Plants fixate nitrogen from the air from lightning or precipitation fixation. Bacteria in the soil also fixate nitrogen in the air. Animals eat the plants and defecate or urinate as they eat. These wastes are incorporated into the soil through decomposition as organic matter. Mineralization occurs, turning the organic nitrate into ammonium. Nitrification then occurs turning the ammonium into nitrites, and another nitrification step turns nitrites to nitrates. These nitrates are absorbed by the plants and the cycle continues.