CONSERVATION OF SOIL MOISTURE 



which surrounds the soil grains in the 

 form of moisture films, and which is also 

 known under the name of capillary 

 water. It is this water which is absorbed 

 by the roots of the plants, and, conse- 

 quently, forms the direct source of sup- 

 ply of all cultivated crops. If you take 

 a pebble and dip it into a basin of water 

 or into the brook, you will observe a film 

 of water closely sticking to the surface of 

 the stone. This is an illustration of what 

 is termed "surface tension," by means of 

 which water, in the form of moisture 

 films, is held in the pores of the soil par- 

 ticles. The existence of this physical 

 force may be made clear by the simple 

 experiment of floating a clean needle, 

 carefully laid, on the surface of water, or 

 by the fact that a drop of any liquid tends 

 to assume the smallest possible space — 

 that is, the shape of a sphere. In short, 

 the free surface of any liquid tends to 

 71 



