Rolling your own fertilizer

Nitrogen is one of the essential building blocks of all life. There is no shortage of nitrogen—it makes up almost four fifths of our atmosphere—but neither plants nor animals can use it directly in its atmospheric form (N₂). It takes energy to convert it to a usable form, for instance, to ammonia or nitrates. Energy from lightning, for example; or energy from the vast amounts of electricity used to create commercial nitrate fertilizers. If a plant could somehow harvest nitrogen directly from the air, it would be able to make its own fertilizer, allowing it to colonize soils that were too poor in nutrients for other plants.

Legumes, including alfalfa, clover, peas, beans, lentils, soybeans, and peanuts, have managed exactly this stunt, called "nitrogen fixing"—and without even solving the problem themselves. Instead, they brokered an amazing agreement with the real inventors of nitrogen fixing: symbiotic bacteria known as Rhizobia. The bacteria grow in nodules on the roots of legumes, supplying the plant with usable nitrogen in return for malates and succinates for energy and carbon. The bacteria combine nitrogen, hydrogen, and electrons like this:

N₂ + 8H⁺ + 8e^– → 2NH₃ + H₂

We could write this more simply as:

nitrogen + hydrogen + electricity → ammonia + hydrogen

The eight hydrogen ions (8H⁺) in this reaction come mostly from water vapor. The electrons (8e^–) can be thought of as electricity created by the bacteria. The results of the reaction include ammonia (NH₃) and hydrogen (H₂).

The created hydrogen reacts pretty quickly with oxygen, forming more water vapor. And the ammonia is very close to what the plant needs. In water, ammonia quickly reacts with hydrogen ions in the water to create ammonium ions:

NH₃ + H⁺ → NH₄⁺

or

ammonia + hydrogen ions → ammonium ions

You are already familiar with this reaction—it is the same one that makes you wrinkle your nose in disgust when you catch a whiff of ammonia. But what smells bad to you is a critical nutrient to most plants.

The ammonium ions created by the bacteria are absorbed directly by the roots of the legumes. The trapped nitrogen feeds the plant, with enough left over after the plant dies to foster the growth of other plants as well. Thus legumes are sometimes called green manure.

So before you get too irritated with the clover growing in your lawn, consider that it is busy fertilizing your lawn.