When your livelihood depends on plants, nitrogen is an obsession. A little nitrogen can make all the difference between a sapling that barely hangs on year after year and a vigorous, healthy tree that strikes its roots deep and firmly into the ground and bears abundantly year after year. So it's no wonder that a lot of people ask if Moringa can fix nitrogen. The short answer is no. If you want my random speculation on why so many people seem to think it does fix nitrogen, and why it would be so nice if it could, then read on. 
Nitrogen is so important, not only to plants but to all living things, because it forms the very heart of proteins. And because proteins make up nearly everything worth having in an organism, life on Earth is inextricably tied to nitrogen. Proteins include the enzymes that catalyze all of life's reactions; histones, the very spools around which our DNA is wound; and the cytochromes that lie at the heart of metabolism. A protein is made up of chains of amino acids strung together one after the other. Amino acids are so called because they include what is known as an amino group. An amino group includes a nitrogen atom bonded to at least two hydrogen atoms. To form a protein, the amino group of one amino acid bonds to the organic acid end of another amino acid, giving up one of its hydrogen atoms in the process. In this way, a protein is made up of amino acids linked by a continual chain of two carbon atoms alternating with a nitrogen atom: N-C-C-N-C-C, and so on. No nitrogen, no proteins, and no proteins, no life. 

The good news is that our atmosphere is mostly nitrogen. So this most precious of elements is also superabundant. The bad news is that only a few organisms on earth have learned the trick of taking gaseous nitrogen out of the air and incorporating it into their proteins. Part of the problem is that nitrogen in the air travels in pairs of atoms, and that they are extremely content in such a configuration. So content that there are no electrons hanging off for organisms to grab onto. This is why we fertilize our fields with things like ammonia. In ammonia nitrogen is in a form that is much more willing to part company with its neighbors and make up part of a new alliance, a protein in the body. 

Taking nitrogen out of the air, then, is a very neat trick. It is known as nitrogen fixation, and only a few kinds of organisms can do it. All of them are very tiny. Modern phylogenetic classifications recognize them as belonging to two different kingdoms, the Bacteria and Archaea. What they all have in common is that they are prokaryotes, single-celled organisms that lack a nucleus. They are, to most of us who like trees and birds and things we can actually see, terribly boring. If you can get ahold of a microscope and get a look at one, you will see nothing but a small round or rod-shaped blob. Boring. 

But what they lack in looks they make up for with their metabolic miracles, and nitrogen fixation is surely an example. So impressive is it that many larger and more charismatic organisms have found ways to set up house together with nitrogen fixing bacteria. The legumes are the most famous ones. Nitrogen fixing bacteria live in nodules on legume roots, almost certainly why legumes do so well even on marginal soils. Other plants also associate with nitrogen-fixing bacteria of various types. The photo below is from a Mexican cycad in the genus Zamia. Cycads have strange roots that grow upward out of the soil rather than down, known as "coraline roots" for their coral-like appearance. These roots are hollow and are home to photosynthetic nitrogen fixing blue-green algae (cyanobacteria). The roots grow out of the soil because the cyanobacteria need light to photosynthesize. 
PictureThe red arrows show coralline roots sprouting out of the soil at the base of this cycad in the genus Zamia

A closer look at some cycad coralline roots reveals their intricately branched structure. They are hollow and the chambers are home to nitrogen fixing cyanobacteria.
If so few plants associate with the bacteria that fix nitrogen, then why do so many people wonder whether Moringa might be among them? The first reason is probably that it would be *really convenient.* Nitrogen fixation not only means saving on fertilizer, it also means that the plants can help enrich rather than deplete the soil of nitrogen. 

But I think that associating Moringa with nitrogen fixation also probably comes from the long tradition of mistaking Moringa for a legume. You may be forgiven for taking a Moringa for a legume. You would be in good company: Linnaeus thought that Moringa oleifera was a legume when he first described the plant (see my post on how Moringa oleifera got its name). For starters, most Moringa species have big pinnate leaves, just like most legumes. Second, Moringa flowers look very much like pea flowers, with a "banner" and a "keel." Third, the fruit in Moringa looks a little like the long pods of many legumes (though Moringa pods open up into three rather than two valves when they shed their seeds). I suspect that a lot of folks associate these general similarities with legumes, and therefore make the link that Moringa species might be nitrogen fixers. 
The leaves of all Moringa species, like this Moringa stenopetala silhouetted against a Jalisco dawn, are pinnate, just like most legumes.
No such luck, though. Moringaceae is a member of the mustard-oil plants, the great group of families that includes the mustards, the capers, the papayas, and a lot else besides. None of these plants seem to have learned the trick of living with nitrogen fixing bacteria. So even a plant as generous as moringa, which gives us protein, oil, powerful antioxidants, vitamins, and grows happily and shockingly quickly in some of the harshest dry tropical habitats in the world, still leaves some things to be desired. 


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Chris Warburton Brown
22/06/2014 3:28pm

I recently discovered that Ceanothus is a non-leguminous nitrogen fixer,a native of North America but which has been growing happily in my garden in north east England for years. For more information here is an academic paper: http://www.plantphysiol.org/content/40/6/1045.full.pdf

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