Other structural phenomena associated with black cotton soil except cracking are
•Structural sphenoid aggregate
•Self mulching in surface soils.
All of the phenomena stated above are also attributed to shrinkage and swelling of soil under moisture change; but actual mechanism is not understood clearly.
The soil movement under moisture variation is not found in only vertical or horizontal planes. This will produce sphenoid aggregate or wedge-shape aggregate and due to movement past each other, the peds becomes polished resulting slickensides.
The development of gilgais is the most interesting phenomena associated with black cotton soils. Gilgais is topographic phenomena where alternate depression and mounds occur at soil surface. The intermediate areas between them are called a shelf.
Many mechanism and many forms of gilgais were described considering uneven swelling & shrinkage of soil. The forms are
• Round or nomal
• Mehen hole
The gilgais are formed by repeated swelling cycles of black cotton soil followed by subsequent shrinkage when moisture is lost. Soil becomes cracked and the cracks are filled with loose materials. When the soil mass swells under next rewetting cycle, the pressure in soil cannot be relieved by cracking which exerts forces sideway and results mounds.
The depressions hold water and make soil wetter and suffer more swelling and subsequent mounds and obviously more shrinkage under drying.
The cracks allow water to penetrate more deep into the soil mass leading more swelling and subsequent shrinkage. The increasing swelling and shrinkage results repeated depressions and mounds.
Thus regular heterogeneity occurs which made the mounds generally more alkaline than depression (shelf); but very few data is available for both shelf and mounds.