534 TEXTBOOK OF BOTANY 



neutral reaction, in addition to the raw materials necessary for certain 

 food syntheses in the bacteria. The bacteria involved in ammonification 

 are common and widespread in most soils, but the nitrifying bacteria 

 are largely restricted to well-aerated, moist, nearly neutral soils. 



Denitrification. The nitrates accumulated in soils through the agency 

 of bacteria may disappear in several ways. They may be used by green 

 plants, carried away by water, or changed into insoluble substances. 

 They may also be reduced by other organisms to nitrous acid and am- 

 monia, or to molecular nitrogen. This reduction of nitrates to gaseous 

 nitrogen is referred to as denitrification. It occurs under certain condi- 

 tions and decreases the nitrogen content of the soil. Some of the chemi- 

 cal reactions involved in the reduction of nitrates may be indicated as 

 follows : 



Nitric acid > Nitrous acid + Oxygen 



2HNO3 > 2HNO2 + O2 



Nitrous acid > Water + Nitrogen + Oxygen 



4HNO2 >^2H20 + 2N2 + 8O2 



The organisms that bring about the reduction of nitrates are prin- 

 cipally anaerobic bacteria, known collectively as denitrifying bacteria. 

 Since denitrification is a reduction process and may occur in the absence 

 of light, the organisms must obtain energy by oxidizing carbohydrates 

 and other organic compounds. 



Denitrification is likely to occur in any soil containing nitrates under 

 anaerobic conditions. It is characteristic of poorly drained soils, and of 

 soils periodically flooded, as in rice fields. In certain regions denitrifica- 

 tion may so deplete the soil of available nitrogen compounds that plant 

 growth is restricted. 



Nitrogen-fixation. Nitrogen constitutes nearly 80 per cent of the earth's 

 atmosphere; the combined nitrogen in all plants is relatively so small 

 that if plants could directly use the free nitrogen of the air the supply 

 would be inexhaustible. Over every acre of land surface the air contains 

 nearly 300 million pounds of nitrogen. It has been calculated that the 

 earth's atmosphere contains the prodigious total of over 5000 million mil- 

 lion tons of nitrogen. In spite of this, nitrogen is the most expensive 

 component of fertilizers. 



Certain bacteria together with a few other organisms directly utilize 

 the free nitrogen of the air. These bacteria transform this free nitrogen 



