What are legumes?
Legumes (Lay-gooms) are plants in the Fabaceae (fuh-BAY-see-ay) family. With distinct elongated pods and many common edible species they are easily recognizable. Think sugar snaps, sweet peas, green beans, edamame, and peanuts. Also clover, alfalfa, lentils...so many to choose from.
Why are they so awesome?
Farmers long ago must have noticed that plants grown in fields previously planted with legumes grew bigger and healthier. Whether they understood that bacteria adapted to live on legume roots are able to sequester atmospheric nitrogen and convert it into plant food (aka nitrogen) remains unclear. It was enough to know that if tilled under, legume plants "fertilize" the soil for the following crop.
Side note on nitrogen. It is an essential ingredient in the proteins that make up your DNA. It is also the essential ingredient in factory-made fertilizers and TNT. For a molecule it has a lot of trapped energy, which is why it is used for explosives, why it makes plants grow tall, and perhaps why my mom's white bean chili (packed with meat proteins and legumes) is so damn satiating.
It is easy to see how legumes, which produce a high protein food source and fertilize the soil, became a major player in our diets and agricultural landscapes. Before the invention of factory-made fertilizers, farmers relied on legumes - or better yet the bacteria that live on their roots - as well as manure, rainwater, floods, ground up bones, and blood to restore the fertility they removed with each harvest.
The Nitrogen Cycle
Have you ever seen illustrations of the nitrogen cycle? I will never forget the first time I opened to the nitrogen cycle page in a text book. There is usually a lightning strike and a horse or cow in a pasture, plus some microscopic soil organisms.
Lightning is depicted because this is one of the short list of ways that atmospheric nitrogen can enter the soil. This gives us an idea of just how much energy it takes to convert atmospheric nitrogen into fertilizer.
They never show the horse or cow actually pooping but it is their way of saying "everybody poops." Manure is nitrogen rich; in concert with legume cultivation, humans successfully grew enough food with these sources to reach a population of around 2 billion.
In the early 1900's scientists hacked the nitrogen cycle and figured out how to extract nitrogen from the atmosphere without having to grow legumes . It's called the Haber-Bosch process. It works, so well in fact that we have drastically altered the amount of nitrogen in our soils and waterways, so well that our population went from a few billion to whatever we're at now? 7 point something billion? This process, discovered by a German scientist around WWI, boosted weapons productions and later became the source of nitrogen for farms across the globe.
Can't help but mention a bit of irony here. The Haber-Bosch process, as it's called, earned Fritz Haber a Nobel Prize in chemistry. It made it possible to both kill (with artillery) and feed (with fertilizer) the masses on a previously unimaginable scale. Haber was a Jewish German chemist who later helped develop chemical warfare for the Kaiser. The same chemicals were later used on his Jewish relatives in the holocause. His wife, also a chemist, shot herself in the heart with his military pistol in the garden. As the story goes, she had qualms with chemical warfare.
Now back to legumes, nitrogen, and gardening
The Haber-Bosch process requires massive amounts of input to create ammonia based fertilizers. The process is too energy intensive to be reproducible on the village scale and with overuse has many negative environmental impacts, including dead zones, destruction of soil biology, factory explosions, and nitrate contamination in wells.
Interestingly, we happen to excrete a nitrogen rich by-product of cellular metabolism on the home scale. When diluted with water it makes a decent fertilizer. It is called urine. You won't get a Nobel prize for peeing in your garden, but you will save a trip to the store and honor an age old tradition.
And if that doesn't suit you, there are always legumes for improving soil fertility. These are some of my favorites:
Scarlet Runner Beans, Sugar Snaps, Haricot Vert, Rattlesnake Pole Beans, Fava Beans. For winter cover crops try vetch, bell beans, field peas.
Note that legume plants add the most nitrogen to the soil when forked or roto-tilled under while still vibrant. Most sources recommend tilling legumes under while they are flowering. Many small scale gardeners I know will chop, remove, and compost the aerial portions of the plants, leaving the roots (and their bacterial nodules) to decompose. Without a rototiller it's a ton of work to chop or mow and til under fava bean plants. Vetch and clover are short and more easily turned under by hand.
Most soils I've worked with are already inoculated with the Rhizobium bacteria and others that make legume roots their home and "fix" atmostpheric nitrogen. To test your soil, go out in your yard with a hori-hori and carefully dig up some clover or a wild growing legume. Look for pinkish nodes on the roots. If present you can go without purchasing an inoculant, if not, find some powdered bacteria for this purpose. They are available at most good nurseries. You don't have to do this every time. The bacteria will establish themselves in your soil if they haven't already.
Seed Saving and Final Notes on Legumes
Hopefully this legume primer has been useful, I put off replacing a sprinkler valve to do this instead. Here are some important notes on legumes for those who wish to save seed:
Mendel, the monk who cleverly demonstrated evolutionary principles of inheritance, chose peas (a legume) to conduct his research because they are mostly self-pollinating, aka "selfers." This means that their flowers have boy and girl parts (a condition known as "perfect" in botanical nomenclature) which allows them to mate with themselves without developing issues usually associated with inbreeding.
For all intents and purposes, individuals of a given variety, say High Scent Sweet Peas, can be considered clones of one another, which is why they say "two peas in a pod" to describe two of the same. All the peas in the pod are nearly genetically identical, and there is little diversity between individual plants.
For seed saving purposes, this means that you can start a new population from just a few individual seeds without compromising the genetic diversity of the population. You will lose some, but all in all the line will continue.
Because they pollinate themselves, different varieties rarely cross (hybridize) with one another without intervention from say, a curious monk, so they naturally make a good control plant for experiments. This also means that you can plant different varieties (or colors) near one another and their seeds will produce the same color flowers as their parents. Recommended isolation distance is 25'. I've used this as a guideline and never noticed any crossing, yet.
TO SAVE SEEDS, simply grab some buckets or bowls or whatever you can find and gather the dried bean or pea pods. Wait until they're brown and snap off easily, though some legumes will "shatter" in hot weather so better to pick a little early - like before you go out of town for a wedding - than lose everything to the mice, rats, squirrels, and crows.
After dinner one night, sit down with loved ones or complete strangers and separate the beans from the pods. It's strangely enjoyable. Discuss current events, love lives, or lack thereof until you're left with a bowl of empty pods and a bowl of clean beans, plus a third bowl for discards. What are discards? If the thought "should I save this one?" enters your mind as it rests in your hand just go with your intuition and discard it. That goes for really small seeds, diseasy looking seeds, or ones that just look at you wrong. Size does matter in seed saving and is a good indicator of vigor.
I dry my peas and beans on a screen for another week or so, occasionally running my had thru the seeds just because it feels good, then I store in a glass jar in the fridge until it's time to plant again. If you want to make sure weevils do not eat your seeds, freeze seeds in the jar for a week. Make sure to let jar return to room temperature to prevent condensation on the seeds.
Hope this is helpful. Happy legume planting.