It is common knowledge among gardening folk in the desert that mesquite and many other desert trees are nitrogen fixing legumes. Here are a few reasons why the desert is full of nitrogen fixers.
Nitrogen is Essential
Nitrogen is one of the most important essential nutrients that plants need to be able to survive and thrive. Nitrogen is an essential ingredient in amino acids, proteins, hormones, etc, but while nitrogen makes up more than three quarters of the gas in our atmosphere, for plants to be able to use this nutrient it must be converted from its gaseous state into its ionized salt forms. Unfortunately, once converted into these usable forms it begins to vaporize back into a gas at a mere 90 degrees.
Here in the low desert of Arizona, we often don’t see the near side of 90 degrees for more than 6 months of the year. The process of converting nitrogen into a usable salt form is very energy intensive. This is because the molecular bonding of nitrogen is very strong, thus there are only three ways that I am aware of by which it can be achieved.
Three Ways Nitrogen is Converted into Usable Salt Forms
First, nitrogen can be converted into a plant usable salt form from mixed natural gases, such as methane through the industrial fertilizer manufacturing process known as the “hagen bosch system”. Again, energy intensive, but possible, and applied on a huge scale by industrial farming systems through the conversion of extracted fossil fuel natural gas into nitrate fertilizer.
Second, nitrogen can be converted by the ionization that occurs from lightning coming in contact with nitrogen gas. Ever wondered why rainwater seems to make plants grow better than municipal water? This is one of many possible reasons why. Again, an intense amount of energy is used to break these strong nitrogen bonds. Lighting is intense.
Lastly, the nitrification process can be accomplished by nitrifying bacteria that colonize the roots of many species of legumes, of which mesquite is one. This colonization thereby aids in the growth and survival of so many other plants in the mesquite or ironwood guilds in the hot, dry, desert climate. Nitrifying bacteria included in this powerful sorcery of ionized nitrogen include many species in the genera Nitrosomonas, Nitrosococcus, Nitrobacter and Nitrococcus. These are some of the smallest and most important superheroes on our planet.
Nitrifying Bacteria
Have you ever seen a tree growing out of a crack in a cliff, like in the photographs on the cover of an Arizona Highways magazine, or on a drive through the mountains? It is unlikely that there is any soil in that crack. No available nutrients via dirt. But the populations of bacteria on the roots of those plants are huge and essential. The bacterias and fungi are breaking down rock and extracting nutrients from the air. They then exchange those nutrients with the plant roots, even colonizing inside the plant’s cells!
So it turns out that this relationship between nitrifying bacteria and plants is a large-scale commonplace operation. Not just on legumes, but on the roots of many other plants too. Some of these plants have direct relationships with species of bacteria that do not fix nitrogen, but those have relationships with some species of bacteria that do fix nitrogen. It quickly becomes a very complex and interconnected web of micro and macro relationships.
But the most well-known relationship of nitrifying bacteria is with the roots of leguminous plants. This understanding that certain species of legumes enrich the soil for other crops is not new. In fact, the practice of using nitrogen-fixing legumes as a cover crop has been a practice in nearly every agricultural civilization probably since the dawn of time, even if all of the details of why it worked may not have been fully understood.
Nitrogen Fixing Cover Crops
Lupines in South America, clover in Europe and Australia, alfalfa in the Moroccan region, and vetch in the United States, cowpeas, bambara beans, and peanuts in Africa, rushpea, pigeon pea, and fenugreek in India, lentils in Western Asia and Canada. Native Americans cultivated pink fuzzy bean and hog peanuts in the woods, as well as Hopniss, a leguminous tuber that in southern regions also produces a reliable bean pod.
In perennial agroforestry applications black locust, acacias, leucaena, sesbania, and hundreds of others have been used worldwide for the same purpose- to increase fertility and production of surrounding crops. Another tree, Red Alder, which is not a legume, but also has a symbiotic relationship with nitrogen fixing bacteria that colonize its root system, has also been used extensively in more recent times in permaculture guilds.
Nitrogen improves fertility and production for the garden and orchard
A few years ago, while in conversation with some county extension agents, I heard of a study that was conducted by the University of Arizona using clover as a living mulch under citrus trees. The scientists found that citrus orchards under-planted with some species of clover performed exceptionally well. The trial resulted in a citrus crop that required maybe a third as much fertilizer as traditional orcharding systems. (The study was terminated early due to an increase in overall biological diversity, which included a sharp increase in the rattlesnake population, thereby putting the scientists, orchardists, and fruit harvesters at greater risk).
Microbial Relationships
Every plant on the planet has essential microbial relationships. Ironically, some of these bacteria and fungi that are found on seed coats (that serve to strengthen a newly germinated root system) can handle boiling water, freezing temperatures, high salt environments, even the high
doses of chlorine found in our municipal water supply, but the microbes that coat seeds are often killed when seeds are coated in ionized nitrogen fertilizer.
And we think that our species is so smart! Sounds like a great idea- package a seed with a bit of “plant food” to give it a good start. However did nature manage without us for so long?!
Some people think that some other people run the world, but I beg to differ. I think that microbes run the planet, even in our hot deserts, and among their superheroes, are nitrifying bacteria. You want a healthy garden, plant some legumes and let the microbes do what microbes do best!