Which geographical factors is responsible for the evolution of black soil?

Which geographical factors is responsible for the evolution of black soil? Why it is considered the most suitable for growing cotton?

Black soils are typical pedogenic features over volcanic rocks. In India black soils are found in a irregularly triangular shaped region covering Deccan trap that includes plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh and Chattishgarh and extend in the South-East direction along the Godavari and Krishna valleys. 70 million years ago there were huge volcanic eruptions taking place in North-Western part of Indian plateau and lava spread North-West to South-East direction covering a huge area. Over these region the rocks are gradually degraded and black soil formed.
The soil is famous for its cotton growing capacity. Conducive conditions for such cultivation is its moisture retaining capacity and extensive fertile layers along its profile. The soil is clayey in nature and rich in soil nutrients like calcium, carbonate, magnesium, potash and lime.