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.]