Front Page > Issues > 2006> July

Nitrogen: It's elemental, my dear

“Water, water everywhere, but not a drop to drink!” In the case of plants, the phrase might be, “Air, air everywhere but not a whiff to breathe!”

Nitrogen is a part of every living cell, so in a world where 78 percent of our atmosphere is nitrogen, you’d think that plants would be as happy as could be, immersed in all that good stuff and “inhaling” it as needed through their leaves day and night. But alas, ‘taint so. Atmospheric nitrogen (N) is in a form that plants can’t use - either from the air or the soil. How, then, is nitrogen converted to a form that plants can use? Hang on and I’ll try to explain ... and keep it simple.

Most nitrogen in the soil is found within the top two feet and is present in three forms. The first is elemental nitrogen. Regular readers will know that one of the components of good soil is pore space, where atmospheric air mingles with roots, micro- and macro-organisms and dirt. But, as mentioned above, atmospheric nitrogen - while plentiful - is unusable by plants in its gaseous state. It needs to undergo a chemical transformation before plants can absorb and use it. Who is our hero in this story? Once again, it’s our friends the bacteria. Certain bacteria (so-called rhizobacteria or rhizobia) form symbiotic relationships with certain plants to form a nitrogen compound that plants can use. Most of the plants that have this relationship are legumes, and they are the only ones we will concern ourselves with in this column. But there are several others, including a lichen found in Pacific Northwest forests and the very common red alder.

It is a most delicious arrangement that the rhizobia have with most (but not all) legumes. To review, common legumes in our gardens include fava beans, crimson clover, Austrian field peas and beans. We often plant some of these legumes as cover crops in the fall because of their ability to “fix” nitrogen - that is, to convert the nitrogen into a usable form. How is this done? Soon after the legume seed germinates and begins to grow, these special rhizobia invade the tiny root hairs and multiply in large numbers. The legume reacts to this invasion by forming a small tumor-like swelling, known as a nodule. The rhizobacteria, now conveniently inside these nodules, absorb air from the soil and chemically convert the gaseous nitrogen (N2) into ammonia (NH3), which is a form of nitrogen the host legume plant can use. The plant uses the ammonia to help produce the proteins, enzymes and chlorophyll needed for growth.

What if you don’t have rhizobia in your soil? If you’ve grown that same type of legume before in your general garden area, chances are good that you already have the correct rhizobia to colonize your plants. If not, you can buy a small packet of rhizobia to inoculate your seeds with before planting. (Moisten the seed in a small container, add the dry inoculant, and then swish it all around for a bit.) The innoculant is not useful in helping your pea and bean seeds germinate, only in supplying the beneficial nitrogen-fixing bacteria.

Almost all the nitrogen that is fixed in a plant’s nodules is used by that specific plant, which means that very little is available for a neighboring non-legume plant. For this reason, cover crops should be cut down or tilled into the soil before they begin to set seeds; otherwise much of the fixed nitrogen will be “used up” in the seed production process. Nitrogen eventually returns to the soil when the plant dies and its roots, leaves and fruit decompose.

But the fixed nitrogen is not free to the plant. The arrangement is symbiotic, and the plant contributes a significant amount of its photosynthesized food to the rhizobacteria in its nodules. It has been estimated that soybean plants divert 20 to 30 percent of their energy to the nodules instead of other plant functions!

The amount of nitrogen that is fixed is less when the plant is under stress, be it from weather factors, such as cold or wet, which are out of our control, or from nutrition, which can be corrected with compost and fertilizers.

The topic of fertilizers brings us to the second form of nitrogen: organic nitrogen, such as manure and other biosolids. That will be the topic of next month’s column.

Glen Andresen hosts “The Dirtbag” heard every second Monday at 10:30 a.m. on KBOO 90.7 FM. He tends his bees on a three-quarter acre organic garden at a retreat in Eagle Creek. He also coordinates Metro’s Natural Gardening Program. Comments and questions may be sent to glan({at))pacifier.com or c/o The Portland Alliance.