SOIL-FORMING PROCESSES 21 



water. There is also a considerable increase in bulk, this 

 being often as much as 88 per cent during the transition 

 of a rock to a soil. Hydrated minerals, while apparently 

 sound, quickly succumb when exposed to forces of weather- 

 ing which are more superficial in their effects. Car- 

 bonation and oxidation usually take place as correla- 

 tive actions with hydration. A simple example of 

 hydration is shown in the change of hematite to limo- 

 nite, which occurs in practically every case when iron 

 is allowed to oxidize from pyrite or a simpler oxide to 

 the higher forms : — 



2 Fe 2 3 (red) + 3 H 2 = 2 Fe 2 3 . 3 H 2 (yellow) 



19. Solution. — As it is now quite evident that weather- 

 ing, especially the chemical manifestations, is largely a 

 simplification of compounds, and that water is almost 

 universally present, some solution must occur. These 

 simple materials are particularly prone to enter solution 

 because of the presence of carbon dioxide, which, by 

 acidifying the soil water, intensifies its solvent action to 

 a considerable extent and consequently increases its power 

 as a weathering agent. The atmosphere contains amounts 

 of this gas ranging from 3.87 to 4.48 parts in 10,000, 

 while considerable amounts are brought down on the 

 rocks and the soil in snow and rain. The carbon dioxide 

 evolved directly into the soil water from decaying organic 

 matter also aids in keeping the soil charged with this gas. 

 This means, then, that solution is largely a process of 

 carbonation, especially after the soluble constituents have 

 been thrown out into the soil solution. It is evident 

 that oxidation, carbonation, hydration, and solution act 

 in unison to bring about the chemical decay of the rock 

 and the soil. This combined action may be represented 



