Black cotton soil which is also called as Regur soil is derivative of trapped lava.  

• These soils are rich in calcium carbonate, magnesium carbonate, potash and lime.  
• Regur soil has high water retention capacity.  
• It cracks in summer and hence oxygen reaches to the deeper layers of soil. 

• Cotton is well grown in this soil and hence is also called as black cotton soil. 

Source: NCERT XI – India Physical Environment