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 alkah 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 dryang 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 



