SOIL WATER 89 



throughout its bulk. But this result is never 

 brought to pass; it is prevented by the frequent 

 downward passage of water, constant evaporation 

 from the surface and continued absorption by 

 roots. 



We are not concerned about checking the current 

 of film moisture toward the roots, except to in- 

 crease it. Usually the larger the loss of film water 

 in this way, the greater the gain to the farmer. 

 But we are greatly concerned about the current 

 of film water that is passing upward to the surface 

 of the soil and is then lost in the air as water vapour. 

 We cannot afford to lose this water; and we can- 

 not afford to lose, even temporarily, the plant food 

 that is dissolved in it. When the water evaporates, 

 this is left upon the surface of the soil where it is 

 useless to plants, until washed down into the root- 

 feeding area. We would rather have the water 

 evaporate, not from the soil, but through the leaves 

 of crops, after it has given to the plants the food 

 that it contains. 



The sun is the mightiest of pumps. The 

 amount of water that is evaporated from the soil in 

 one summer day is astonishing even to those who 

 have observed how quickly the soil becomes dry 

 in midsummer after a heavy rain. King found 

 that each square foot of an ordinary farm soil 

 lost 1.3 pounds of water daily by evaporation 

 from the surface. 



Capillary Action. The movement of film water 

 in the soil is frequently called "capillary action." 

 The soil being made of millions of tiny grains, 

 there are likewise millions of tiny spaces between 

 the grains, as in a pile of wheat; so it follows that 

 there is a more or less continuous passage from one 

 space to another, making many small and very 



