200 



SOIL PHYSICS AND MANAGEMENT 



sure is exerted in any direction. If curved the tension will cause 

 a pressure in the direction of the center of curvature and in pro- 

 portion to the radius of curvature. The pressure is equal to two 

 times the tension divided by the radius. The greater the curvature 

 the less will be the radius and consequently the greater the pressure. 

 If soil particles are in contact the water will be in two forms: 

 (1) as a film around the particles, and (2) as a waist between the 

 particles as shown in figure 90. The pressure is always in the direc- 

 tion of the center of curvature and varies inversely as the radius. 

 The pressure of the film around the soil particle is inward, while 

 that of the waist is outward. The force will then be the difference 

 between these two. As a general rule the waist film exerts the 

 greater force because it has the greater curvature or the lesser 

 radius. The pressure due to difference of curvature is well shown 

 by two soap bubbles x a and d that have a free air passage between 

 them as in figure 91. The curvature of the smaller, d, should give 



FIG. 90. Soil particles showing films 

 and waists of capillary water. 



Fia. 91. Large and small bubble con- 

 nected by a tube 6. The greater curvature 

 of d forces the air into a until the curva- 

 ture of c is the same as a. 



it greater pressure. That this is true is shown by the fact that the 

 air is forced from it into the larger bubble, <z, till the film, c, across 

 the end of the tube, has a curvature the same as that of the large 

 bubble. 



If soil particles are in contact so that the water films coalesce, 

 the films will adjust themselves so as to be in equilibrium. If, how- 

 ever, water is removed from one of the particles, as at d, the equilib- 

 rium will be destroyed, a pull will be .set up toward d, and water 

 will move from other particles until equilibrium is restored. In 

 figure 90 the film around d is thinner than at a, and this may make 

 a slight difference in the curvature of the films, hut more particu- 

 larly of the waists, the curvature being much greater between c and 

 d than between a and 6. The greater outward force at e and / 

 would draw water from the other films. 



If water is added at a the equilibrium will be destroyed and 

 readjustment will take place. The smaller the amount of water 

 present in the soil the greater will be the curvature of the films 



