EVIDENCE OF HISTORY 247 



been taken from the soil by the combination 

 of cropping and drainage. 



Extend this calculation not over four years 

 or forty years, but over four hundred or four 

 thousand years and one's amazement is that 

 we find any minerals left in the soil at all. 

 Leaching has been going on for ages, in vir- 

 gin prairies as well as on cultivated land. So 

 even gravitational water, with its low content 

 of plant food, becomes an enormous item in 

 time, if we subscribe to the bookkeeping 

 theory. 



Gravitational water becomes capillary water 

 when evaporation begins at the surface. It 

 begins an upward journey. It does not travel 

 now by the broad highways that hurried it 

 downward when the upper soil was too satu- 

 rated to hold it. 



Instead, it moves from grain to grain of soil, 

 picking up its ration of solutes from the film 

 water surrounding each grain as it passes. Its 

 mode of passage can best be illustrated by 

 water flowing across a screen door. This 

 water, as we frequently have opportunity of 

 observing, moves in squares, blocking off the 

 meshes of the screen. So the capillary water 

 deep in the earth moves among the interstices 



