166 



THE PRINCIPLES OF SOIL MANAGEMENT 



80. Gravitational movement. Gravitational move- 

 ment is the result of the gravity pull upon the soil 

 water. The slower the downward movement of water, 

 the longer the water will be in the root zone of the crop, 

 and therefore the greater use will the plant be able 

 to make of that particular supply of moisture. This 

 gravitational movement concerns primarily the gravi- 

 tational water, and is not effective to move either the 

 hygroscopic or the capillary forms of water, although 

 these are subject to the same gravity pull. The reason 

 is, so far as these forms of moisture are concerned, 

 that the gravity pull upon them is overbalanced by 

 other forces. It will be noticed, in fact, that gravita- 

 tional water is defined as that part of the soil water 

 which is free to move under the influence of gravity, 

 Such movement constitutes percolation. 



The rate of percolation depends upon two primary 

 conditions. These are: (1) The texture of the soil. 

 (2) The structure of the soil. The rate of movement 

 depends directly upon the diameter of the individual 

 soil spaces. The larger the size of spaces, the more freely 

 will the water descend. King has observed the following 

 movement of water through sands of different texture 

 in twenty-four hours: 



Table XXVIII 



