SAVING WATER BY CULTIVATION 



51 



at any depth, water is evaporated, until, if the soil is 

 moist, the pores of the soil are filled with air saturated 

 with vapor water. This saturated soil air moves, how- 

 ever, very slowly into the atmosphere. Buckingham 

 has shown that water vapor escapes from the soil air 

 only by the slow process of diffusion; that is, the particles 

 of water vapor find their way, one by one, into the atmos- 

 phere, while there is a corresponding movement of 

 atmospheric gases into the soil air. This interchange of 

 gases between the soil air and the atmosphere is so small 

 as to be of little or no consequence in the loss of soil water 



Fia. 12. Evaporation losses from soils protected with mulches of different depths. 



by evaporation. Practically all soil water is lost by evap- 

 oration at the soil surface. 



Buckingham has shown, also, that very little water 

 is lost by direct evaporation 2 inches below the surface. 

 From below a 12-inch layer of dry soil the evaporation- 



