174 SOIL ORGANISMS IN RELATION TO SOIL FERTILITY 



pounds, and much of it finally into ammonia (gas) before the 

 nitrogen can be converted into available form suitable for plants. 

 This breaking-down process is brought about by fungi as well as 

 by bacteria. As soon as the ammonia is formed, other bacteria, 

 called nitrifying bacteria or nitrifiers, convert it into soluble 

 nitrogen-containing salts, called nitrates. 2 This conversion of 

 ammonia into nitrates by nitrifying bacteria (Fig. 114-B) is termed 

 "nitrification." The opposite of nitrification is denitrification, 

 which means the breaking down of nitrates by certain organisms 

 which work only when the air is shut out of the soil. Under good 

 soil management we need not concern ourselves about this destruc- 

 tion of nitrates. 3 



What Becomes of the Nitrates Formed. Nitrification is 

 usually most rapid during the growing period. The nitrates then 

 formed are almost completely and immediately absorbed by the 

 growing crops, unless too much is manufactured. If no crops are 

 present to utilize the nitrates, or when too much is formed, they 

 are leached out of the soil and lost. Ordinarily more nitrates are 

 produced in a rich, cultivated soil than are used by the crop. 



An idea as to the rate at which nitrification goes on in some 

 soils may best be gained by studying the nitrogen needs of good 

 crops of corn, sugar beets, and cabbage. During cold weather 

 nitrification ceases. 



The Use of Catch and Cover Crops. Since certain crops are 

 harvested early, some soils are left bare during the fall months. 

 It has been found that much nitrates are formed during this time 

 and lost from the soil. To conserve this nitrogen, rye or some other 

 crop is sown immediately after the harvesting of these early crops. 

 Such crops are commonly called " catch" or " cover" crops. They 

 not only prevent nitrogen losses, but they prevent soil washing and 

 blowing, and improve the soil when plowed under. In the South, 

 rye, either alone or with hairy vetch or crimson clover, is frequently 

 sown as a winter cover crop. In cultivated orchards, the most 

 common cover and catch crops are rye and clover. 



2 Ammonia is a gas composed of one atom of nitrogen and three atoms of 

 hydrogen, hence the chemical formula, NH 3 . The strong smell of washing 

 ammonia-water is due to this gas. 



The last stage of decomposition in which ammonia is formed is called 

 " ammonification." 



3 It is to be noted that the decomposition is a breaking-down 

 process, and nitrification is a building-up process. A common nitrate formed 

 in soils is calcium nitrate [Ca(NOs)2], which is a more complex substance 

 than ammonia. 



