GRAVITATIONAL MOVEMENT 167 



The columns were one-tenth of a foot in cross-section 

 and fourteen inches high, and a head of two inches of 

 water was maintained above the top of the soil. These 

 figures show very clearly the reduction in the flow of 

 water as the texture becomes finer. 



Under field conditions, the percolation of water 

 through the soil is much facilitated by the presence of 

 numerous cracks, root passages, and worm and insect 

 burrows, because of their relatively large diameter. 



Several other factors affect the percolation of water. 

 The entrance of rain or irrigation water into the dry soil 

 where it is applied in a sheet over the surface is hindered 

 by the presence of the air in the pores in the soil. If 

 the subsoil is dense, or is filled with water, this inter- 

 mediate band of air-filled soil serves to hold back the 

 surface water, except as the air may escape in bubbles 

 through the upper layer. For this reason, in part, a 

 heavy shower of rain sinks into the soil to a very small 

 depth, and is relatively ineffective. Entrance of the 

 water may be greatly facilitated by a loose condition 

 of the soil, which affords quite large as well as small 

 spaces. The large spaces are less likely to be entirely 

 filled with water, and hence afford means for the escape 

 of air, while the water passes in through the smaller 

 pores. There is another hint here in the conservation 

 of rainfall. If the soil is in a very loose condition to a 

 depth of eight or ten inches, the water will percolate 

 into this layer, and its movement will be so much re- 

 tarded that a larger part will find its way into the deep 

 subsoil and be permanently retained than if the surface 

 soil is uniformly fine. 



