38 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
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. White alkali 
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 
