CAPILLARY WATER 205 



is of greater importance than an increased tension, at least within 

 the limits of the experiment. While an increase in viscosity gives 

 greater surface tension, the rate of movement is decreased becau.se 

 of greater resistance. Under higher teni}>eratures the capillary 

 limit would be reached sooner, yet the height attained would be 

 less than at lower temperatures. 



Somewhat dry soils frequently become moist in late autumn 

 without rain. This is probably duo to two things less evap- 

 oration due to lowering of temperature and increased capillary pull 

 at the surface due to greater surface tension of the water at this 

 temperature. 



(b) Substances in Solution. The viscosity and likewise the 

 surface tension of water are affected more or less by substances in 

 solution. Some ' increase, while others decrease tension. The 

 height to which liquids rise in soils varies with the surface tension, 

 the densities of the liquids being the same. The rapidity of rise 

 of those liquids in the same soil depends upon the viscosity : the 

 more viscid the liquid the slower the movement. Mineral sub- 

 stances added to a soil generally increase the surface tension. If 

 potassium chloride is added to the surface of a soil the capillary 

 pull of the surface moisture will be increased, and more water will 

 be brought up from the subsoil. This is true of most mineral 

 fertilisers. 



Organic substances lower the surface tension so that they would 

 not cause so great capillary pull, but would increase the rapidity of 

 movement. Soil solutions have a low surface tension. 



Rains wetting a few inches in depth have a tendency to draw 

 water up from the deeper soil, as observed by King.-"' He found 

 that .'}."> pounds per square foot in one trial and .'5.0!) in another 

 had been transferred from the lower soil '2\ to 18 inches to the upper 

 in 20 hours after wetting. This is due in part to the fact that the 

 surface tension of rain water is greater than that of soil solutions. 



From the next table it will be seen that common salt gives the 

 greatest tension and that soil solutions are low. If any of the 

 chemical substances mentioned in the table should be applied to 

 the surface of the soil when it passed into solution, the tension 

 would be increased sufficiently to draw up water from below. Some 

 recent experiments by Karraker * indicate that substances in solu- 

 tion play little part, in moisture movement. The strengths of the 

 solutions in the table are much greater than probably ever occur 

 in soils, unless it. should be in the immediate surface just after an 



