CONDITIONS OF EVAPORATION 149 
by the plow, and the surfaces of which are sun-baked, 
are often found to possess large percentages of water 
at lower depths. 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 con- 
ditions, 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 experimen- 
tal 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 ap- 
pears ‘‘that under very arid conditions a soil auto- 
matically protects itself from drying by the forma- 
tion 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. An ad- 
mixture of limestone seems to favor the formation of 
such a natural protective mulch. Ordinarily, the 
farmer can further the formation of a dry topsoil 
layer by stirring -the soil thoroughly. This assists 
