If you cultivate a vegetable garden, a flower garden, or a small farm, sooner or later you realize that the Nitrogen is the fuel for plant growth.When nitrogen is lacking, plants turn yellow, grow slowly, and produce less. The good news is that you don't always need to use bags of chemical fertilizer: many species can capture nitrogen from the air or mobilize nitrogen deep in the soil and make it available to the roots.
Over the past few years, a great deal of research has been done on nitrogen-fixing plants and soil-improving speciesThis applies to both small-scale organic farming and large farms linked to the CAP and the well-known greening payments. Furthermore, their effects on arid ecosystems, their role in biodiversity, and how to use them in crop rotations, cover crops, and green manures have been studied. We'll weave all this information into a single, practical, and comprehensive article so you can make the most of them on your land.
What does it mean for a plant to contribute or fix nitrogen to the soil?
When we talk about plants that "provide nitrogen," we don't always mean the same mechanism, but they all help to ensure there is More nitrogen available for crops and less dependence on fertilizersIt is advisable to distinguish between several groups in order not to mix concepts.
On one side there are the plants symbiotic nitrogen-fixing organismswhich form nodules on the roots with bacteria of the genus Rhizobium (and others similar): alfalfa, broad beans, peas, lentils, clovers, lupins, soybeans, peanuts, etc. These bacteria transform atmospheric nitrogen (N₂) into forms that can be assimilated by plants.
Secondly we have the plants non-symbiotic or not associated with other types of bacteriaThese also contribute to soil stabilization but without such visible nodules. This group includes, for example, species such as alder, casuarina, and ceanothus, which are widely used in soil restoration and as barriers around crops.
Finally, there is a group of plants that, although they do not fix nitrogen from the air in the classic way, They "pump" nutrients from deep layers or recycle nitrogen which would otherwise be lost, thanks to their deep roots and the large amount of biomass they generate (comfrey, turnip, fodder radish, sorghum, oats, etc.).
Key benefits of using nitrogen-fixing plants
The first positive effect is that these species allow improve soil fertility Unlike many mineral fertilizers, biological nitrogen fixation has no environmental costs. It is a natural process, supported by soil bacteria and fungi, that enriches the soil with a slow and constant "fertilizer."
Another very interesting point is that they facilitate the reduction in the use of chemical fertilizersThis translates into medium-term cost savings and a reduced risk of water contamination from nitrate leaching. Especially on professional farms, introducing legumes into the crop rotation can make a significant difference to the bottom line.
These plants are also a basic tool in the crop rotation and organic farmingAfter a crop that is very demanding in nitrogen, such as winter cereals or corn, introducing a legume or a green manure rich in biomass helps to recover the soil and prepares the land for the next season.
If we look beyond the individual plot, soil-fixing and soil-improving species contribute significantly to the biodiversity, erosion prevention and restoration of degraded soilsMany are used as plant cover that protects the surface from rain and sun, stabilizes slopes, and provides shade and food for beneficial fauna.
Types of nitrogen-fixing plants and their role in the garden
In the vegetable garden and in extensive agriculture we usually work with two main categories: nitrogen-fixing legumes and non-leguminous species that, nevertheless, participate in the fixation or mobilization of this nutrient. Each group has different uses and management practices.
Among the most typical legumes we find those that are already well-known in any kitchen: beans, lentils (which can be used as liquid manure), peas, broad beans, kidney beans, chickpeas…All of them can function as a crop for human consumption while working underground with their symbiotic bacteria.
At another level are the forage and cover crop legumessuch as alfalfa, clover, sainfoin, sulla, vetch, fenugreek, or lupins. Their main role is not so much to produce grain for the market as to generate abundant biomass and enrich the soil, in addition to serving as pasture for livestock.
On the side of non-leguminous species relevant to nitrogen fixation or recycling are the alder, casuarina, ceanothus and other pioneer treesIn addition to plants such as buckwheat, lambsquarters or dandelion, which have been observed fixing nitrogen or associating with microorganisms that do so.
Examples of legumes that enrich the soil
If we're talking about plants that fix nitrogen symbiotically, the list is extensive, but there are a few. key players worth getting to know well because they are the most used and are also recognized in regulations such as green payment.
The broad beans (Vicia faba) They are a classic in winter gardens: they withstand the cold well, develop strong, deep roots, and produce a large amount of aboveground biomass. They work great as a food crop and, at the same time, as a soil improver thanks to nitrogen fixation.
The peas (Pisum sativum) And while different types of beans offer tasty and abundant harvests, their true treasure lies underground, in the nodules teeming with bacteria. When their cycle is complete, if the roots are left in the soil and the crop residue is incorporated as mulch or green manure, they release a significant portion of the accumulated nitrogen.
In extensive crops or as forage, the queen is the alfalfa (Medicago sativa)Its very long roots associate with soil-fixing fungi and bacteria, and the plant produces an enormous amount of green matter. Furthermore, it contains triacontanol, a substance that acts as a growth stimulant in other species, so a alfalfa infusion It can work as a homemade biofertilizer.
We must not forget the role of clover, sainfoin, sulla, vetch, fenugreek and lupinsThese are used as living ground cover in meadows, vineyards, orchards, and rotational grazing systems. They simultaneously protect the soil, provide nitrogen, and offer food for pollinators and beneficial insects.
Non-leguminous plants that also provide nitrogen and improve the soil
Although we usually associate nitrogen fixation with legumes, there are non-legume species that also do an impressive job improving the soil, either by mobilizing nutrients from deep layers, generating large amounts of organic matter, or fixing nitrogen in a less well-known way.
