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). 



Wlienever 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 



