THE FORMS OF SOIL WATER 223 



two conditions : (1) if the soil column is in contact with 

 free water; and (2) if no gravity water is present, the 

 movement being merely from a moist soil to a drier one, 

 an inexhaustible supply of water not being present. In 

 the first case the lower portion of the soil is entirely 

 saturated for a short distance above the free water sur- 

 face, due to the functioning of the pore spaces as true 

 capillary tubes; above this the film movement becomes 

 dominant. The second condition of capillary adjustment 

 is the one most commonly found in a normal soil, since a 

 water table a short distance below the surface is not 

 usually conducive to the best crop growth. In studying 

 the rate and height of capillary rise in any soil, however, 

 the maintenance of a supply of free water at the lower end 

 of the column is usually provided for, since this allows a 

 near approach to the maximum capillary capacity for any 

 point in the column. 



151. Factors affecting rate and height of capillary 

 movement. To persons familiar with the habits of grow- 

 ing plants it is evident that capillary movement must 

 play an important part in their nutrition, since the root- 

 lets are unable to bring their absorptive surfaces in con- 

 tact with all the interstitial spaces where the bulk of the 

 available water is held. Consequently a consideration 

 of the movement of capillary moisture is necessary, not 

 only as to its mechanics, but also as to the factors influ- 

 encing its rate and height of movement. These factors 

 are four in number: (1) thickness of water film; (2) sur- 

 face tension ; (3) texture ; and (4) structure. 



152. Effect of thickness of water film on capillary 

 movement. It has been repeatedly noticed, in the 

 study of the capillary adjustment between two soils, that 

 the lower the percentage of water, the slower is the rate 



