Construction of under-drains 



253 



drains. But, as the excess of water is removed, and the 

 soil structure is improved, they assume more fully the 

 function of deep drains by removing water from the 

 joints, or checks, which extend deeply into the soil. 

 Where deep-rooted crops and trees are to be grown, 

 deeper drainage is 

 necessary than where 

 shallow-rooted crops 

 are grown. In gen- 

 eral, it is not desir- 

 able to lower the 

 water-table so much 

 in sandy as in clay 

 soils, because of the 

 lesscapillary capacity 

 of the former. The 

 water-table should be 

 lowered to from three 

 to five feet below 

 the surface, but it is 

 not always necessary 

 to place tile at this 

 depth, to attain suf- 

 ficiently thorough 

 drainage. Where 

 there is a distinct 

 change from sand to 

 clay, or vice versa, 

 within from two to 

 four feet of the sur- 



. . 11 i ^ig. 83. Laying double-sole drain- 



tace, it is usually best tile by hand. 



