Answer:
Arid land can be turned arable to at least a large portion of crops by irrigation. In fact, canals have been used by desert people for thousands of years to irrigate their farmlands. These include the Mesopotamians (in the Middle East), who grew dates and wheat, as well as Native American groups like the Navajo, who grew beans, corn, and squash.
However, some plants don't grow well in desert soil or the heat of the desert. Soil can be altered using fertilizers or compost, but little can be done about temperature. However, the process of changing desert soil is obviously too expensive to be used on a commercial scale.