A primary factor limiting plant species diversity in desert grasslands seems to be soil texture. Loamy soils consistently support more species than do adjacent sands. This is probably related to the increased micro-environmental differences associated with heavy textured soils. Such soils would be expected to have more microtopographic and soil moisture variations than sandy soils.
Wyckoff, John W.
"The effects of soil texture on species diversity in an arid grassland of the eastern Great Basin,"
Great Basin Naturalist: Vol. 33
, Article 4.
Available at: http://scholarsarchive.byu.edu/gbn/vol33/iss3/4