250 THE PRINCIPLES OF SOIL MANAGEMENT 



the adjacent subsoil. This, in turn, is determined by 

 the texture and structure of the soil, and upon all those 

 other factors which determine the efficiency of closed 

 drains, later to be discussed. 



To be efficient, an open drain should be properly 

 graded, should have a smooth bottom and sides, should 

 have sufficiently tenacious walls to resist incidental 

 erosion, and should have a shape approximately that 

 of a semicircle, which is the form giving the greatest 

 carrying capacity per cross-sectional area. Since this 

 exact shape is difficult to maintain, it is common in 

 practice to make the depth and bottom width, respec- 

 tively, one-half the width of the top, with sloping sides. 

 The farm and grade of the ditch must be governed by 

 the character of the soil. The steepness of the sides will 

 be determined by the ability of the soil to form resistant 

 walls. Clay soil will maintain a much steeper bank than 

 sand. The fall must not be so great as to produce 

 serious erosion. A loam or sand soil is much more suscep- 

 tible to erosion than a clay. The fall should be uniform, 

 in order that there be no undue accumulation of sediment 

 at any point. Sedimentation may be reduced by pre- 

 venting the growth of vegetation in the bottom. 



As deep-soil drains, open surface ditches have a 

 number of disadvantages, some of which are: (1) They 

 are seldom of sufficient depth. (2) As ordinarily con- 

 structed, they have a small carrying capacity, due to 

 their uneven grade and rough bottom and sides. (3) 

 They are expensive to maintain. (4) They waste much 

 land. (5) They greatly interfere with cultural opera- 

 tions. (6) They may be subject to serious erosion. 



