38 BULLETIIs^ 355, U. S. DEPAETMENT OF AGEICULTURE. 



of the filling may sometimes be done quickly and efficiently with a 

 team and scraper. 



Cost and 'profits of drainage. — The final questions which are always 

 considered in connection with any needed drainage are (1) the cost, 

 and (2) whether the accruing profits from increased production will 

 warrant the cost of putting in the drainage system. The conditions 

 which determine these two factors are so varied that no discussion 

 of the matter will be entered into in this lesson. The different items 

 of cost and profit are separately considered by Elliott (Ref. No. 8, 

 pp. 121-138) and should be carefully studied by those who con- 

 template carrying out a drainage project. 



Drainage of irrigated and alkali lands. — It frequently happens in 

 arid lands where irrigation is practiced that soil areas which are 

 adjacent to or somewhat below the level of irrigation canals or 

 irrigated fields, and which have been fertile and productive for 

 years, finally become unproductive and practically useless for agri- 

 cultural purposes. Investigation has shown that almost invariably 

 the changed condition is due to the subsurface soil being water- 

 soaked from seepage from irrigated areas or irrigation canals or 

 from excessive use of irrigation water. These areas are often at 

 considerable distances from the source of the trouble. Where such 

 a condition exists the surface soil also frequently becomes laden with 

 soluble salts which are harmful to the crops commonly grown upon 

 the land. This is because the seepage and other waters have carried 

 quantities of these salts in solution which later become deposited 

 at the surface of the soil upon the evaporation of the salt solu- 

 tion. Such deposits of salts, including sulphate, chlorid, and 

 carbonates of sodium, magnesium sulphate (epsom salts), calcium 

 sulphate (gypsum), and calcium chlorid, are also commonly found 

 in spots of the surface area of extremely arid lands not irrigated. 

 This is because the rise of water from capillarity, leaving the salts 

 deposited upon evaporation, exceeds the downward movements from 

 the rainfall. Areas containing harmful quantities of soluble salts 

 in the surface soil are called alkali lands. If sodium carbonate is 

 present in considerable quantity the alkali is usually dark colored 

 due to action of the alkali on organic matter and is known as black 

 alkali. The sodium carbonate, besides being harmful to plant growth, 

 often causes the soil particles to puddle and to form an impenetrable 

 hardpan a few inches below the surface of the soil. Wliite alkah 

 is that in which sodium sulphate and similar neutral salts which 

 do not blacken organic matter predominate. This is much less 

 harmful than black alkali. Underdrainage is one of the best ways 

 of preventing the accumulation of alkali in soils and of reclaiming 

 watersoaked and seeped areas, it being frequently established as a 



