220 SOILS 



above it would be but poorly supplied with film 

 water after the water-table is lowered into it. In 

 the Northern States it is absolutely necessary to 

 lay tiles below the frost line anyway, for they are 

 easily heaved and cracked by frost. This means 

 a depth of two to three feet, and even four 

 feet in the northern prairie states. In the 

 Red River Valley of Minnesota and the 

 Dakotas the ground freezes six feet deep, 

 and there is much doubt as to whether tile 

 drains are practicable there. It is not prac- 

 ticable to try to place drains so deep that the 

 roots of ordinary crops will not enter them, for 

 these roots commonly run from five to ten feet 

 deep. 



The best depth for drains on average soils is 

 three and one-naif to four feet. There are few 

 cases where lateral drains should be five feet, or 

 over, but mains are frequently laid at that depth. 

 It is rarely expedient to lay deep drains in stiff soils ; 

 shallow drains are much better, for water moves 

 slowly through heavy soils. Land that is to be 

 permanently in grass may have the drains laid 

 more shallow, not only because the grass will 

 prevent the ground from freezing so deep, but also 

 because grass thrives when the water-table is 

 nearer the surface than is best for most tilled crops. 

 Under-drains in such lands are often laid two and 

 one-half to three feet deep with excellent results, 

 especially if the soil is not very heavy. Thirty 

 inches is about the minimum depth, under any 

 circumstances, at which it is practicable to lay 

 drains. When laying drains in peaty land 

 make allowance for the settling and shrinking 

 of the soil from the decay of the vegetable 

 matter. 



