156 AMMONIA IN THE AIR HOW DECOMPOSED. 



ing; we have, however, no satisfactory evidence either from theory or 

 experiment that it has undergone any sensible diminution in our time.* 



§ 4. Of the supply of ammonia to plants. 



In a previous lecture it has been shown that in our cultivated fields 

 plants derive a portion of their nitrogen from the manure which is added 

 to the soil. But the quantity of this element present in the manure, 

 supposing it all taken up and appropriated by the plant, is seldom equal 

 to that contained in the series of crops which this manure assists in raising. 



Thus, in the experiments of Boussingault already described (p. 144), 

 the manure added previous to the first, or four years' course, contained 



157 parts of nitrogen, while the crops contained 251 parts, — or nearly 

 two-thirds more than could be derived from the artificial manure. 



Whence is this excess of nitrogen derived, and in what form does it 

 enter into the plant? Liebig replies to these questions, that the whole 

 of the nitrogen absorbed by plants enters in the state of ammonia, and 

 that the excess above what is present in the manure is drawn either 

 from the soil or from the air. This opinion, advanced by so high an 

 authority, demands our attentive consideration. 



Ammonia has been detected in many clays, and traces of it may be 

 discovered in most soils, but it is not known to be a natural or essential 

 constituent of any of the solid rocks of which the crust of the globe is 

 composed. These clays and soils, therefore, may be supposed to have 

 derived their ammonia from the atmosphere ; and Liebig ascribes the 

 fertilizing action of the air upon stiffclays when fallowed, of burned clay 

 when applied as a top-dressing, and of gypsum on grass lands [see note 

 to page 53], to the larger quantity of ammonia which the surface of the 

 soil is by these means caused to absorb and retain. 



There is no question that ammonia is present in the atmosphere in 

 small and variable quantity (p. 37). Whence is this ammonia derived, 

 and is its quantity sufiicient to supply the demands of the entire vegeta- 

 tion of the globe ? 



When animal substances undergo decay, nearly all the nitrogen they 

 contain is ultimately separated from the other constituents in the form of 

 ammonia. During the decay of plants also, a portion of their nitrogen 

 escapes in the state of ammonia. Of the ammonia thus formed, much 

 ascends into the air, chiefly in combination with carbonic acid as carbonate 

 of ammonia (smelling salts), and much remains in the soil. Were the 

 whole of the nitrogen contained in plants and animals to assume the" 

 form of ammonia when they decay, and to remain in the soil or in the 

 air, it would always be within the reach either of the roots or leaves of 

 the living races; and thus the same ammonia [or ammonia containing 

 the same nitrogen — supposing the hydrogen to have been changed] 

 might again and again return into the circulation of new vegetable tribes, 

 and be always alone sufficient to supply all the demands of the exist- 

 ing vegetation of the globe. 



But of the ammonia thus formed, a portion is daily washed from the 

 soil by the rains and carried to the sea, and much more probably is 



In another work {Chemical Geology) now preparing for publication, I have discussed 

 this question in connection with purely Geological considerations and without reference to 

 oiir time ; but it would be out of place to introduce here any train of reasoning which is not 

 calculated to throw light on the phenomena of the existing vegetation ol the globe. 



