ALKALI LANDS AND THEIR RECLAMATION 



281 



creases the volume, producing a slight depression in which water is 

 likely to stand. It also tends to form tough and impervious strata 

 at different depths in the soil. 



The white alkalies have no injurious effect on the soil, but, on 

 the other hand, tend to produce a granular character that is very 

 favorable to tilth. 



Vertical and Horizontal Distribution. The distribution of 

 alkali salts is very irregular, both in amount and kind. The follow- 

 ing table gives the vertical distribution in one place, which may be 

 somewhat representative of most alkali areas. There is a zone of 

 greatest concentration at about the depth of annual percolation. 

 This zone is moved downward slightly by the winter and spring 

 rains and is brought upward by summer evaporation. In heavy soils 

 it will be nearer the surface than in permeable ones. 



Vertical Distribution of Alkali Before and After Irrigation at Various Depths, 

 Tularc, California. Pounds per Acre (Hilgard.-) 



The amount of alkali in an area or even in a small field varies 

 almost infinitely. It seems to move from place to place, so that an 

 area with abundant alkali may in short time, perhaps not over a 

 week or two, have much less. The kind of alkali varies even more 

 than the quantity. A spot of black alkali may change to white, and 

 vice versa. Low places in irrigated land will usually contain most 

 alkali, and are frequently called alkali marshes. 



Effect of Irrigation on Rise of Alkali. The tendency of irri- 

 . gation is to increase the amount of evaporation from the surface 

 of the soil. The water applied enters the soil, dissolves the salts and 

 carries them downward. When evaporation begins the water moves 

 upward, carrying the salts with it and depositing them at the sur- 

 face. The effect of successive irrigations and the excessive evapora- 

 tion that follows is to transfer large quantities of salts to the sur- 



