CAPILLARY MOVEMENT AND STRUCTURE 183 



The stronger such a membrane, the larger the pull it can 

 exert under a given strain. Consequently, in a soil of 

 uniform texture, and in moisture equilibrium, any- 

 thing which changes the surface tension may set up 

 motion of the soil water. The introduction of fertilizers 

 may set up such a movement, and this addition to a soil 

 may enable it to draw and permanently retain more 

 water than the adjacent soil of same texture. Appli- 

 cations of magnesium chloride, salt and muriate of 

 potash, are observed to keep the soil more moist in dry 

 weather, and a similar effect of some alkali salts has 

 been noted. These materials all raise the surface tension. 

 High temperature reduces the surface tension, and there- 

 fore, in a soil in moisture equilibrium, if one part, as 

 the surface, is heated, the water will be drawn away 

 from that region to the cooler zone, where the tension 

 is higher. 



88. Condition of surfaces of particles. The condition 

 of the surface of the soil particles affects the tenacity 

 with which water adheres to them. The application 

 of oil to a soil tends to destroy its capillary capacity; 

 and any substance in the soil which will bring about 

 such a condition reduces the capillary efficiency of the 

 soil. 



The action of capillarity is not limited to any one 

 direction. It may take place in any direction. It has 

 usually been measured vertically upward. But it oper- 

 ates vertically downward, as well, and it moves water 

 horizontally. The vertical upward movement of capillary 

 water is modified by the influence of gravity, as is capil- 

 lary retention. (See page 149.) 



