CHAPTER II. 



THE CHEMICAL PROCESSES OF SOIL FORMATION. 

 Chemical Disintegration, or Decomposition. 



IT may be said that in general, the physical agencies of dis- 

 integration are most intensely active in the dry or arid regions 

 of the globe, while chemical processes of decomposition are 

 most active in humid climates. 



The chemical decomposition of rocks is primarily due to the 

 action of the atmosphere, the average composition of which 

 may be stated as follows : 



I'KR ('IN r. 



WEIGHT PER Ci \ r. 



Nitrogen 



Oxygen 



Carbonic dioxid. 



Ammonia 



Water vapor.... 



oo 



2I.OO 



03-.04 



i to 4 millionths 

 Variable ; .|S to 83 grams per 

 i uhic meter, when satu- 

 rated between O and 

 50C. 



75-55 

 23.22 



.04 5- .060 



In addition to the above, air contains minute amounts of the 

 very indifferent and therefore practically negligible elements, 

 argon, krypton, neon, xenon and helium, the aggregate amount 

 of which in air is somewhat less than one per cent, of which the 

 greater part is argon. So far as known these elements take 

 no part whatever in vegetable or animal life, and possess no 

 known chemical action or affinity. 



The primary active agents in effecting chemical changes 

 in rocks by which soils are formed, are water, carbonic acid. 1 

 and oxygen ; all therefore ingredients of the atmosphere. 

 Hence the chemical changes so brought alxmt are in the most 

 general sense comprehended within the term weathering, as 



1 Owing to the universal presence of water (M 3 O) in air as well as in soils, it is 

 usual and convenient to speak of carbonic dioxid (CO?) gas when so occurring as 

 carbonic acid (HaCOs), of which it produces the effects 



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