant, i. e., that con- 

 tained in nitrates, is changed so that it is less available or 

 rendered unavailable. There are a number of processes 

 by which this is accomplished. The process may be the 

 reverse of nitrification, the nitrates being reduced to ni- 

 trites and to ammonia. The bacteria responsible for this 

 -action can act only in the absence of air and, while they 

 are always present in the soil, they do not usually mani- 

 fest themselves. During a wet spell in the summer, the 

 soil may be so saturated with water and hence contain so 

 little air that these bacteria can destroy the nitrates pres- 



