From what I've read, vertisols in dry climates are virtually impossible to work for most of the year, due to the tendency for the soil to both shrink and become hardened, causing trouble for even nomadic grazers if nothing is done to address the problem. However, I don't believe that this is the end of the story. I've read reports from the British in their attempts at turning a profit in East Africa (via peanut plantations) which indicate the land can be worked if certain trees or thorn-bushes are present to break up the soil. Additionally, during the 3-to-5-month rainfall season, the ground is softened and made ready for work. Due to the high clay content of the soil, moisture is retained well, and it should permit the growth of many types of food, if I understand correctly.
I'd like to know what sorts of trees and bushes one could use to break up the soil, including crops from other continents.