SOIL WATER AND SOIL AIR 



off from the roots and the plant suffers. It is largely for this 

 reason that the water of the soil in the region of the roots 

 should be capillary or film water, and not free water. 



Amount of water held by the soil. We have seen that 

 roots of plants rely on root-hairs to secure water and that the 

 water must be in the form of capillary or film water. The 

 quantity of capillary water held by the soil is important, 

 for the amount that can be removed by the plant will depend 

 upon the supply within easy reach of the roots. 



Soils differ greatly in their capacity to hold capillary water, 

 due to differences in the 

 size of soil particles. If a 

 piece of solid substance one 

 cubic inch in size is put in 

 water and then removed, a 

 certain amount of water will 

 adhere to each of its six 

 sides or surfaces. If the 

 cube be cut in half, it will 

 present two additional sur- 

 faces capable of holding a 



Diagram showing relative amount of 

 surface exposed by sand and clay. 



A. Sand. B. Clay. 



layer of water. By dividing the cube its power for holding 

 film water is increased, although the amount of solid material 

 remains the same. Each division adds more surface; the 

 greater the number of divisions, the greater the total surface 

 provided for contact with water. The total surface of all the 

 particles of a cubic foot of sandy loam has been estimated 

 at 1.39 acres; of clay, 3.54 acres. 



A certain mass of coarse particles, such as sand, will have 

 less total surface than a similar mass of silt or clay which 

 consists of finer particles. Consequently, sand will hold less 

 film water than either silt or clay. This fact is easily demon- 



