170 NATURE AND PROPERTIES OF SOILS 



laboratory, the maintenance of a supply of free water is 

 usually provided for, since this allows a nearer approach to 

 the maximum capillary capacity for any point in the column 

 and also gives the most rapid capillary adjustment. 



To persons familiar with the habits of growing plants, it is 

 evident that capillary movement must play an important 

 part in their nutrition, since the rootlets are unable to bring 

 their absorptive surfaces in contact with all the interstitial 

 spaces in which the bulk of the available water is held. Con- 

 sequently a consideration of the movement of capillary mois- 



FiG. 31. — Conventional diagram showing the mechanics of the movement 

 of the film portion of the capillary water. The readjustment takes 

 place in the direction of (A) due to the tension developed by the 

 greater film curvature at that point. 



ture is necessary, not only as to its mechanics,- but also in 

 respect to the factors influencing its rate and height of move- 

 ment. These factors are as follows: (1) surface tension and 

 viscosity; (2) thickness of capillary film; (3) texture; and 

 (4) structure. 



Surface tension and viscosity. — As the force developed by 

 surface tension is the activating factor in capillary adjust- 

 ment, any change in the former will influence this movement. 

 Theoretically, a rise in temperature or the presence of soluble 

 salts would decrease the rapidity of the capillary activity of 

 soil-water. In a normal soil, however, the change of surface 

 tension is generally not sufficient to have any very great prac- 

 tical influence. Viscosity, on the other hand, is much more 

 important. If the viscosity of water at 0° C. is taken as 100, 



