Finally, the plants growing in soils are powerful agents of soil formation.  First, the roots forcing their way into the soil exert a strong pressure which helps to pulverize the soil grains; secondly, the acids of the plant roots actually dissolve the soil, and third, in the mass of decaying plants, substances are formed, among them carbon dioxid, that have the power of making soils more soluble.

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 the desired degree of filth.  Moreover, clay is very retentive of water, gases, and soluble plant-foods, which are important factors in successful agriculture.  Soils, in fact, are classified according to the amount of clay that they contain.  Hilgard suggests the following classification:—­

Very sandy soils 0.5 to 3 per cent clay
Ordinary sandy soils 3.0 to 10 per cent clay
Sandy loams 10.0 to 15 per cent clay
Clay loams 15.0 to 25 per cent clay
Clay soils 25.0 to 35 per cent clay
Heavy clay soils 35.0 per cent and over

Clay may be formed from any rock containing some form of combined silica (quartz).  Thus, granites and crystalline rocks generally, volcanic rocks, and shales will produce clay if subjected to the proper climatic conditions.  In the formation of clay, the extremely fine soil particles are attacked by the soil water and subjected to deep-going chemical changes.  In fact, clay represents the most finely pulverized and most highly decomposed and hence in a measure the most valuable portion of the soil.  In the formation of clay, water is the most active agent, and under humid conditions its formation is most rapid.