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 
Clayloams .. . . . . . 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 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 pur- 
poses. The fact is, that heavy clay soils are very 
rare in arid regions; if found at all, they have prob- 
