Seriously? We're running out of dirt? You know it's getting pretty bad when we're to that point.
"Really, Peak Dirt- the world is losing soil 10 to 20 times faster than it is replenishing it.
Drake Bennett in the Boston Globe tells us that dirt is complicated stuff, made from sand or silt, then years of plants adding nutrition, bugs and worms adding their excrement, dying and rotting.
"The resulting organic matter feeds a whole underground ecology that aerates the soil, fixes nutrients, and makes it more hospitable for plant life, and over time the process feeds back on itself. If the soil does not wash away or get parched by drought, it very gradually thickens. It takes tens of thousands of years to make 15 centimeters of topsoil, about 6 inches' worth."
Till it and plant a monoculture like corn on it and that soil gets depleted rapidly, so farmers add fertilizer, lots of it. The philosophy was "Well, if your soil's degraded, just put some more fertilizer on, or till it another time and you can get the same crop yield," says David Laird, a soil scientist."
From here.