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.
A soil having so high a degree of alkalinity (pH 8.5 or higher), or so high a percentage of exchangeable sodium (15 per cent or more of the total exchangeable bases), or both, that plant growth is restricted.