Plot · field guide
Best Nitrogen Fixing Plants for the Garden
Nitrogen is the nutrient your plants need most, and it is the one that runs out first. Bagged fertilizer replaces it, but that costs money and can stop coming. Legumes solve the problem for free. Bacteria on their roots pull nitrogen straight out of the air and lock it into the soil. Best of all, most nitrogen fixing plants feed you too. Here are seven to grow, ranked by how much nitrogen they leave behind.
Photo: Photo: Ezra S F (CC BY)
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Why nitrogen runs out and legumes bring it back
Plants build leaves and stems out of nitrogen. Every crop you harvest carries some away, so a busy garden bed loses nitrogen year after year. Pale, slow, yellow-leaved plants are the sign it is gone.
The air above your garden is about 78 percent nitrogen, but plants cannot use it in that form. Legumes can, with help. Bacteria called rhizobia move into little bumps on the roots, called nodules, and trade the plant sugar for usable nitrogen. Scientists call this nitrogen fixation. For you it means free fertilizer grown in place.
The numbers below come from SARE, the USDA-backed Sustainable Agriculture Research and Education program. They are pounds of nitrogen added per acre. A home bed is far smaller, so treat these as a ranking of which crops give back the most, not a promise for your plot.
Pigeon pea: 40 to 200 lb of nitrogen per acre
Pigeon pea tops the list. SARE puts its nitrogen contribution at 40 to 200 pounds per acre, the widest and highest range of any food legume here. It is a tall, woody plant that shrugs off heat and drought once it takes hold.
The peas are a staple food across the tropics, eaten fresh or dried like any bean. In a warm-climate garden it doubles as a soil builder and a long-season protein crop.
Cowpea and black-eyed pea: 40 to 150 lb per acre
Cowpea, also sold as black-eyed pea or Southern pea, adds 40 to 150 pounds of nitrogen per acre by SARE's figures. It loves heat, handles poor sandy soil, and grows fast enough to slot in after a spring crop comes out.
You get a dependable pot of beans and a green cover in one planting. That combination makes it one of the most useful nitrogen fixing plants for a hot-summer garden.
Peanut: 40 to 100 lb per acre
Peanuts fix 40 to 100 pounds of nitrogen per acre, per SARE. The plant flowers above ground, then pushes its pods down to ripen in the soil, so it needs loose ground and a long warm season of about 120 to 150 days.
The payoff is a high-protein, high-fat food that stores well once dried. Few crops feed both your soil and your pantry as completely.
Fava bean: 45 to 200 lb per acre
Fava beans, also called broad beans, match pigeon pea at the top end, with SARE listing 45 to 200 pounds of nitrogen per acre. Unlike the others here, favas thrive in cool weather, so they fill the fall-to-spring gap when warm-season legumes cannot grow.
Sow them in fall in mild regions or early spring in colder ones. You get an early crop of large, mild beans and a soil charged with nitrogen for the summer garden that follows.
Crimson clover: 70 to 150 lb per acre (cover crop, not food)
Crimson clover is the one plant on this list you do not eat. It is a cover crop, grown only to feed the soil. SARE puts its nitrogen contribution at 70 to 150 pounds per acre, and its red blooms feed bees on the way.
Sow it over an empty bed in fall, let it grow through the cool months, then cut it and turn it in before you plant a heavy feeder in spring. Think of it as the pure soil-building option when a bed would otherwise sit bare.
Lima bean: 30 to 80 lb per acre
Lima beans, or butter beans, add 30 to 80 pounds of nitrogen per acre by SARE's numbers. They want steady warmth and a longer season than snap beans, but they reward you with a rich, meaty bean that dries and stores well.
Grow the pole types up a trellis to save space and pick over a long stretch of summer.
Dry beans: 30 to 50 lb per acre
Common dry beans, the pintos, kidneys, and black beans of the pantry, sit at the bottom of this range with 30 to 50 pounds of nitrogen per acre, per SARE. They fix less than the others, but they are the easiest to grow and store almost forever once dried.
For a survival pantry that also builds soil, a bed of dry beans is a low-effort win even at the small end of the nitrogen scale.
How to actually capture the nitrogen
The nitrogen lives in the roots and the whole plant, not just the pods. If you pull the plant out and throw it away, most of that free fertilizer leaves with it. To keep it, you have to return the plant to the soil.
Time it around a heavy feeder like corn, which is hungry for nitrogen. Grow your legume first, then plant the corn where it grew.
- Cut and drop: chop the plant at soil level and leave the roots in the ground. The roots and their nodules break down right where the nitrogen is.
- Turn it under: cut the top growth and dig it into the top few inches, 2 to 3 weeks before you plant the next crop, so it has time to break down.
- Harvest, then return: pick your beans, then cut the plant and lay it on the bed as mulch instead of pulling it.
- Follow with a heavy feeder: plant corn, squash, or leafy greens next in that spot to use the nitrogen while it is fresh.
Questions, answered straight
Nitrogen fixing means a plant pulls nitrogen gas out of the air and turns it into a form the soil can use. Legumes do it with the help of rhizobia bacteria that live in nodules on their roots. The bacteria trade usable nitrogen for sugar from the plant, so the crop fertilizes the soil as it grows.
It depends on the crop. SARE lists ranges by the pound per acre: pigeon pea 40 to 200, cowpea 40 to 150, peanut 40 to 100, fava bean 45 to 200, crimson clover 70 to 150, lima bean 30 to 80, and dry beans 30 to 50. A home bed is much smaller, so use these as a ranking, not a promise for your plot.
Only if the right bacteria are not already in your soil. Where legumes have grown before, they usually are. In new or poor ground, coating the seed with a matching rhizobia inoculant, sold cheaply in garden stores, helps the plant form nodules and fix more nitrogen. It is a small cost for a bigger payoff.
Partly. Legumes add real nitrogen, but only if you return the plant to the soil by cutting and dropping it or turning it under. They add little phosphorus or potassium, so poor soil may still need those. Treat nitrogen fixing plants as one free tool, not a full replacement for feeding your soil.