Source : Webster's Revised Unabridged Dictionary (1913)

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.