92 PLANT PHYSIOLOGY 



trates from organic compounds but only from the nitrogen after 

 it has reached the stage of ammonia. 



The conditions necessary for good nitrification are good aeration 

 of the soil, a medium water supply, a soil temperature of 30° C. 

 with a maximum of not more than 40°, the presence of some 

 basic compound such as calcium carbonate, and only small amounts 

 of organic matter. It will be observed that these are exactly the 

 proper conditions of good tillage for the majority of field crops. 

 When the soil is in good condition for the roots of the growing 

 crop, it is also in proper condition for good nitrification. The re- 

 quirements for nitrification and fertility are nearly identical. 



Ammonification. — The production of ammonia from the or- 

 ganic matter in the soil is the work of various organisms among 

 which are Bacillus mycoides, B. ramosus, and B. vulgaris. All 

 these bring about the decomposition of organic bodies, producing 

 ammonia along with the other by-products of their activity. 

 They are saprophytes and, in fact, ammonia is doubtless a by- 

 product of the activity of most saprophytes. Omelianski (1899) 

 has thus shown that to produce nitrates from organic matter all 

 three groups of bacteria must be present, viz., the ammonifying, 

 those which produce nitrites, and those which produce nitrates. 

 If we call these A, B, and C, then, as shown in the table below, 

 when only A and B are present the action goes no further than 

 nitrites. If A and C are present, only ammonia is produced ; while 

 if only B and C are in the nutrient solution, there is no change of 

 the organic material whatever. All three must be present to get 

 any nitrates produced. 



A, B, and C A and B A and C B and C 



Organic nitrogen 

 Ammonia 

 Nitrites 

 Nitrates. 



J 



1 



Denitrification. — While the previously described processes in- 

 crease the amount of available nitrates in the soil, there are un- 

 fortunately other organisms which cause the reverse process to 

 take place. This antithetic process is called denitrification and 

 consists in the reduction of nitrites and nitrates with the forma- 

 tion of free nitrogen and its resultant loss to the soil. Denitrifica- 

 tion is accomplished by a large number of organisms which nor- 

 mally live aerobically and get their oxygen from the air, but 

 which, when forced to live under anaerobic conditions, obtain 



