The plant families list below focuses on two different ways to helping balance out a garden’s soil: the soil cleaners, and the soil rebuilders. I also include some common landscape plants for the home gardener’s reference, and tips I know from companion planting and permaculture.
Legumes: clover, pole beans, runner beans, bush beans, peas, sugar snaps, and a whole smattering of trees: redbuds, Kentucky coffee tree, honey locust, black locust, yellowwood, mimosas… check out the seed pods on these folks, that’s where the family really tells.
The glory of legumes is their “nitrogen fixing capacity.” That’s how it’s usually phrased, but truth be told, No Plants Fix Nitrogen. There are soil microbes that convert (fix) atmospheric nitrogen into a plant soluble form. There are also plants whose roots make a veritable soil spa for those critters.
The legumes (and a few non-legumes) secrete a particular sugar from their roots, sugar they made in their leaves through photosynthesis. The plants give away some of that sugar; enchanted, the soil microbes go forth and multiply.
After a bit, the soil microbes have converted so much nitrogen that there is extra. The plant’s roots will store the excess nitrogen in the form of nodules on their roots. Those roots stay in the soil and help feed the next crop in.
Nightshades: Tomato, potato, eggplant, peppers (not black pepper like salt & pepper, that’s a citrus(!)), tobacco, petunias, tomatillo.
The nightshades make it in here because of the potato, most specifically because (a) potatoes are hard to interplant with another crop, (b) they are cousins of the tomatoes and so will get every disease the tomato had and then some if planted where those were, and (c) potatoes take up a lot of space. (Unless you do potatoes in a barrel, which looks super cool but I haven’t done it myself yet.)
Potatoes are also heavy feeders: they clean up the soil of any excesses, making it a good spot to put lettuces next year. Thomas Jefferson used them as “cleaners” too. Potatoes were his last crop in before letting a bed lay fallow for two years under clover.
Grasses: (This family used to be Graminae but then became Poaceae. Alas, that’s far less light on the tongue.) Maize*, wheat, rice, barley, oats, rye, millet, sorghum, bamboo, teff… the list is endless. I’ll point out here that rye (annual) and ryegrass (perennial) are two different plants.
Grasses are here for a variety of reasons, mostly to do with cover crops. The one non-cover crop is maize, which can be used as a soil cleaner the way potatoes are, or it can be interplanted in a Three Sisters polyculture.
The cover crop grasses are usually interplanted with a nitrogen fixer. Because grasses have such intensive root systems, any extra nitrogen fixed in the soil by the-critters-that-co-habitate-with-legumes is taken up into the grass roots. When the grass dies (annual grasses killed by cold are the subset of choice for cover cropping where I live), the roots remain the soil.
Over the course of the next year, those roots will break down, releasing that stored carbon and nitrogen into the soil. Where the roots were becomes little pathways of air, much to the delight of the soil critters.
*Why “maize” instead of “corn”? In most of the world, maize is maize and corn is any grain crop. To reduce confusion, and because maize is a fun word, I’m using that nomenclature here because there are so many grains being discussed and so few pictures.