146 THE PRINCIPLES OF SOIL MANAGEMENT 



It must be remembered that the hygroscopic ca- 

 pacity of these soils also increases with their fineness, 

 and that the strictly capillary moisture is represented 

 by the difference between the total moisture content 

 given above and the hygroscopic moisture. 



The above figures are the most exact available 

 which show the influence of texture upon moisture 

 retention. But, while they show the relative effect of 

 texture, they do not indicate the amount of water 

 retained by field soils; because these samples have 

 been subject to a force almost 3,000 times that of 

 gravity. When under the influence of gravity alone, 

 these same soils will retain much more water than is 

 indicated by the figures. However, this influence of 

 gravity introduces a modification in the moisture content 

 of the soil which must be constantly kept in mind. 

 Moisture is retained in the soil as a result of two sets 

 of forces. These are, first, the attraction of the soil 

 for water, or adhesion. For example, if a marble is 

 dipped into water and withdrawn, it carries with it 

 a film of water over its entire surface. This shows that 

 for a certain small distance from its surface, the marble 

 exerts a stronger pull on the water than the water 

 exerts for itself. If the marble were dipped into mercury 

 instead of water, it would come out with a dry surface, 

 because in this case the attraction of the mercury for its 

 own substance is greater than the attraction of the 

 marble for the mercury. Quinke estimates the appreci- 

 able range of this attraction to be approximately .002 

 millimeter, which, it will be remembered, is equivalent 

 to the diameter of a medium-sized clay particle. Its 



