58 DRY-FARMING 
ably been formed under abnormal conditions, as in 
high mountain valleys, or under prehistoric humid 
climates. 
Sand.— The sand-forming rocks that are not 
capable of clay production usually consist of uncom- 
bined silica or quartz, which when pulverized by the 
soil-forming agencies give a comparatively barren 
soil. Thus it has come about that ordinarily a clayey 
soil is considered ‘‘strong” and a sandy soil ‘‘ weak.” 
Though this distinction is true in humid climates, 
where clay formation is rapid, it is not true in arid 
climates, where true clay is formed very slowly. 
Under conditions of deficient rainfall, soils are nat- 
urally less clayey, but as the sand and silt particles 
are produced from rocks which under humid condi- 
tions would yield clay, arid soils are not necessarily 
less fertile. 
Experiment has shown that the fertility in the 
sandy soils of arid sections is as large and as available 
. 4 ‘ i ene 
to plants as in the clayey soils of humid regions. 
Experience in the arid section of America, in Egypt, 
India, and other desert-like regions has further 
proved that the sands of the deserts produce excel- 
lent crops whenever water is applied to them. The 
prospective dry-farmer, therefore, need not be afraid 
of a somewhat sandy soil, provided it has been formed 
under arid conditions. In truth, a degree of sandi- 
ness is characteristic of dry-farm soils. 
The humus content forms another characteristic 
