62 SOILS AND FERTILIZERS 



parched the crops, there is always a thin film of moisture 

 surrounding each particle or aggregation of particles, al- 

 though plants may not be able to obtain it. The thin film 

 that is absorbed from the air and condensed on the surfaces 

 of the particles, when no other source of supply is at hand, 

 is termed hygroscopic water. If the film becomes somewhat 

 thicker by reason of another supply like rainfall or under- 

 ground water, the additional supply is termed capillary 

 water. These two forms are much alike, both being held 

 as a film around the particles, partly by the attraction of the 

 soil for the water and partly by the attraction of the particles 

 of the water for each other, which prevents the film from 

 breaking and running away. One other difference between 

 hygroscopic water and capillary water is that the former is 

 always stationary, while the latter may move. 



A further increase in the quantity of water in a soil gives 

 rise to the third form — gravitational or free water. With 

 the advent of more water, the films become so thick that 

 the attraction by which they adhered to the particles is 

 overcome by gravity and there is a downward movement 

 through the pore spaces, or else the pore spaces are com- 

 pletely filled and the soil becomes saturated by reason of 

 the inability of the water to escape from the soil. 



64. Hygroscopic water. — From a practical viewpoint, 

 hygroscopic water is not of much importance because 

 plants cannot use it. A plant may die for want of water 

 when the soil in which it grows contains its maximum of 

 hygroscopic moisture. The forces that hold the water in 

 the soil are greater than those that tend to draw it into 

 the plant. 



The quantity of hygroscopic moisture that a soil will 

 hold depends largely on its texture and on the quantity of 

 partially decomposed organic matter that it contains. Fine 

 particles have a greater absorptive power for water than do 



