<P> Deserts contain substantial mineral resources, sometimes over their entire surface, giving them their characteristic colors . For example, the red of many sand deserts comes from laterite minerals . Geological processes in a desert climate can concentrate minerals into valuable deposits . Leaching by ground water can extract ore minerals and redeposit them, according to the water table, in concentrated form . Similarly, evaporation tends to concentrate minerals in desert lakes, creating dry lake beds or playas rich in minerals . Evaporation can concentrate minerals as a variety of evaporite deposits, including gypsum, sodium nitrate, sodium chloride and borates . Evaporites are found in the USA's Great Basin Desert, historically exploited by the "20 - mule teams" pulling carts of borax from Death Valley to the nearest railway . A desert especially rich in mineral salts is the Atacama Desert, Chile, where sodium nitrate has been mined for explosives and fertilizer since around 1850 . Other desert minerals are copper from Chile, Peru, and Iran, and iron and uranium in Australia . Many other metals, salts and commercially valuable types of rock such as pumice are extracted from deserts around the world . </P> <P> Oil and gas form on the bottom of shallow seas when micro-organisms decompose under anoxic conditions and later become covered with sediment . Many deserts were at one time the sites of shallow seas and others have had underlying hydrocarbon deposits transported to them by the movement of tectonic plates . Some major oilfields such as Ghawar are found under the sands of Saudi Arabia . Geologists believe that other oil deposits were formed by aeolian processes in ancient deserts as may be the case with some of the major American oil fields . </P> <P> Traditional desert farming systems have long been established in North Africa, irrigation being the key to success in an area where water stress is a limiting factor to growth . Techniques that can be used include drip irrigation, the use of organic residues or animal manures as fertilisers and other traditional agricultural management practises . Once fertility has been built up, further crop production preserves the soil from destruction by wind and other forms of erosion . It has been found that plant growth - promoting bacteria play a role in increasing the resistance of plants to stress conditions and these rhizobacterial suspensions could be inoculated into the soil in the vicinity of the plants . A study of these microbes found that desert farming hampers desertification by establishing islands of fertility allowing farmers to achieve increased yields despite the adverse environmental conditions . A field trial in the Sonoran Desert which exposed the roots of different species of tree to rhizobacteria and the nitrogen fixing bacterium Azospirillum brasilense with the aim of restoring degraded lands was only partially successful . </P> <P> The Judean Desert was farmed in the 7th century BC during the Iron Age to supply food for desert forts . Native Americans in the south western United States became agriculturalists around 600 AD when seeds and technologies became available from Mexico . They used terracing techniques and grew gardens beside seeps, in moist areas at the foot of dunes, near streams providing flood irrigation and in areas irrigated by extensive specially built canals . The Hohokam tribe constructed over 500 miles (800 km) of large canals and maintained them for centuries, an impressive feat of engineering . They grew maize, beans, squash and peppers . </P>

What type of vegetation is found in desert region