from
The Collaborative International Dictionary of English v.0.48
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.
[Webster 1913 Suppl.]