170 MANURES. 



that which is occupied in the production of food for 

 man. 



If these gases were not manures ; if there were no 

 means by which they could be used by plants, the 

 fertility of the soil would long since have ceased, and 

 the earth would be unfertile. That this must be 

 true, will be shown by a few moments' reflection on 

 the facts stated in the first part of this book. The 

 fertilizing gases in the atmosphere being composed 

 of the constituents of decayed plants and animals, it 

 is as necessary that they should be again returned to 

 the form of organized matter, as it is that constitu- 

 ents taken from the soil should not be put out of 

 existence. 



AMMONIA. 



The ammonia in the atmosphere probably cannot 

 be appropriated by the leaves of plants, and must, 

 therefore, enter the soil to be assimilated by roots. 

 It reaches the soil in two ways. It is either arrested 

 from the air circulating through the soil, or it is ab- 

 sorbed by rains in the atmosphere, and thus carried 

 to the earth, where it is retained by its clay and car- 

 bon, for the uses of plants. In the soil, ammonia is 

 the most important of all organic manures. In fact, 

 the value of the organic parts of manure may be 

 estimated, either by the amount of ammonia which 

 they will yield, or by their power of absorbing am- 

 monia from other sources. 



The most important use of ammonia in the soil is 



