SOIL MOISTURE 339 



as eaves-troughs to prevent flood water of surrounding 

 uplands from entering lower areas. 



Deeper ditches are required when the main drain is to 

 receive the discharge of lateral drains. The size and depth 

 will depend on the territory to be drained, and the fall of 

 the ditch. In the Middle West, open ditches, many of them 

 miles in length, are being constructed, each farm served 

 paying its share of the expense. Open lateral ditches or 

 underground tile then empty into this main drain. Under 

 average conditions, the cost of opening a ditch seven feet 

 deep and twenty feet wide at the top by means of a dredging 

 machine is about one thousand dollars a mile. 



Underdrainage. Underdrainage has the advantage 

 of carrying off the ground water to any desired depth. 

 This is an important matter in the growth of most crops. 

 For where the level of ground water is near the surface, 

 plants will not strike their roots deep in the soil, but spread 

 them out near the top. This leaves the crop at the mercy 

 of drought later in the season, when the upper layers of 

 soil dry out. Deep rooting is also necessary to make full 

 use of the plant food of the soil. 



Tile underdrains. Burnt clay and cement are the 

 materials chiefly used for underdrains in most regions. 

 These materials are made into cylinders from three to thir- 

 ty-six inches in diameter, and from twelve to thirty inches 

 in length. For lateral drains, tiles four or five inches in 

 diameter are most used. The main outlet drain usually 

 requires tiles from eight to twelve inches. 



Placing the drains. Moderately heavy clay soils re- 

 quire laterals about four rods apart to carry off the rainfall. 

 If the subsoil is sandy, the laterals may be as much as eight 

 rods apart. Sometimes the marshiness of a piece of ground 

 is caused by seepage leading to it from some higher area. 



