Nitrogen & fertiliser

With every breath you mainly pull one gas into your lungs that your body does not even use: nitrogen. And yet this very gas is in your breakfast bread, in your pasta and in your salad. How does nitrogen get from the air onto your plate? This story is one of the most important in all of chemistry.

A lazy giant

Air is about 78 % nitrogen and only about 21 % oxygen. Nitrogen floats through the air as the molecule N₂: two nitrogen atoms holding on to each other extremely tightly. This bond is so strong that N₂ almost never reacts on its own. We say nitrogen is inert. That is exactly the problem for plants. They urgently need nitrogen to build leaves and proteins, but they cannot take up the inert N₂ from the air. They need it fixed, meaning in compounds like ammonia (NH₃) or nitrates in the soil. In nature only lightning and special bacteria on the roots of peas, beans and clover manage this conversion. For big harvests that is nowhere near enough.

Bread from air: Haber-Bosch

In the early 20th century, Fritz Haber and Carl Bosch achieved something that had seemed impossible: they forced the inert N₂ to react on an industrial scale. At high pressure, high temperature and with a catalyst, nitrogen reacts with hydrogen to form ammonia: N₂ + 3 H₂ → 2 NH₃. Ammonia becomes synthetic fertiliser, and with fertiliser the same field grows much bigger harvests. It is estimated that about half of humanity could not be fed without this process. That is why people say Haber-Bosch makes bread from air.

Too much of a good thing

Fertiliser is a blessing, but the dose makes the poison. Whatever the plants do not take up gets washed by rain into streams, rivers and lakes. There the nitrogen fertilises algae, the algae grow wildly, die off, and their decay uses up the oxygen in the water. Fish and other creatures suffocate. Just like with recycling, the idea of a cycle applies here too: nitrogen should go from the field into the plant, into our food, and back to the field via compost or manure, instead of escaping into the water. Smart fertilising means the right amount at the right time.