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 



