\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.