There is a sad irony to floating in a lifeboat in the ocean. Despite being surrounded by water, a person is likely to die from lack of water, because salt water just isn’t drinkable for a human. Plants encounter a similarly ironic situation when it comes to one of the most important components in their diet: nitrogen.

Nitrogen is the most common element in our atmosphere. Almost 80% of the air around us is nitrogen. But despite its abundance, lack of nitrogen is, after lack of water, the factor that limits most plant growth. Nearly every terrestrial plant outside the tropics would grow larger with more nitrogen. Early farmers recognized this fact and would bury dead fish along with their corn crops. As the fish decomposed, the rotting organic material increased the soil nitrogen that was available for the plants’ use, significantly improving the size and health of the corn plants. Today, people regularly treat their crops, flowers, and lawns with nitrogen-rich fertilizers.

Adding nitrogen to the soil commonly increases a plant’s growth, because every protein the plant builds contains nitrogen, every bit of DNA the plant synthesizes contains nitrogen, and every molecule of chlorophyll it makes contains nitrogen. In order to grow and survive, plants need nitrogen. If nitrogen is so plentiful in the air, why can’t plants make use of it directly?

The problem is that nitrogen in the atmosphere exists as a molecule of two nitrogen atoms (N₂) bound very tightly and stably together. The nitrogen becomes usable for plants and animals only when it is converted to a molecule with a single nitrogen atom, such as ammonium (NH₄⁺) or nitrate (NO₃^–). Breaking apart the strong, stable bond between the two nitrogen atoms in the nitrogen molecule is no easy task, however, and plant evolution did not produce the metabolic know-how to do it. Bacteria, on the other hand, are metabolic wizards, and several species can break nitrogen molecules apart, using an enzyme they produce called nitrogenase, and put the nitrogen into a plant-usable form—a process called nitrogen fixation (FIGURE 17-26). Most nitrogen-fixing bacteria are free-living in the soil. Some, however, live within plants in a mutually beneficial relationship that enables plants to gain access to the nitrogen fixed by these bacteria.

Figure 17-26The conversion of atmospheric nitrogen into a form that plants can use.

The alliance between plants and nitrogen-fixing bacteria can occur in several ways. In legumes—alfalfa, peas, beans, and soy, for example—the symbiosis develops in several simple steps (FIGURE 17-27).

• 1. A plant’s roots secrete a bacteria-attracting compound.
• 2. Bacteria enter the root inner cells, multiplying to form, in some cases, a big, round lump called a nodule.
• 3. The bacteria split apart nitrogen molecules absorbed from the soil—an energetically expensive process that is fueled by the plant’s sugars.
• 4. The bacteria convert the nitrogen into a usable form and release this into the plant. Any excess of the amount required by the plant is released into the soil.
• 5. The nitrogen is absorbed throughout the plant body and used for growth. (And, we should add here, animals eat plants, from which all of their nitrogen ultimately comes.)

Figure 17-27Nitrogen fixation: a product of the symbiotic relationship between plants and bacteria.

Figure 17-28Another way to acquire nitrogen: the Venus flytrap.

Not all soil conditions are conducive to the growth of nitrogen-fixing bacteria, and some plants have alternative ways to acquire nitrogen. Insect-eating plants, for example, generally grow in very acidic soils in which nitrogen-fixing bacteria do not thrive. The Venus flytrap (FIGURE 17-28) captures insects rich in nitrogen-containing molecules, digests them, and recovers the nitrogen for its own uses.

Because so many plant species are not able to form the mutually beneficial alliance with nitrogen-fixing bacteria and instead must scavenge for usable nitrogen in the soil, farmers commonly rotate their crops every few seasons. They alternate between growing plants that have the nodules of nitrogen-fixing bacteria (such as peas, soybeans, and alfalfa) and those that don’t. As excess nitrogen fixed by the bacteria is released, usable nitrogen in the soil is replenished like a natural fertilizer. Also, because these plants are often plowed under before the next crop rotation, they decay and add even more nitrogenous compounds to the soil. Called “green manure,” these compounds can reduce the need for additional fertilizer (and the costs to farmers). The soil is now ready for a non-nitrogen-fixing crop.