 The Vertisol great group has a light-coloured A horizon that is not readily distinguishable, and the Humid Vertisol great group has a dark-coloured A horizon enriched in organic matter that is clearly distinguishable from the underlying soil material.These acid soils have a B horizon containing accumulations of amorphous materials composed of humified organic matter associated with aluminum and iron.They develop most commonly in sandy materials in areas of cold, humid climate under forest or shrub vegetation.