Building Nitrogen from the Soil Up: The Power of Legumes

Soil is the bedrock of any garden. While sun and water are important when it comes to ensuring your garden thrives, a long term consideration for soil health is nitrogen. This is an essential pillar of plant health, and it accumulates over time with proper management by increasing the organic matter in your soil. Gardeners must always consider how much nitrogen their plants need, and more importantly, how nitrogen becomes available to plants.

The air we breathe is approximately 80% nitrogen. So, why do we amend soil to introduce nitrogen for plants? Though there is plenty of nitrogen to go around, not all of it is available to plants. Many organic nitrogen sources need to decay first, in order to become available to plants. Ammonium and nitrate are the most common decayed forms of nitrogen that can be readily taken up by plants. Decay takes time, and in some ecosystems, the soil may be too nitrogen poor to support much plant growth. In lieu of better soil conditions, some plants have evolved to fill this gap. 

Legumes (Fabaceae) have developed a unique relationship with soil bacteria to “fix” non-available nitrogen from their environment. Nitrogen fixation is the term used to describe how these bacteria convert atmospheric nitrogen into bioavailable forms. Legumes develop nodules in their roots which house these symbiotic bacteria. Interestingly, this turns these bacteria into “bacteroids”, which cannot reproduce and are, more or less, fused to the plant.  This symbiotic relationship is incredibly important in agroecology, as it allows for nutrient poor soils to be farmed, and is an essential element of crop rotation. 
Legumes overall improve the soil surrounding them, and allow for a larger diversity of soil microbiota to thrive. This can be contrasted with lawns and industrial agriculture, where depleted soils gain nitrogen primarily through artificial inputs such as fertilizers. Studies have shown that native plants benefit most from soils which contain a large variety of fungal and bacterial life to support them, though these soils may not necessarily be rich in nitrogen. On the other hand, invasive plants tend to do best when there are large additions of nitrogen to the soil. This is one reason we recommend against fertilizing native plantings, and instead, suggest improving soil through natural amendments by including legumes and graminoids. When planning your garden, a helpful reminder to keep in mind is that diverse plant life fosters broader biodiversity.

There are a large variety of legumes. Perhaps the common bean (Phaseolus vulgaris) is most familiar, since its spread worldwide after being cultivated in south and central America. Runner beans (Phaseolus coccineus), another annual bean, are a similar species with bright blossoms and enormous pods. Legumes you are more likely to see in a native plant garden include Canada milk vetch (Astragalus canadensis), wild indigos (Baptisia spp.), leadplant (Amorpha canescens), roundheaded bushclover (Lespedeza capitata), and purple prairie clover (Dalea purpurea). All of these plants help benefit the soil as described above, and are all uniquely beautiful. 

The wild indigos act like bushes, whereas the other species are nice accent plants to support your prairie or forest edge. The Phaseolus species can climb as non-destructive vines, while providing nectar for hummingbirds and interesting (and delicious) seed pods. Due to their higher protein content, legumes are favored by herbivores, so be sure you plant them among native grasses and asters to make them less-than-easy targets. Alternatively, if you are looking for a tree that can improve the soil around it, you may be interested in redbuds (Cercis canadensis) and honey locusts (Gleditsia triacanthos). An important factor to consider with all native plants is whether they are a cultivated type or not. Cultivated native plants, called cultivars or “nativars”, can have a scattering of less-beneficial qualities than their wildtype counterparts. There are many species out there, and always ensure you are selecting a native-type over a cultivar.

Nitrogen fixing plants are a mainstay of many ecosystems and of cultivated gardens. Their ability to unlock an important resource for use is helpful to the soil they grow in, and for the people that steward them. They contribute greatly to soil organic matter and are important sources of food for birds and herbivores thanks to their nutritive qualities. While many people know of ideas like crop rotation, the effects of legumes on their environment is much more storied than that, and they are a key to sustainable gardening. On top of their practical benefits, legumes are some of the most beautiful and interesting plants you’ll find in a garden. From fuzzy leaves to delicate flowers, they are sure to please anyone who sees them. If you’re looking for another native plant to grow, we highly recommend including legumes in your garden.

Written by,

Ben Krueger