Nitrogen is a vital nutrient for all plant life. It is a major component in chlorophyll, the compound by which plants take sunlight and CO2 to produce carbohydrates (carbon sequestration!). These carboyhydrates are essential building block for vegetative growth and almost all the foods we depend on.
Nitrogen fixing plants are those whose root systems form a symbiotic relationship with a bacteria (Rhizobium in the case of legumes) that can take atmospheric nitrogen and convert it into a form that plant roots and soil organisms can absorb. When the host plant is cut back or tilled under, the roots slough off their tips or decompose, releasing the fixed nitrogen into the soil for other plants around them to use.
To get the most out of your N-fixers, plant them next to trees and perennials and cut them back at times of the year you want to encourage vegetative growth. It’s a common misconception that the ‘fixed’ nitrogen is in the upper portion of the plant, and that chopping N-fixers down and using as mulch elsewhere will provide nitrogen where they are dropped. Unfortunately this is not the case – you want to plant your N-fixers so their roots are where you will be using the nitrogen in the long run. There is some nitrogen available in the green vegetation, but unless it is immediately composted or incorporated into the soil, the nitrogen will quickly release back into the atmosphere once cut.
Crops like peas and beans that are allowed to ripen seed release significantly less nitrogen through their roots, as those nutrients are sent into producing seed pods. That’s why chop-and-drop of N-fixers work so well, as you hack back the plant several times a year, which prunes the roots and releases that nitrogen, preventing it from being used by the host plant towards fruit and seed.
We have many nitrogen-fixers in the orchard, such as goumi (pictured above on left), sea buckthorn (pictured above top right), ground nut, siberian peashrub, honey locust, false indigo, vetch, clover, peas, and beans, which are strategically planted where the soil could use the extra nitrogen. Our pathways are a mix of clovers and grasses, which our ducks and chickens keep mowed to help build soil that benefits our perennial row crops. Many perennial nitrogen fixers alternate with our trees and berries in our rows, and are hacked back or coppiced annually to keep feeding the plants around them.
Lupines (pictured below) are a favourite at Thimble Hill, as they are a native low-maintenance perennial that provides early-season forage for beneficial insects. We let them bloom each spring and cut them to the ground before they set seed, which releases the nitrogen at a time when nearby perennials and trees are needing those extra nutrients for growth. The cut material makes a great mulch when left in place that helps to retain moisture and keep tree roots cool through the heat of summer, as well as add organic matter to the soil. A word of caution though, cutting back nitrogen fixers in late summer can sometimes promote excessive late-season growth in perennials that won’t harden off enough before winter. Late spring or when plants are dormant are the best times for significant cutting back of nitrogen fixers to give nearby trees a chance to lignify new growth before fall frosts.
Nitrogen fixers are beneficial for building soil and feeding nearby plants, however it is possible to overdo it. Excess nitrogen can over-nitrify your soil, which can end up contaminating nearby water sources. Mycorrhizal fungi will not establish well in soils with too much nitrogen, as the plant hosts do not require the same services from the fungi to source nitrogen. These fungi are a great way to get a long-term supply of nitrogen, phosphorus, micronutrients and water to your plants, so avoiding over-nitrification soil is important. We use these plants sparingly as soil builders in our pathways and to help get new plantings established when they’re putting on heavy growth. But once trees are mature and we don’t need so much vegetative growth, some of these nitrogen fixers will be replaced with other plants. If your soil is lacking in organic matter and nitrogen, these can be a valuable tool. However if you have really rich soil (especially if you use a lot of manure or compost), it is important to monitor your nitrogen levels and exercise moderation.
Many N-fixers are resilient pioneer species that are adaptable to many soils and poor growing conditions because they can produce a lot of their own nutrients. As a result, some can be considered invasive species in certain regions – black locust, siberian peashrub, scotch broom, and autumn olive are examples. If you have these on your land and don’t use them, chopping them back yearly before they are able to set seed inhibits their invasive habit, keeps their roots pruned back to avoid long-distance suckering, and releases nutrients into the soil to benefit other species that want to move in. Some livestock will forage on the leaves, fruit, and seeds of locusts, peashrub, and autumn olive, which can provide a valuable perennial food source while at the same time reducing these plants’ invasive habit through frequent browsing of suckers and seedlings.
When using legumes for their N-fixing abilities in a new area, it’s often recommended to inoculate your seed with Rhizobium bacterium in case it is not present in your soil. I’ve never found this necessary, but you can check to see if the bacteria is present in your soil by finding some wild vetch or clover in the area, pull it up, and look for the small clusters or bumps of the Rhizobium nodules on the roots (see photos below of tiny white nodes on clover roots on left, larger nodes on lupine on right). If the nodes are pinkish in colour, it means the are actively fixing nitrogen, whereas tan or white mean those nodes are no longer active. If you have any forms of these nodes, the Rhizobium bacterium is in your soil and there’s no need to inoculate.
Why bother growing your own N when you can buy it by the bag for cheap? Synthetic nitrogen fertilizers are an environmental nightmare. Leaching of nitrogen into watersheds from excessive fertilizer use in agriculture and landscaping (and the nursery industry) has disastrous impacts for our aquatic ecosystems, as well as the people who depend on that water downstream. In many regions, those most impacted by nitrate contamination in drinking water are disproportionately low income and minority communities. Nitrate runoff overloads aquatic ecosystems which can decimate marine life, and has caused the dead zone in the Gulf of Mexico. Manufacturing of nitrogen fertilizer is a major contributor to climate change. It is made through an energy-intensive process requiring large quantities of natural gas (which come with the footprint of fracking and groundwater contamination) and produce massive amounts of nitrous oxide, a greenhouse gas that depletes the ozone layer and is 265 times more effective at trapping heat in the atmosphere than carbon dioxide.
We should not be supporting the synthetic nitrogen industry when the atmosphere provides this vital nutrient for free. We just need to get our plant and bacteria friends to help us capture it. There are so many great nitrogen fixers that provide this service, as well as providing fruit, food for livestock, forage for pollinators, and biomass for our compost piles. Nitrogen fixation is a fascinating example of symbiosis that reminds us that diversity and cooperation are what make ecosystems function.