What is "alkali soil"

Alkali soil \Alkali soil\ Any one of various soils found in arid and semiarid regions, containing an unusual amount of soluble mineral salts which effloresce in the form of a powder or crust (usually white) in dry weather following rains or irrigation. The basis of these salts is mainly soda with a smaller amount of potash, and usually a little lime and magnesia. Two main classes of alkali are commonly distinguished: black alkali, which may be any alkaline carbonate, but which practically consists of sodium carbonate (sal soda), which is highly corrosive and destructive to vegetation; and white alkali, characterized by the presence of sodium sulphate (Glauber's salt), which is less injurious to vegetation. Black alkali is so called because water containing it dissolves humus, forming a dark-colored solution which, when it collects in puddles and evaporates, produces characteristic black spots.

Alkali, or alkaline, soils are clay soils with high pH (> 8.5), a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate, which causes the soil to swell and difficult to clarify/settle. They derive their name from the alkali metal group of elements, to which sodium belongs, and that can induce basicity. Sometimes these soils are also referred to as (alkaline) sodic soils.
Alkaline soils are basic, but not all basic soils are alkaline; see alkali for the difference between alkali and base.