Most of the organisms that decompose organic matter are aerobic—they need air to survive. As the pile decays, the air inside the pile is consumed and the rate of decomposition declines. Bacteria that require less air to function take over. These bacteria work more slowly than aerobic bacteria. That’s why a pile that’s never turned and doesn’t have some other way of gaining access to air takes such a long time to decay. Learn the best techniques for introducing air into a compost.