54 DRY-FAEMING 



dry-farm districts. Constant heavy winds, which 

 as soil formers are second only to temperature 

 changes and freezing water, are also usually more 

 common in arid than in humid countries. This is 

 strikingly shown, for instance, on the Colorado 

 desert and the Great Plains. 



The rock powder formed by the processes above 

 described is continually being acted upon by agencies, 

 the effect of which is to change its chemical compo- 

 sition. Chief of these agencies is water, which exerts 

 a solvent action on all known substances. Pure 

 water exerts a strong solvent action, but when it 

 has been rendered impure by a variety of substances, 

 naturally occurring, its solvent action is greatly 

 increased. 



The most effective water im]3urity, considering 

 soil formation, is the gas, carbon dio.rid. This gas 

 is formed whenever plant or animal substances 

 decay, and is therefore found, normall}^ in the 

 atmosphere and in soils. Rains or flowing water 

 gather the carbon dioxid from the atmosphere and 

 the soil; few natural waters are free from it. The 

 hardest rock particles are disintegrated by carbon- 

 ated water, while limestones, or rocks containing 

 lime, are readily dissolved. 



The result of the action of carbonated water upon 

 soil particles is to render soluble, and therefore more 

 available to plants, many of the important plant- 

 foods. In this way the action of water, holding in 



