138 now onops FEEti. 



matter, and the more soluble substances and albuminoids 

 it contains, the more rapidly does it decay or humify. 



It has been shown by a chemical examination of what 

 escapes in the form of gas, as well as of what remains as 

 humus, that the carbon of wood oxidizes more slowly 

 than its hydrogen, so that humus is relatively richer in 

 carbon than the vegetable matters from which it origin- 

 ates. With imperfect access of air, carbon and hydrogen 

 are to some extent disengaged in union with each other, 

 as marsh gas (CHJ. Carbonic oxide gas (CO) is proba- 

 bly also produced in minute quantity. The nitrogen of 

 the vegetable matter is to a considerable extent liberated 

 in the free gaseous state ; a portion of it unites to hydro- 

 gen, forming ammonia (NHJ, which remains in the de- 

 caying mass ; still another portion remains in the humus 

 in combination, not as ammonia, but as an ingredient of 

 the ill-defined acid bodies which constitute the bulk of 

 humus ; finally, some of the nitrogen may be oxidized to 

 nitric acid. 



Chemical Nature of Humus. — In a subsequent chapter, 

 (p. 224,) the composition of humus will be explained at 

 length. Here we may simply mention that, under the in- 

 fluence of alkalies and ammonia, it yields one or more 

 bodies having acid characters, called humic and ulmic 

 (also geic) acids. Further, by oxidation it gives rise to 

 crenic and apocrenic acids. The former are faintly acid 

 in their properties ; the latter are more distinctly char- 

 acterized acids. 



Influence of Humus on the Minerals of the Soil.^ 



a. Disintegration of the mineral matters of soils is aided 

 by the presence of organic substances in a decaying state, in 

 so far as the latter, from their hygroscopic quality, main- 

 tain the surface of the soil in a constant state oi tnoisture. 



h. Organic matters furnish copious supplies of carbonic 

 acid, the action of which has already been considered 



