54 DRY-FARMING 
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 impurity, considering 
soil formation, is the gas, carbon dioxid. This gas 
is formed whenever plant or animal substances 
decay, and is therefore found, normally, 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 
