GRAVITATIONAL WATER 



161 



of the hygroscopic and capillary capacity of a soil. 

 It is not retained by the same forces, and is, therefore, 

 free to move under the influence of gravity, in so far 

 as the condition and character of the soil will permit. 

 The amount of gravitational water depends on the total 

 pore space of the soil on the one hand, and on the total 

 hygroscopic and capillary capacities on the other hand. 

 It is the difference between the total capacity of the 

 soil for water and that held in the other two forms. 

 It is measured by that amount which will flow from a 

 soil having all of its pores filled with water. 



The maximum water capacity of a soil refers to the 

 total amount of water which can be put in a given 

 volume of soil. It is therefore determined directly 

 by the total pore space of the soil. The pore space may 

 range from 35 per cent in a clean sand to GO or 70 per 

 cent in a well-granulated clay, and to 80 or 90 per cent 

 in a muck soil. If we assume the weight per cubic foot 

 for these materials given on page 155, the maximum 

 per cent of water is as follows: 



Table XXVI 



Dune sand 



Coarse sand 



Fine sandy loam 

 Light silt loam. . 



Clay 



Humus 



Weight 



per cu. ft. 



Pounds 



80 

 81 

 83 

 83 

 68 

 15 



II 



Per cent 

 pore space 



52 

 51 

 50 

 50 

 59 

 80 



III 



Pounds of 



water 

 per cu. ft. 



32.5 

 32.0 

 31.5 

 31.5 

 37.0 

 50.0 



IV 



Per cejit 



of water 



in soil at 



saturation 



40.5 

 39.5 

 38.0 

 38.0 

 54.5 

 333.0 



