236 soils: properties and management 



if such a law was in force a sand having a diameter of 

 .5 millimeter would exhibit a flow 10,000 times greater 

 than that through a clay loam with a diameter, say, of 

 .005 millimeter; whereas the actual ratio, as observed 

 experimentally by King, was less than 200. 



Evidently, therefore, while it can be stated as a general 

 thesis that the flow varies with the texture, no governing 

 law can be deduced for soils since structure exerts such a 

 modifying influence. The percolation in a heavy soil 

 takes place largely through lines of seepage, which are 

 really large channels developed by various agencies. 

 If in the drainage of average soil, the farmer depended 

 on the movement of water through the individual pore 

 spaces, the soil would never be in a condition for crop 

 growth. These lines of seepage are developed by the 

 ordinary forces that function in the production of soil 

 granulation, as freezing and thawing, wetting and drying, 

 lime, humus, plant roots, and tillage operations. 



A clear understanding of the factors governing the 

 flow of gravitational water is of especial importance in 

 tile drainage operations, particularly regarding the depth 

 of and interval between tile drains. Since percolation 

 is so slow in a heavy soil, it is evident that the tile must 

 be near the surface in order to secure efficient drainage. 

 In a sand the depth may be increased, because of the 

 slight resistance offered to water movement. The depths 

 for laying tile in a heavy soil range from one and a half 

 to two and a half feet, while in a sand the tile may often 

 be placed as deep as four feet below the surface. It is 

 evident also that the less deep a tile drain is laid, the 

 less distance on either side it will be effective in removing 

 the water; consequently on a clay soil the laterals must 

 be relatively close, as compared to the interval generally 



