PHYSICAL PROPERTIES OF SOILS 21 



pull on the free water below and can raise it to quite a height. 

 This capillary movement does not necessarily always move 

 upward. In general it moves toward the point where the soil 

 is the dryest. If the ground becomes quite dry and there 

 comes a heavy shower, the capillary movement may be 

 downward, the tendency being always to equalize the thick- 

 ness of the film around the adjoining soil grains. 



Percolation of Water. — By percolation is meant the down- 

 ward movement of free water through the soil. Some of the 

 spaces between the soil grains are too large to hold water by 

 capillarity, so that whatever water comes to them is allowed 

 to pass on. In this way water which falls on the land as rain 

 soaks into the soil and moves downward until it comes to the 

 water table. It is plain that if the soil particles are coarse 

 the spaces between them will be larger than if the particles 

 are fine, so that in the coarse soil water can percolate more 

 readily than in the fine-grained soil. For this reason sandy 

 soils allow the rains to soak into them and drain away more 

 easily than clay soils. The way in which water percolates 

 through soils determines the amount of tile draining that is 

 needed. Clay soils need more draining than any other, 

 because water moves through them very slowly. 



Power of Soils to Retain Moisture. — When soils have 

 been saturated and then allowed to drain without any evap- 

 oration from the surface they are still found to hold different 

 amounts of water. Taking sandy, loamy, and clayey soils it 

 will be found that this amount will vary in the order that the 

 soils are named, the sandy holding the least and the clayey 

 the most. The amount held is partly dependent upon the 

 size of the soil grains and partly upon the amount of organic 

 matter in the soil, so that loam often holds more than clay. 



