Colorful lupines are some of the prettiest garden flowers that add nitrogen to soil.
Nitrogen is one of the most essential plant nutrients, and one of the best ways to boost nitrogen in your soil is to grow nitrogen “fixing” plants. This amazing group of plants naturally add nitrogen into the soil by taking nitrogen from the air and converting it into a usable form in the soil. And many are common garden plants that you may already grow, like peas, beans, bayberry, or clover.
Why Grow Nitrogen Fixers?
Beans and peas are vegetable garden standbys that fix nitrogen. (Image by Jessie Keith)
Legumes, like beans or peas, are the classic example. Experienced veggie growers know that legumes are the ideal crops to combine or alternate with nitrogen-guzzling gluttons such as tomatoes, corn, or melons. What the cantaloupes take away, the scarlet runner beans restore (at least partially).
Of course, the legumes work their magic only if their remains remain in the veggie garden. If you rip out their roots and discard their stems after harvesting your crop, you’ll lose the atmospheric nitrogen that they captured. The trick is to leave the roots in the ground, and to compost or till in the top growth. Your garden will reap a significant nitrogen dividend as a result. Moreover, it’s an ecologically friendly dividend, released gradually as the organically bound nitrogen works its way slowly through the soil’s natural food web.
How Plants Fix Nitrogen
Rhizobium root nodules on bean roots. (Image by Dave Whitinger)
Only a relative few plant species have developed the capacity to split atmospheric nitrogen molecules into individual atoms and to “fix” the freed nitrogen atoms into soilborne compounds that are available to plants. But, these plants don’t do it alone. Nitrogen-fixing plants partner with a narrow range of specially adapted microbes that do the actual splitting and synthesizing of nitrogen.
When present in the soil, these microbes enter a host plant’s feeder roots, which triggers the formation of round nodules. The nodules provide a cozy, nutrient-rich environment for the microbes as they set about converting atmospheric nitrogen into a soilborne form that’s available to plant roots. It’s to this partnership (technically referred to as a symbiotic relationship) that we owe much of our soil fertility, and our food. Not to mention our flowers.
Long before biologists teased out the relationship between plants and microbes and nitrogen cycles, farmers already used nitrogen-fixing plants to boost the productivity of their fields. Although they knew nothing about soil microbes and atmospheric nitrogen, they were well aware that certain plants replenished the soil and enhanced the performance of others crops.
Astute gardeners still use this principle to bring the best out of their soil. Got a garden niche that could use a nitrogenous pick-me-up? Plant a nitrogen-fixer!
Garden Plants that Fix Nitrogen
Red clover is a great cover crop with colorful flowers that bees love.
Many vegetable gardeners think big when it comes to nitrogen fixers. They maximize the dividend by using legumes as cover or rotation crops to be mowed and composted rather than harvested. As a result, the hefty portion (as much as 80 percent) of fixed nitrogen that would have gone to your table will return to the soil instead. Crimson clover (Trifolium incarnatum), small-seeded fava beans (Vicia faba), and garden peas (Pisum sativum) are among the legumes that make first-rate seasonal cover crops. Sow them in early spring or late summer, before or after most other crops are in the ground. Till or compost them at least 4 weeks after the shoots emerge, and at least 3 weeks before sowing another crop onto any tilled areas. For optimum nitrogen fixation, use seed that’s been inoculated with a compatible nitrogen-fixing bacterium from the genus Rhizobium (available from seed merchants and agricultural suppliers). These microbes might not be present in your soil, especially if the cover crop is new to your garden.
White clover feeds lawns and bees! (Image by Ivar Leidus)
Lawns also benefit from nitrogen-fixing plants. Dutch white clover (Trifolium repens) is an excellent case in point. Time was when most lawn seed mixes contained a quotient of this low-growing, nitrogen-packing legume. A superior alternative to the humus-depleting fertilizers that are used on all too many lawns, it provides hungry turf grasses with a steady, sustainable, organic source of nitrogen that won’t contaminate neighboring ecosystems with runoff and leaching. An over-seeding of Trifolium repens in spring or late summer, followed by a light top-dressing of Fafard Premium Topsoil, may be just what your lawn needs if you’re looking to get it off chemical life support. Dutch white clover’s symbiotic Rhizobium bacterium is present in many areas of the United States, but inoculants are available from lawn and garden suppliers if needed.
False indigo is an attractive, nitrogen-fixing perennial. (Image by Jessie Keith)
Perennial, shrub, and tree plantings can also benefit handsomely from the inclusion of nitrogen-fixers, particularly if your soil is too lean to support prima donna plants. Outstanding legumes for perennial borders include false indigo (Baptisia spp.), wild senna (Senna spp.), yellow lupine (Thermopsis spp.), lupine (Lupinus spp.), and leadplant (Amorpha spp.). The roster of leguminous shrubs and trees is also lengthy, boasting such standouts as bush clover (Lespedeza spp.), yellowwood (Cladrastis kentuckea), Kentucky coffee tree (Gymnocladus dioicus), pagoda tree (Styphnolobium japonicum), and Amur maackia (Maackia amurensis). In addition to adding beauty to the garden, these legumes will improve the performance of neighboring plants by adding nitrogen to the soil. As with other legumes, inoculation of their seed or the soil with a compatible Rhizobium bacterium may be required for optimum nitrogen fixation.
Bayberry is a tough shrub that naturally adds nitrogen to soils. (Image by Jessie Keith)
Equally meritorious are a number of non-leguminous ornamental shrubs and trees that host symbiotic bacteria from the genus Frankia. These plants, which include bayberry (Morella pensylvanica), New Jersey tea (Ceanothus americanus), sweet fern (Comptonia peregrina), and alders (Alnus spp.), rank among the best plants for improving the soil and beautifying the garden. Many do not require an inoculant, partnering happily with one or more of the Frankia species that naturally occur in soils throughout most of the temperate and tropical latitudes.