Whitney recorded this observation with considerable
surprise, many years ago, and other observers have found the same
conditions at nearly all points of the arid region. This matter has
been subjected to further study by Buckingham, who placed a variety
of soils under artificially arid and humid conditions. It was found
in every case that, the initial evaporation was greater under arid
conditions, but as the process went on and the topsoil of the arid
soil became dry, more water was lost under humid conditions. For the
whole experimental period, also, more water was lost under humid
conditions. It was notable that the dry protective layer was formed
more slowly on alkali soils, which would point to the inadvisability
of using alkali lands for dry-farm purposes. All in all, however, it
appears "that under very arid conditions a soil automatically
protects itself from drying by the formation of a natural mulch on
the surface."
Naturally, dry-farm soils differ greatly in their power of forming
such a mulch. A heavy clay or a light sandy soil appears to have
less power of such automatic protection than a loamy soil.
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