634 SOILS: PROPERTIES AND MANAGEMENT 



those changes, and therefore the basic importance of good 

 drainage of the soil, is indicated by this summary of 

 effects. Even where ordinary yields of crops can be 

 grown, improved drainage will usually increase the yield 

 10 per cent or more; and increases of several hundred 

 per cent are in many cases realized where the conditions 

 before drainage were particularly bad. Land in need of 

 drainage is in many cases fertile in all other respects, and 

 when the soil moisture is properly adjusted it responds 

 with large yields. Proper drainage should be the starting 

 point in any permanent improvement of the soil. 



534. Methods of drainage. — Two general methods of 

 drainage are employed : (1) open ditches, and (2) closed 

 drains, or underdrains. 



Open ditches are most satisfactory where the volume 

 of water to be moved is very large. The general drainage 

 of a region is usually carried in open ditches. They are 

 used where the land is exceedingly flat, and especially if 

 the land level is very near the level of the water in the 

 outlet channel so that only a small head can be developed. 

 They are used also where a temporary result is desired. 



There are many objections to open ditches, either large 

 or small, especially as applied to tilled land. They waste 

 a considerable area of land in the channel and on the 

 banks, and they interfere with free tillage operations. In 

 the case of small field ditches this interference is serious. 

 The ditch bank promotes the growth of weeds. The 

 shallow surface trenches commonly used to remove stand- 

 ing water from the land are of very low efficiency, since 

 they do not remove the water from the subsoil and often 

 are so shallow that the surface soil remains almost satu- 

 rated. Water flows slowly in such rough, irregular 

 channels. 



