Nitrogen is the most abundant element in our atmosphere and we breath it in and breath it out without absorbing it at all. And yet, nitrogen is a precious resource that both governs and limits the growth of living organisms. Plants rely on nitrogen to grow leaves and photosynthesize. Animals rely on it in the form of amino acids that make proteins that build muscles, DNA, and other critical components of their bodies.
Where do living beings get the nitrogen they need? Animals get it by eating plants or other animals, whether it’s worms eating cellulose, elephants eating tree leaves or tigers eating an elephant. After eating, they shed the nitrogen in bodily waste. As a waste product, nitrogen can also be harmful, especially in water. High levels of nitrates encourage algae and bacteria to thrive, disrupting ecosystems and affecting water quality. Plants have a unique ability to recycle that waste into valuable foliage and fruit that is once again eaten by animals and turned back into nitrogen to feed the plants.
There is one family of plants, however, that long ago charted a different course. Legumes are plants that have a symbiotic relationship with a special type of bacteria called Microrrhyzia. The bacteria have the unique ability to pull nitrogen out of the air and store it in tumor-esque growths on the plant roots.
Legume’s unique qualities have given them several ecological advantages over other plants. First, they can survive and thrive in soils where there are insufficient nitrates for other plants to grow. Over time, they gradually build the soil to the point where other plants can gain a toehold.
Second, legumes use the nitrogen in their roots to produce seeds that are unique among plants in their high concentration of the right combination of amino acids to make so-called complete proteins. Their high nutritional content encouraged birds and other animals to eat them and spread their seeds around the world, and finally for humans to domesticate them as crop and forage species.
Among the most common legumes that humans rely on for survival are the ones you probably eat occasionally: beans, peas, and lentils. But forage legumes used to feed livestock, such as alfalfa and clover, are just as important. While alfalfa is not technically “food”, it is grown on more acres of farmland in the U.S. than any other crop. Then there are also dozens of types of leguminous trees, many of which have historically been used as food, such as mesquite and tamarind.
At Terra Firma, we grow fresh peas and green beans to harvest for our customers. In addition to being a delicious addition to our CSA boxes, edible legumes are a critical part of our crop rotation — giving the soil a rest after crops like broccoli or tomatoes. Edible legumes don’t leave much nitrogen in the soil — most it all goes into the part of the plant that is harvested. But they don’t take any out, either.
But the most important role of legumes at our farm is one I haven’t even mentioned yet: as cover crops or “green manure”. Varieties used for this purpose produce lots of lush, nitrogen-storing foliage and roots that we plow back into the soil for the following crop. Growing cover crops is one of our principle ways of improving the soil. But it also provides wildlife habitat, reduces water pollution in storms, and protects the soil from the wind.
For millions of years, our planet had a mostly closed loop of nitrogen use. In the last hundred years, civilization has thrown a wrench into this system by pulling using fossil fuels to pull millions of tons of nitrogen from the atmosphere to make nitrogen fertilizer that is pumped into the soil to grow crops. And instead of being recycled back into beneficials uses, that nitrogen has been treated as a waste product and polluted our ground and surface waters. At some point in the future, this will cause our planet as big or bigger a problem than the carbon dioxide causing global climate change.
Postscript: NPR’s Radiolab did an amazing story on the 19th century scientist who “invented” nitrogen fertilizers that I didn’t hear until after I had finished this newsletter. It has a really shocking ending and is worth checking out: “The Bad Show”. http://www.radiolab.org/series/podcasts/