646 soils: properties and MANAGEMKX I 



Tile drains operate best on a grade of one or two feet 

 in a hundred. Larger grades are permissible, but in 

 such cases the earth should be carefully packed around the 

 tile in filling. Tile will operate even on the very slight 

 grade of one or two inches in a hundred. In this case 

 the minimum size of tile should be larger than on high 

 grades, and the distribution of the fall should be very 

 uniform. Every part of the operation of planning and 

 construction should be guided by readings of an accurate 

 level. 



548. Depth of drains. — The depth of tile drains should 

 ordinarily be from two feet to three and one half feet. 

 The former depth should be the one for clay loam 

 or other moderately impervious soil, and is adequate 

 for most crops having a shallow root penetration. The 

 greater depth should be used on sandy and gravelly soil 

 and where deep-rooted perennials are to be grown. Under 

 special conditions the drains may be laid deeper or less 

 deep than these figures. On very dense clay or where "a 

 very impervious hardpan exists, the drains may be placed 

 a little nearer the surface, since their function is primarily 

 to remove the water trapped near the surface. To 

 intercept deep underground flow or to secure an outlet 

 for it, or where especially deep rooting of crops is desired, 

 drains may be laid deeper than the normal. 



Where the soil is sufficiently porous to permit reasonably 

 free percolation of water, as in gravelly and sandy sub- 

 soils, the deeper drains operate earlier after a rain and are 

 the more efficient. The number of drains necessary is 

 also reduced by laying them deeper. Where the subsoil 

 is relatively impervious, shallow drains should be in- 

 stalled and placed as near the top of the impervious layer 

 as is practicable. A shallow trench should be formed in 



