400 IRRIGATION PRACTICE 



240. Washing out alkali. In the attempt to remove 

 alkali, lands are often flooded with a large quantity of 

 water flowing under a high head. The theory has been 

 that the rapidly moving water passing over the soil will 

 dissolve the alkali and carry it off. This, however, has 

 been found to be ineffective, for, the moment the water 

 dissolves the alkali, it sinks into the soil and only the pure 

 water runs off the surface. 



A better method is to apply irrigations so large that 

 the water seeps into the country drainage. When this 

 can be done it is very satisfactory, but only on naturally 

 well-drained lands, or on open soils, can it be made 

 effective. Occasionally, the soil is underlaid by a hard- 

 pan, and it is found helpful to make holes through this 

 impervious layer to connect with the more permeable soil. 

 In any case, when much water is used on alkali land, irri- 

 gation should be followed by careful cultivation. 



241. Underdrainage the final remedy. The only 

 really satisfactory treatment for alkali is one that removes 

 the alkali permanently from the soil. This is accomplished 

 best by underdrainage, since few soils permit of natural 

 drainage. 



The feasibility of underdrainage has been demon- 

 strated in all parts of the world, and the only consider- 

 ations, with respect to alkali lands, are the cost of instal- 

 lation and the disposition of the drainage water. The cost 

 is no higher in irrigated than in humid regions; and irri- 

 gated lands are fully as valuable as those in humid regions. 

 The disposition of the drainage water depends on local 

 conditions, and must be carefully determined upon, for 

 the drainage from alkali lands is unfit for agricultural 

 uses. Drainage from such lands need not, however, be 

 continuously great, for lands, underdrained for the re- 



