56 DRY-FARMING 
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 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 gx59 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 
very varying size. 
