If "desert soil" sounds like a contradiction in terms to you, you're not alone. For most people, the word soil conjures up images of rich brown dirt that's great for growing crops. For geologists, however, soil is the loose uppermost part of bedrock weathering; it's the set of chemical reactions between air, rain, and the bedrock and sediment on the earth's surface. So what makes Midwestern farm belt soil and desert soil so different? Basically, the intensity and rate of the chemical weathering process. This all depends on the presence or absence of water.
All of the chemical reactions involved in weathering require water to take place. What's more, soils in areas with plenty of water are also rich in organic matter. Soil animals, like moles, worms, and insects, mix minerals in the soil with dead leaves, roots, and fallen branches. Microbes finish the job by breaking down this mixture into nutrients, which, in turn, allows vegetation to thrive.
In the desert, the absence of water means little or no chemical weathering can take place. Instead, erosion, frost, sedimentation, and the huge temperature fluctuations between day and night break down the rocky surface into sand or gravel. The low moisture also means that silt and sand-sized particles are easily blown away, while the remaining particles eventually form a tightly packed layer known as desert pavement. Finally, because of the negligible rainfall, there isn't a lot of biological activity and organic matter in the surface material. The result: a dry place, full of sand and desert pavement.