120 GROWTH AND WORK OF PLANTS 



humus it is usually in the form of ammonia (NH 3 ). In the fields 

 the ammonia would soon volatilize and pass off in the gaseous 

 form in the air and be lost. Ammonia is often applied to the 

 soil in fertilizing (in stable manure). "Ammonia is also formed in 

 the decay of plant parts and animals (especially in the absence of 

 air which is the case in bulky parts, and in soil). The proteid 

 substances are split into ammonia by the ferment action of cer- 

 tain bacteria. This process is called denitrification, and is the 

 opposite of nitrification. The denitrifying bacteria can only act 

 on the protein substances and nitrates in the absence of oxygen. 

 They obtain oxygen from that combined in the nitrogenous sub- 

 stance which they denitrify. This is another example of anaero- 

 bic respiration. (See paragraph 191.) For the conservation of 

 this ammonia it is very important that, as it is formed, it shall be 

 converted into a more stable form which will not volatilize, and 

 which will still be available as plant food, for while it has been 

 shown that corn and a number of other plants can thrive as well 

 when fed on ammonia compounds as when fed on nitrates, in 

 practice a large part of the ammonia would be lost if it were not 

 immediately changed to nitrates (i.e., nitrified). The process by 

 which ammonia is nitrified is termed nitrification, and it is one of 

 the most important processes in nature for the nutrition of plants. 

 Nitrification is brought about by two different kinds of very 

 minute bacteria, called nitrite bacteria (Nitromonas) and nitrate 

 bacteria (Nitrobacter). These bacteria are widely distributed in 

 the soil over the earth (though not so plentiful in the forest). 

 The nitrite bacteria convert the ammonia into nitrous acid, and 

 then the nitrate bacteria convert this, into nitric acid which unites 

 with another substance * and forms nitrates. This process supplies 

 them with energy so that they are able to assimilate free oxygen 

 from the air. 



FIXATION OF NITROGEN. 



201. If the free nitrogen of the air were available as such for 

 food by all plants, one of the serious problems of the agriculturist 

 would be satisfactorily solved, and many plant and animal foods 

 * This substance is called by chemists a base. 



