If you were to imagine a desert, you might picture vast expanses of sand. But actually, most of Earth’s deserts aren’t very sandy. Why are virtually all deserts composed of sand?…does all normal soil turn to sand if it doesn’t receive rain? Will the Sahara Desert someday have regular soil if it starts raining there again?
Actually, sand covers less than a third of the world’s deserts. The Sahara, the world’s largest desert, is made of only about 20 percent windblown sand. Desert soil doesn’t turn into sand. But without water or plants to hold dirt down, wind can sweep fine particles of clay and silt high into the air. The coarser grains –sand and rock–are left behind.
What defines a desert is a very arid climate. In a desert, more water is lost than gained. Rich soil needs silt, sand and gravel — combined with organic debris from plants and the action of bacteria. Still, most of the soils of the warm deserts –including the Sahara — were formed during wetter periods of the Earth’s history. These soils were nutrient-rich when they formed, but have lost most of their nutrients over the centuries.
North American deserts have very little sand. Deserts in China and Arabia are sand richer. Almost half of the worlds deserts are polar deserts. Sand is a grain size — the product of erosion and weathering. But it would take more than rain to make regular soil. Soils also take minerals, plants, organic matter — and lots of time.