SPECIAL ASPECTS OF NUTRITION OF PLANTS 127 



the form of nitrates, while in the forest where there is much 

 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 with 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 protein 

 substances are converted into ammonia by the action of certain 

 bacteria. This process is called denitriftcation, and is the opposite 

 of nitrification. 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 (Chapter XXVII), called nitrite bacteria (Nitro- 

 monas) and nitrate bacteria (Nitrobacter). These bacteria are 

 widely distributed in the soil over the earth (though not so plenti- 

 ful 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 carbon dioxide from the air. 



203. Fixation of nitrogen. 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 for man would be greatly reduced 

 in price. In former years it was held by some scientific men, that 

 the ordinary green plants could use directly the free nitrogen of 

 the air for food. Careful experiments have demonstrated, how- 

 * This substance is called by chemists a base. 



