All Nitrogen Fixers Are Not Created Equal: By Eric Toensmeier

Eric Toensmeier is the author of Perennial Vegetables and the co author of Edible Forest Gardens Volumes 1/2. He has a wealth of knowledge when it comes to food forestry, permaculture design, and useful plants. What follows is an essay Eric recently wrote about the nitrogen fixing capacities of a variety of plants from different regions. This one’s especially good for the plant geek within you! Learn more by visiting Eric’s website

All Nitrogen Fixers Are Not Created Equal

Nitrogen fixing species are a cornerstone of food forestry and other permaculture practices.  Through a partnership with symbiotic organisms in their roots, these plants can turn atmospheric nitrogen into nitrogen fertilizers useful to themselves but also becoming available to their neighbors over time through root die back, leaf fall, and chop and drop coppice management. While it does not replace the need to bring in phosphorus, calcium, and other nutrients depleted by harvests, this strategy provides a free source of an essential fertilizer.

Martin Crawford’s Creating a Forest Garden and Nitrogen Fixing Plants for Temperate Climates are excellent resources for calculating the percentage of nitrogen fixtures needed in order to supply all required nitrogen just from plants. Martin estimates this at 25 to 40% of plants in full sun or 50 to 80% of plants and shade, depending on the nitrogen needs of the crops being grown.

Now I’m going to throw another wrench into your calculations. I’ve known for some time that the amount of nitrogen fixed varies widely among species, but I recently discovered that the USDA plants database gives information about the amount fixed about many, many species native and naturalized to the United States. Check out their advanced search page to select species for your area. They classify species as HIGH (160+ lbs/acre), MEDIUM (85-160lbs/acre), and LOW (1-85lbs/acre). Note that there are a few species that might represent data entry errors. For example Phragmites is listed as a nitrogen fixer, though I’ve been unable to find another reference to this being the case.

It’s interesting to note that many of the most hated naturalized species turn out to be incredibly efficient nitrogen fixers. In fact the “high” nitrogen fixtures category is a rogues gallery of successful dispersive species, like Russian olive (Eleagnus angustifolia), kudzu (Pueraria lobata), and Scotch broom (Cytisus scoparius). I decided to use the database to generate lists of native and non-native nitrogen fixers and categorize them by their efficiency.

Red alder, a “high” nitrogen-fixing native, coppiced with sea kale in my garden

In many cases there’s a strong temptation to use nitrogen fixing species which are also edible. I’d like to point out that if you harvest a heavy crop from a nitrogen fixer, you’ve probably taken most of the nitrogen with you, though this may not be the case with fruits as much as it is with beans and leaves. This is another good reason to use efficient native nitrogen fixers even if they’re not edible. On the other hand, if what you really need is nitrogen there are very few native species in the “high” category, making a good case for use of white clover and other non-natives.Though not all native plant enthusiasts would believe it, I’ve spent decades promoting underutilized native plants. While you may choose to grow non-native pears and peaches in your food forest, there is no particular reason to grow a non-native nitrogen fixer over a native one, with all things being equal. In fact I tend to assume that native plants have a network of visible and invisible relationships with other organisms of all kingdoms, making them more desirable to use whenever possible. I think with the information these databases have provided, we are in an excellent position to promote particular native nitrogen fixers for use in permaculture projects.

With that said, let’s look at a few tables I put together for different regions of the country and then review some of our top native candidates. The astute reader will note that there are very, very few natives in the “high” category. I would speculate that there may be few anywhere, but that they are spreading around very, very successfully.

I’d like to profile a few of these US native nitrogen fixers that have broad range of applicability.

Red Alder (Alnus rubra) grows throughout much of Western North America, particularly near the coast. It coppices readily, at least if you start when it is young and do it frequently. Unlike most alders it does not require wet feet. It can also handle some partial shade. Here in my garden in Massachusetts is killed the ground during winter and re-sprouts up to 10 feet high the following year. According to the database, this is the only tree native to North America that fixes over 160 pounds of nitrogen per acre per year. Though you might think that other alders are equally powerful, the genus actually shows up in the high, medium, and low categories.

White Prairie Clover (Dalea candida) is a native clover of the prairies that extends some into the Eastern Forest region. It is used to make a tea, but had never crossed my mind as a particularly significantly given at all of the hundreds of species that seemed to grow in the prairie. Now I know that the USDA database states that this is the only herb native to North America that fixes over 160 pounds of nitrogen per acre per year. Though it wants full sun and (and can handle fairly dry soils), I’m going to try to find room for some of this little–used native that is deserving of a place in the spotlight. Interestingly, like alder, members of this genus can be found in the high, medium and low categories.

