High Quality Content by WIKIPEDIA articles! In both the FAO and USA soil taxonomy, a vertisol is a soil in which there is a high content of expansive clay known as montmorillonite that forms deep cracks in drier seasons or years. Alternate shrinking and swelling causes self-mulching, where the soil material consistently mixes itself, causing vertisols to have an extremely deep A horizon and no B horizon. (A soil with no B horizon is called an A/C soil). This heaving of the underlying material to the surface often creates a microrelief known as gilgai.