It may be noted that moisture, carbon dioxid, and vegetation, the
three chief agents inducing chemical changes in soils, are most
active in humid districts. While, therefore, the physical agencies
of soil formation are most active in arid climates, the same cannot
be said of the chemical agencies. However, whether in arid or humid
climates, the processes of soil formation, above outlined, are
essentially those of the "fallow" or resting-period given to
dry-farm lands. The fallow lasts for a few months or a year, while
the process of soil formation is always going on and has gone on for
ages; the result, in quality though not in quantity, is the
same--the rock particles are pulverized and the plant-foods are
liberated. It must be remembered in this connection that climatic
differences may and usually do influence materially the character of
soils formed from one and the same kind of rock.

Characteristics of arid soils

The net result of the processes above described Is a rock powder
containing a great variety of sizes of soil grains intermingled with
clay. The larger soil grains are called sand; the smaller, silt, and
those that are so small that they do not settle from quiet water
after 24 hours are known as clay.

Clay differs materially from sand and silt, not only in size of
particles, but also in properties and formation. It is said that
clay particles reach a degree of fineness equal to 1/2500 of an
inch. Clay itself, when wet and kneaded, becomes plastic and
adhesive and is thus easily distinguished from sand. Because of
these properties, clay is of great value in holding together the
larger soil grains in relatively large aggregates which give soils