DENITRIPICATION 



407 



entrance of air. Lime is sometimes added 



to soil to neutralize its acidity so that the 



nitrifying activities may be increased. 



Denitrification, If soil is poorly drained, 



poorly aerated, and contains fresh organic 



matter, denitrifying bacteria thrive. They 



convert the ammonia, formed in the decay 



process, into free nitrogen. The free nitrogen ■""^lilL^'e'dtrg^^sms!'" 



then escapes into the air and is lost to the 



plants. The denitrifying bacteria can also break down nitrates into 



nitrites, then into ammo- 

 nia, and finally into free 

 nitrogen. Soil should 

 not be covered thicklv 

 with unrotted manure as 

 the activity of these bac- 

 teria are then promoted. 

 From the point of view of 

 conserving soil fertility, 

 the denitrifying bacteria 

 are undesirable. Certain 

 other bacteria are able, 

 how^ever, to take free 

 nitrogen from the air 

 and again change it into 

 a form in which it can be 

 used. These are called 

 the nitrogen-fixing bac- 

 teria. 



Nitrogen-fixation. The 



^ nitrogen-fixing bacteria 



Nitrogen-fixing bacteria form nodules or small swellings , l j u A' 



on the roots of pod-bearing plants. The countless bac- liave already DCen (lis- 

 teria in these nodules fix free nitrogen of the air into a i • j.i 



usable form, nitrates. CUSScd in the prcVlOUS 



