THE SOIL 49 



give up a little water to a dry atmosphere by evaporation. 

 When water no longer evaporates from a soil, it is said to be 

 air dry, and the water remaining is called hygroscopic water, 

 most of which can be removed by heating the soil. 



The following simple experiment tests the water-holding 

 powers of soil. Pulverize a quantity of soil and dry it for 

 several days in the hot summer sunshine or in an oven at 

 about boiling temperature for a day. Gently tamp two 

 pounds of the dried soil in a tin can, of about one quart 

 capacity, with a few perforations in the bottom. Slowly add 

 water, an ounce at a time, until it is well soaked and the ex- 

 cess water drips from the can. The percentage of water held 

 by the dried soil is known as the ''water-holding capacity" 

 of the soil. A good soil should hold 50 to 100 per cent of its 

 weight in water. The water that was not held by the capil- 

 lary forces of the soil, but passed through the perforations, 

 was the gravitational water. 



The movement of the water in the soil depends on the 

 fact that it is in continuous films from soil particle to soil 

 particle and is held to the soil particles with a force propor- 

 tional to the thickness of the film. If water enters a root 

 hair, the film at that point becomes thinner and the pulling 

 force of the soil increases, which causes the water to move in 

 that direction. This is the movement of capillary water and 

 it is similar to capillary movement in a finely drawn glass 

 tube or even the slight curve of the water at the surface on 

 the sides of the tumbler at the dinner table. The reverse 

 holds true; when water is added to the soil, the films become 

 thicker and the water moves by capillarity to where they are 

 thinner. This movement is very slow as can be seen by 

 digging into the soil after a rain. Since some force is neces- 

 sary to cause the water to move through the soil there is a 

 tendency for it to accumulate near the surface after a rain, 

 by increasing the thickness of the films, instead of becoming 

 equally distributed on all the soil particles even at greater 