Buffalo Berry (Shepherdia argentea) it is native from New York through California. It is fairly drought tolerant and suckers extensively. It can produce very high volumes of edible fruit, though you need both male and female plants to get it. It can coppice, though again Jerome Osentowski reports that at his site it does not do so reliably. USDA rates this as a “medium” nitrogen fixer. The related S. canadensis and Elaeagus commutataare also “medium” N-fixers and widely native.

I’d love to hear about your experiments using the database (or this article) to select native nitrogen fixers for your area. Myself, I feel like I have a new tool to make sure that the nitrogen fixers I select will be the best available for the job. I also feel like I can make a strong case for growing some native species that are currently very underutilized.

Comfrey For The Garden & Orchard

Comfrey is a plant with a multitude of uses not only in the home apothecary but also in the garden. There are several species in the genus Symphytum, all of which merit special attention, but generally S. officinale, or common comfrey, and a hybrid species S.x uplandicum are most often used. The ladder of which is particularly suitable for the home garden because it is sterile and doesn’t spread by seed. The most available variety is called ‘Bocking 14’. These are upright comfreys which can grow 5′ tall. There are also rhizomatous species which only grow 6″-1′ high and spread to form a dense groundcover.

Comfrey is prized amongst herbalists for its incredible healing powers. Comfrey leaves and roots

Comfrey leaves and roots chopped for making an herbal salve.

contain a high concentration of allatoin– a substance that speeds cell renewal. It got the colloquial name ‘knitbone’ for its use in treating wounds and reducing inflammation from broken bones and sprains.  It’s not a surprise that comfrey fulfills a similar ecological niche working to heal wounded and degraded soils. Comfrey is referred to as a mineral accumulator or dynamic accumulator for its ability to mine nutrients with its deep roots (which also loosen compacted soils). Those nutrients are then deposited in the aerial parts of the plant; being especially high in potassium, phosphorus, calcium, and nitrogen. When the aerial parts die back in the fall those nutrients assimilate into the surrounding soil. Gardeners can facilitate this process by intentionally cutting the plant down at strategic times for use as mulch around fruit trees, berry bushes, or in the veg garden. In the permaculture world we

Close up showing comfrey mulch around trunk of tree.
Comfrey mulch chopped around the base of a young peach tree.

refer to this process as ‘chop-n-drop’. Plant comfrey at the base of your fruit trees and simply chop-n-drop the material right in place; breaking the leaves and stalks into smaller pieces will accelerate decomposition but is not necessary. This is an effective way to build topsoil rapidly and reduce off-site inputs. From my experience I’ve been able to get anywhere from 3-5 cuttings throughout the season. One fella recently told me he cuts his back 7-8 times! I like to wait for the first flush of flowers in early summer because they provide excellent bee fodder.

Preparing comfrey tea brew.
Finished comfrey tea will be slimy on top. Discard slimy material and use as mulch or add to compost.

Another strategy for utilizing comfrey’s amazing mineral accumulating capacity is to brew a fermented comfrey tea. This is a very easy procedure which entails chopping up a few handfuls of comfrey leaves and placing them in a 5 gallon bucket with water. Cover with a lid and let the mixture ferment outdoors for 1-2 weeks. Alternatively, start the batch by pouring boiling water over the plant material and letting it steep for a few hours then adding room temp. water to fill the rest of the bucket. The hot water method seems to extract more of the nutrients. You will know its ready when it has a strong stank…a good stank! Apply with a backpack sprayer diluted or undiluted. This fermented tea can be used a foliar feed or applied directly to the soil.

‘Dwarf Yellow’ comfrey

I grow comfrey around my compost bins and periodically add it to new piles or existing piles to jump-start the decomposition process. I also like to think that the comfrey roots capture any leached nutrients from the compost pile. One reason some people bash on comfrey is because its nearly impossible to get rid of as the tiniest piece of root will put on adventitious buds and sprout into a new plant. Easy to propagate? Ooooh yes! Aside from comfrey’s amazing medicinal qualities and numerous garden uses, it is also a very beautiful plant that can tolerate almost any soil type and will grow well in partial shade. Some ornamental varieties include ‘Goldsmith’, ‘Hidcote Blue’, and ‘Dwarf White’. I would love to hear how you’re utilizing comfrey in your garden! Cheers.

Short video clip: Comfrey Chop-N-Drop Timing

Since adolescent pawpaw trees require dappled shade, comfrey is a great multipurpose companion.
Comfrey and mint at base of young persimmon tree.
‘Bocking 14’ comfrey flowers
Comfrey showing itself in early spring by a compost bin.