226 SOIL PHYSICS AND MANAGEMENT 



(h) The effectiveness of thorough drainage in preventing 

 erosion has heen ohserved in many instances, but this point is dis- 

 cussed under the subject of erosion. 



(i) Drainage is one of the most effective means for removing 

 alkali from land under irrigation, and thus preventing its " rise '' 

 and consequent injury to crops. It, in conjunction with flooding, 

 is also an effective method for reclaiming land that contains so much 

 alkali as to render it unproductive. 



Types of Drainage. Two general types of drainage have been 

 employed, open and tile drains. 



(a) Open Drains. In a great many cases, the open drain is 

 an absolute necessity, because the large amount of water to be 

 carried off precludes the possibility of using covered drains at a 

 reasonable cost. Hence there will always be a large number of 



Fio 97 Alfalfa that was completely killed by heaving. Note roots 'lying on surface. 



(S. V. Holt.) 



open drains, such as dredge ditches. In some cases open ditches 

 are necessary because quicksand is present which enters the drains 

 through the openings between -the tiles and. fills them so as to re- 

 duce their efficiency or even clog them entirely. In other places 

 the fall or slope of the land is so slight that tile drains would be 

 of very little use and hence the open ditch becomes a necessity. 



A form of open or surface drainage that is effective and adapted 

 to certain types of soil is that practiced on soils with hardpan or 

 tight clay substrata. Such soils occur in various parts of the 

 country and the form of drainage adapted to them is that of dead 

 furrows or shallow ditches about 20 or 25 feet apart. These are 

 employed to a large extent on areas with tight clay subsoils. 



There are several serious objections to open drains. They aie 

 almost invariably expensive forms, because constant care is needed 



