SAVING WATER BY CULTIVATION 43 



supply that lost by evaporation at the top and to place 

 the remaining water in full equilibrium with all active 

 forces. Such loss of water is felt to the full depth of soil 

 concerned in plant-growth. 



As evaporation proceeds from the top soil, the water 

 in every soil layer diminishes to the full depth of root- 

 action. The process may be likened roughly to the behav- 

 ior of cotton packed loosely in a box. If a small quantity 

 is removed from the top, the remainder expands and 

 fills the box again, the difference being that the whole 

 mass is looser from the top downward than it was before. 

 So, after evaporation has occurred, and water has moved 

 upward to replace the loss, there is a thinner soil-water 

 film throughout the soil. This process may go on until 

 the soil-water film has been reduced to the minimum 

 thickness that allows capillary movement. When this 

 degree of dryness has been reached, it does not follow that 

 the film is of the same thickness at every point to the full 

 depth involved. On the contrary, the lower layers, to a 

 depth of 8 to 10 feet, contain more water than do the 

 upper soil layers. At first, as evaporation proceeds, the 

 tendency is to distribute the water evenly over the soil 

 sections below the upper one, which is immediately 

 exposed to the atmosphere and therefore always drier. 

 As the lento-capillary point is approached, the upward 

 movement becomes more and more sluggish; and, in 

 fact, it is ordinarily very difficult to reduce the lower 

 soil layers below this point, though the upper layers may 

 be brought considerably lower in their moisture content. 

 When living plant-roots fill the soil, this distribution 

 does not hold, for the roots draw moisture directly from 

 the soil, and the percentage of soil moisture is in inverse 

 proportion to the distribution of the roots. 



