THE CHEMICAL PROCESSES OF SOIL FORMATION, 2 I 



efficient in promoting the transformation of mere rock powder 

 into soil material proper, the efficacy does not end with the 

 life of the plant. In the natural process of decay to which the 

 roots are subject after death, and which also affects the 

 leaves, twigs and trunks falling on the surface, the vegetable 

 matter suffers a transformation which must be considered more 

 in detail hereafter, and results in the formation of the com- 

 plex mixture of dark-tinted substances known as vegetable 

 mold or humus; the remnant of vegetation that imparts to 

 surface soils their distinctive dark tint. Its functions in soils 

 are both numerous, and important to vegetable growth ; as re- 

 gards soil formation, it assists disintegration of the rock min- 

 erals both by the formation of certain fixed, soluble acids ca- 

 pable of acting on them with considerable energy, and by the 

 slow but continuous evolution of carbonic acid under the in- 

 fluence of atmospheric oxygen, which has been alluded to 

 above. 



Causes influencing chemical action and decomposition. The 

 chemical processes causing rock decomposition are of course 

 continued in the soil, and there also are materially influenced 

 by climatic and seasonal conditions, which bring about great 

 differences in the kind and intensity of chemical action. 



Within the ordinary limits of solar temperatures it may be 

 said that, other things being equal, the higher the temperature 

 the more intense will be chemical action in soil formation. 

 Since, however, water is a potent factor in the majority of 

 these processes, the presence or absence of moisture at the same 

 time with heat will cause material differences in the kind and 

 intensity of chemical action. In view of the importance 

 of carbonic acid as a chemical agent, the presence or absence 

 of vegetable matter or humus, from which by oxidation or 

 decay carbonic and humus-acids are formed, will likewise be 

 of material influence. 



The presumption that climatic and seasonal conditions must 

 greatly influence both the kind and rapidity of the soil-form- 

 ing processes, is fully borne out by observation and practice. 

 Especially is the amount and distribution of rainfall of great 

 importance in this respect, and should therefore be first con- 

 sidered. 



