56 



DRY-FARMING 



in this connection that climatic chfferences 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 njck 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 



Fig. 17. Soil is a mixture of particles of tllCV do UOt Settle 

 verv varj'iDg size. ^ ' . ^ ^ 



from cjuiet water 

 after 24 hours are known as clay. Compare Fig. 17. 

 Clay differs materially from sand and silt, not only 

 in size of i3articles, but also in properties and forma- 

 tion. It is said that clay ])articles reach a degree 

 of fineness eciual to ^'ioo "f ^^i^ inch. Clay itself, 

 when wet and kneaded, becomes plastic and adlie- 

 sive and is thus easily distinguished from sand. 

 Because of these projjerties, clay is of great value 

 in holding together the larger soil grains in relatively 

 large aggregates which give soils the desired degree 

 of tilth. ^loreover, clay is very retentive of water, 

 gases, and soluble plant-hiods, which are important 



