There is abundant nitrogen in nature, but very little is available for plant growth. The atmosphere is loaded with nitrogen gas (N2), with a whopping 3.8 X 1015 tons of molecular nitrogen circulating in the air we breathe. The rocks and sediments in the earth’s crust contain even more nitrogen, with about 18 X 1015 tons locked up in the lithosphere. But only a small fraction of lithospheric nitrogen is found in the soil, and an even smaller proportion of soil-nitrogen is available to plants. So plant-available nitrogen is often the limiting factor for plant growth, especially for vegetable production.