In places, glaciers moving slowly down the canons crush and grind into powder the rock over which they pass and deposit it lower down as soils.  In other places, where strong winds blow with frequent regularity, sharp soil grains are picked up by the air and hurled against the rocks, which, under this action, are carved into fantastic forms.  In still other places, the strong winds carry soil over long distances to be mixed with other soils.  Finally, on the seashore the great waves dashing against the rocks of the coast line, and rolling the mass of pebbles back and forth, break and pulverize the rock until soil is formed._ Glaciers, winds, and waves are also, therefore, physical agencies of soil formation.

It may be noted that the result of the action of all these agencies is to form a rock powder, each particle of which preserves the composition that it had while it was a constituent part of the rock.  It may further be noted that the chief of these soil-forming agencies act more vigorously in arid than in humid sections.  Under the cloudless sky and dry atmosphere of regions of limited rainfall, the daily and seasonal temperature changes are much greater than in sections of greater rainfall.  Consequently the pulverization of rocks goes on most rapidly in 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 composition.  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 carbonated 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 solution carbon dioxid and other substances, tends to make the soil more fertile.

The second great chemical agency of soil formation is the oxygen of the air.  Oxidation is a process of more or less rapid burning, which tends to accelerate the disintegration of rocks.