NATURE OF DRY-FARM SOILS 57 



factors in successful agriculture. Soils, in fact, are 

 classified according to the amount of clay that they 

 contain. Hilgard suggests the following classifi- 

 cation : — 



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 re]3resents the most finely pulverized 

 and most highly decomposed and hence in a measure 

 the most valuable portion of the soil. In the forma- 

 tion of clay, water is the most active agent, and under 

 humid conditions its formation is most rapid. 



It follows that dry-farm soils formed under a 

 more or less rainless climate contain less clay than 

 do humid soils. This difference is characteristic, 

 and accounts for the statement frequently made that 

 heavy clay soils are not the best for dry-farm ]3ur- 

 poses. The fact is, that heavy clay soils are very 

 rare in arid regions; if found at all, tliey have prob- 



