106 DRY-FARMING 
Capillary soil-water 
The so-called capillary soil-water is of greatest 
importance to the dry-farmer. This is the water that 
clings as a film around a marble that has been dipped 
into water. There is a natural attraction between 
water and nearly all known substances, as is witnessed 
by the fact that nearly all things may be moistened. 
The water is held around the marble because the 
attraction between the marble and the water is 
greater than the pull of gravity upon the water. 
The greater the attraction, the thicker the film; 
the smaller the attraction, the thinner the film will 
be. The water that rises in a capillary glass tube 
when placed in water does so by virtue of the 
attraction between water and glass. Frequently, 
the force that makes capillary water possible is 
called surface tension (Fig. 28). 
Whenever there is a sufficient amount of water 
available, a thin film of water is found around every 
soil grain; and where the soil grains touch, or where 
they are very near together, water is held pretty 
much as in capillary tubes. Not only are the soil 
particles enveloped by such a film, but the plant 
roots foraging in the soil are likewise covered; that 
is, the whole system of soil grains and roots is 
covered, under favorable conditions, with a thin 
film of capillary water. It is the water in this form 
upon which plants draw during their periods of 
