SOIL IMPROVEMENT BY DRAINAGE. 63 



water courses, opening ditches, running furrows 

 through the fields, etc, have long been known and 

 such work has generally been done to some extent in 

 all civilized countries. Sometimes improving the sur- 

 face drainage may be all that is necessary or desirable. 

 It is better, however, that the surplus water, unless in 

 great excess, should pass down through the soil rather 

 than over it. 



Water flowing over the surface washes away more 

 or less of the best of the soil and of manures which 

 have been applied. It may seriously wash the surface. 

 It is also true that while stagnant water in the soil is 

 almost always injurious, moving water, in reasonable 

 quantity, is beneficial. While the stagnant water 

 makes the soil cold, the rains in spring are often 

 warmer than the soil and increase its temperature by 

 passing down into it. The rain carries with it am- 

 monia and carbonic acid from the air. It aids in the 

 preparation of plant food. The air will freely follow 

 the water as it could not if the water remained saturat- 

 ing the soil. When the surface has become dryer 

 than the deeper soil water will be carried up again by 

 capillary action, which cannot go on in a soil full of 

 water. 



Underground or covered draios have advantages 

 over open ditches. They do not lessen the area to be 

 cultivated nor interfere with the passage of teams. 

 They do not invite the growth of weeds. Oftentimes 

 they are more effective than open ditches of equal 

 depth. 



Material for ]>raiiis. — Many classes of material 

 have been used in making underground drains, but 

 nothing is so good as round tile made of clay, usually 



