. Dry-farming; . Fig. 17. Soil is a mixture of particles of very varying size. Characteristics of arid soils The net result of the soil-forming 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. Compare Fig. 17. Clay differs materially from sand and silt, not only in size of particles, but also in properties and forma- tion. It is said that clay particles reach a degre
. Dry-farming; . Fig. 17. Soil is a mixture of particles of very varying size. Characteristics of arid soils The net result of the soil-forming 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. Compare Fig. 17. Clay differs materially from sand and silt, not only in size of particles, but also in properties and forma- tion. It is said that clay particles reach a degree of fineness equal to 2^5Vo of an inch. Clay itself, when wet and kneaded, becomes plastic and adhe- sive 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 tilth. Moreover, clay is very retentive of water, gases, and soluble plant-foods, which are important
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