The Soil and Soil Water 



H'. T. Stilling 

 FIG. 19. Measuring the water-holding power of different soils. 



into the air, it will be covered with a film of water so How water 



... . , . , j w held in 



thick that a few drops quickly run to the lower side and ^ so n 



drop off (Fig. 20). The remaining water is so firmly 

 held that the force of gravity is not strong enough to 

 pull it loose from the rock. Now if the rock is allowed 

 to dry in the sunshine until no more moisture is visible, 

 there still clings to it a thin, invisible film of water. 

 This last trace of water can be removed only by heating 

 the rock to several degrees above the boiling point of 

 water. 



Since the soil is composed mainly of small rock parti- Free water 

 cles, each one of these particles behaves toward water 

 just as the rock does. If soil is very wet, a certain waier 

 amount of moisture drains down from one particle to 

 another by the force of its own weight. This is called 

 " free water." As the stone that has been wet holds 



