SOIL AS WATER RESERVOIR 25 



pull the moisture downward, while the attraction between 

 the soil and the water tends to hold it as a film around the 

 soil grains. The actual movement of a particle of water 

 in a soil is a resultant of these forces. 



In general, water moves from the thicker to the 

 thinner soil film. Immediately after an irrigation, when 

 the upper soil layers are wettest, the water moves down- 

 ward; immediately before an irrigation, when the plants 

 have largely exhausted the upper soil layers of water, 

 the soil water is moving slowly upward. The down- 

 ward movement, aided by gravity, is more rapid than 

 the upward movement against gravity. The film of soil 

 moisture is usually in a state of motion, attempting to 

 place itself in equilibrium with the many contending 

 forces in the soil. As examples, the moisture in the soil 

 moves in all directions toward a point at which a root- 

 hair is absorbing water; and, as evaporation occurs at 

 the soil surface, there is a general upward movement of 

 the soil moisture to supply the loss. 



The drier the soil, the slower does the soil moisture 

 move. Under the point of lento-capillarity, soil-mois- 

 ture movements occur with great difficulty; above this 

 point, they occur with great freedom. One proof of this 

 is that at depths of 8 to 10 feet, where plant roots pene- 

 trate in small numbers only, the moisture is seldom 

 reduced below the lento-capillary point, while nearer 

 the surface and abundant root-action the moisture is 

 often reduced to the wilting coefficient. 



23. The distribution of soil moisture. After a sur- 

 face irrigation of a soil with a water content near the 

 lento-capillary point, the upper soil layers are invariably 

 wetter than the lower ones. Any deviation from this rule 

 is only apparent and is due to the fact that the subsoil 



