Topsoil regenerates at the rate of 1 centimeter every 200 years (according to [[Soil]], it's 1 centimeter every thousand years). In the US, this is being used up at 10 times the rate of replenishment. In Europe, it is seventeen times. And in China, it is 57 times. Only 7% of the Earth's surface is available for growing food. The loss has been covered up the the use of manufactured nitrogen fertilizer, so much so that half the nitrogen in our crops comes from artificial fertilizer. Nitrogen runoff drains into the oceans and causes uncontrolled algae blooms, which use up the oxygen needed by other forms of life. There are now more than 400 of these dead zones in coastal waters around the world, some extending more than 20,000 square miles.