DRY-LAND AGRICULTURE 



243 



the range of capillary action. Therefore loss by percolation is in- 

 significant. Percolation into the upper soil layers must take place 

 very rapidly to check complete saturation of the surface soil, he- 

 cause this would result in more or less run-off. For this purpose 

 the looser the soil the hotter. 



Fia. 106. A deep, medium-grained soil well adapted to dry funning. Utah 

 Farming, Widtsoc. Courtesy MacmillaD Company.) 



(Drv 



(c) Evaporation. Arid conditions arc very well adapted to 

 evaporation of water from the soil surface, due to the very low rela- 

 tive humidity, the rarity of the atmosphere and the large air move- 

 ment taking place in arid regions. This is the most serious source 

 of loss. At Salt Lake City the relative humidity in summer is about 

 35 per cent, while in humid regions the average is from ?"> to 80 



