188 LAND DRAINAGE 



should be used. A representative of the college visited 

 the farm. He found that the crop (corn) growing upon 

 the field was very pale and lacking in vigor. The symp- 

 toms all indicated a wet soil. The owner was sure the 

 land was naturally well drained. An examination re- 

 vealed the fact that in many places the water-table stood 

 within a foot of the surface. 



The difficulty in these cases is due to the presence of 

 an underlying impervious layer of subsoil. It is most fre- 

 quently a stratum of clay. It is sometimes a layer of 



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FIG. 77. To illustrate the conditions described in paragraph 248. S 

 represents soil, which may range from a few inches to several feet in 

 thickness. /, impervious or semi-pervious layer. It may be clay, 

 hard-pan, or rock, which may range from a few inches to many feet 

 in thickness. It usually occupies horizontal position. 



sand-iron hard-pan. Sometimes it lies within two or three 

 feet of the surface. If within three feet, it is usually 

 desirable to set the tile down in this subsoil sufficiently 

 to give to the drain a total depth of at least three feet. 

 (See Fig. 77.) 



249. Considerable slopes of light soil. In Fig. 78 is 

 illustrated an interesting case. The soil occupies an 

 irregular slope and is a rich sandy loam underlaid, as 

 shown, by a sandy hard-pan. The hard-pan permitted 

 only slight movements of water downward, and while the 

 soil was a sandy loam and, therefore, fairly open, it did 

 not permit a sufficiently rapid movement of soil water, 

 laterally to provide the necessary drainage. The result 

 was that while the field appeared to have excellent natural 



