410 ANALOGOUS DECOMPOSITION OF ALL ORGANIC SUBSTANCES. 



ammonia or of nitric acid. What takes place on the confined area of a 

 nitre bed, may take place to some extent also in the wider area of a 

 well-limed and well-manured field. 



In the action of alkalies in the nitre bed, disposing to the produc- 

 tion of nitric acid, we observe the same kind of agency, which we 

 have already attributed to lime, in regard to the more abundant ele- 

 ments which exist in the vegetable matter of the soil. It gently per- 

 suades ah the elements — nitrogen and carbon alike — to unite with 

 the oxygen of air and water, and thus ultimately to form acid com- 

 pounds with which it may itself combine. 



The action of hme upon such organic matters containing nitrogen as 

 usually exist in the soil, may, therefore, be briefly stated as follows : — 



1^. These substances, like all other organic matter, undergo in moist 

 air — and, therefore, in the soil — a spontaneous decomposition, the ge- 

 neral result of which is the production of ammonia, and of an acid 

 substance with which the ammonia may combine. This change is 

 precisely analogous to that which takes place in such substances as 

 starch and woody fibre, which contains no nitrogen. In each case, 

 one portion of the elements unites with oxygen to produce an acid, 

 the other with hydrogen to form a compound possessed of alkaline or 

 indifferent properties. Thus, — 



With oxygen,— vegetable matter produces carbonic, ulmic, and other 

 acids. 

 " animal matter produces carbonic, nitric, ulmic, and 



other acids. 

 With hydrogen, — vegetable matter produces marsh gas or other 

 carburetted hydrogens, 

 " animal matter produces ammonia. 



If the ammonia happen to be produced in larger relative quantity 

 than the acids with which it is to combine, or if the carbonic be the 

 only acid with which it unites, a portion of it may escape into the air. 

 This rarely happens, however, in the soil, the absorbent properties of 

 the earthy matters of which it consists being in most cases sufficient 

 to retain the ammonia, till it can be made available to the purposes 

 of vegetable life. 



When caustic (hydrate of) lime is added to a soil in which ammonia 

 exists in this state of combination with acid matter, it seizes upon the 

 acid and sets the ammonia free. This it does with comparative slow- 

 ness, however — for it does not at once come in contact with it all — 

 and by degrees, so as to store it up in the pores of the soil till the roots 

 of plants can reach it, or till it can itself undergo a further change 

 by which its nitrogen may be rendered more fixed (p. 411.) 



Carbonate of lime, on the other hand, still more slowly persuades 

 the ammonia to leave the acid substances (ulmic, nitric? &c.,) with 

 which it is combined, and yielding to it in return its own carbonic 

 acid, enables it in the state of soluble carbonate of ammonia to be- 

 come more immediately useful to vegetation. 



2°. But in undergoing this spontaneous decay, even substances con- 

 taining nitrogen reach at length a point at which decomposition appears 

 to stop — an inert condition in which, though nitrogen be present as in 

 peat, they cease sensibly to give it off in such a form or quantity as to 



