Explain why desert soils tend to be rich in minerals but poor in topsoil

Desert soils tend to be rich in minerals because of the low levels of rainfall that occur in these climatic regions. Rainfall facilitates leaching which is the washing away of minerals in the soil. Since there is little to no rainfall, the minerals will stay in place. These regions have poor topsoils because the lack of vegetation means that there is little to no organic matter in the soils. 

Yes it would be a physical change.