A striking example is the buckwheat or buckwheat (Fagopyrum esculentum)This plant, like lamb's quarters or dandelion, can contribute to nitrogen fixation without being a legume. Furthermore, its seed is highly nutritious and valued in human food, thus combining agronomic and economic interest.
El lamb's quarters (Chenopodium album) It has a very vigorous root system that draws nutrients deep into the soil and a tall stature that protects other plants from the wind. Its leaves can be eaten like spinach, and the seeds have traditionally been used in baking. The roots contain saponins, so they can even be used as natural soap.
Among the large biomass plants are the sorghum or Guinea maize (Sorghum halepense)oats and rye. All of them create a network of roots that loosens, aerates and structures the soil, provides carbon and leaves a thick cover that releases nutrients, including some of the accumulated nitrogen, as it decomposes.
There are species like the comfrey (Symphytum officinale) They literally act as nutrient pumps: their roots reach deep and draw up nitrogen, potassium, calcium, magnesium, and other elements from areas where most crops cannot reach. Mulching with their leaves is one of the most valued practices in organic horticulture.
Companion plants that protect, attract beneficial fauna and contribute biomass
Beyond direct nitrogen fixation, many companion plants help to make the soil more nutrient-rich. more fertile and resilient thanks to its biomass, roots and effect on beneficial faunaThey are interspersed among main crops or allowed to grow wild on the margins.
La borage (Borago officinalis) And calendula (Calendula officinalis) are great allies in the garden: they develop deep roots, produce abundant foliage, and attract a multitude of pollinators and other beneficial insects. Borage also has edible leaves that add texture to soups and stews, and calendula is used to make skin creams.
La nasturtium (Tropaeolum majus) It covers the ground very well with its creeping habit, protects against erosion, and offers edible flowers with a slightly spicy flavor. Its flowers also attract beneficial insects, reducing pest pressure on nearby crops.
Other interesting “living ground cover” species include the purslane (Portulaca oleracea), very resistant to drought and edible raw, and the cosmos (Cosmos bipinnatus), which forms a veritable carpet of flowers that serves as shelter and food for a multitude of beneficial insects.
plants like the sunflower (Helianthus annuus) They have a mixed role: they provide a good windbreak, act as a support for climbing species such as vetch, and contribute a significant amount of biomass when shredded and returned to the soil, in addition to giving us the prized sunflower seeds.
Deep roots, green manures and breaking up compacted soils
A very valuable group of plants in any agroecological system is the one formed by species with taproot or very deep root systems that are capable of breaking compacted layers, improving subsoil drainage, and bringing nutrients to the area where crop roots explore.
El turnip (Brassica rapa) And forage radish is excellent for this: they produce thick roots that penetrate the soil and naturally break it up, while accumulating nitrogen absorbed from lower layers. When the crop is cut and incorporated into the soil, much of that nitrogen is gradually released.
La white mustard (Sinapis alba) It forms large plants with powerful roots that also help to loosen compacted soil. Its bright yellow flowers attract a legion of beneficial insects, a highly valued trait for ecological pest control.
Among the cereals, the rye (Secale cereale) It is famous for its ability to create a dense mulch that benefits the subsequent cultivation of legumes. At the same time, its fibrous root system works the soil, aerates it, and prevents erosion. Oats perform a similar function, with the added advantage of being an excellent forage crop.
El melilotus (Melilotus officinalis) and other clovers with yellow or white flowers, in addition to nitrogen-fixing legumes, create a significant aboveground mass that, if used as green manure, returns a large amount of organic nitrogen and other nutrients to the soil.
Trees associated with the orchard that fix or mobilize nitrogen
It's not all about herbaceous plants. Many diversified garden and farm designs also incorporate... trees and shrubs capable of fixing or mobilizing nitrogentaking advantage of its wood, its shade and its shredded leaves as mulch.
El alder (Alnus cordata and common alder) It is a good example: its ability to associate with nitrogen-fixing bacteria has been studied and it is often used in riverbank restoration or as a windbreak, while enriching the soil with nutrient-rich leaf litter.
Other trees linked to soil improvement and nitrogen fixation include the carob tree, false acacia, silk tree, and Judas treeMany of them produce melliferous flowers, provide shelter for wildlife, and when pruned, the chopped branches can be used as nutritious mulch around fruit trees and perennial crops.
This combination of tree layer and vegetable garden crops It creates small agroforestry systems that combine shade, wind protection, recovery of poor soils, and food production for people and animals.
Greening and nitrogen-fixing crops
Within the Common Agricultural Policy, the so-called green payment or greening It is financial aid granted per hectare, linked to basic payment entitlements, provided that the farm respects certain practices beneficial to the environment.
These practices include the crop diversification according to farm size, the maintenance of permanent pastures and the presence of ecological interest areas (EIAs), which include fallow land, green cover, forested areas and, very importantly, plots dedicated to nitrogen-fixing crops.
Not all nitrogen-fixing species count for greening purposes: only those that are considered are. intended for human or animal consumptionThe list includes crops such as beans, chickpeas, lentils, peas, broad beans, lupins, carob, grass peas, vetch, bitter vetch, fenugreek, broad beans, alfalfa, sainfoin, sulla, clover, soybeans, and peanuts.
In order for these areas to be calculated correctly, the plants must remain in the field at least until the start of floweringAnd if they are sown mixed with other non-fixing species, the nitrogen-fixing component must make up more than 50% of the mixture. Furthermore, it is not permitted to leave the plot fallow immediately after a nitrogen-fixing crop, to avoid nitrogen losses through leaching.
Another key requirement is that, when a plot with nitrogen-fixing crops is declared as an IE, Plant protection products cannot be used From preparing the land for sowing until after the harvest (or throughout the entire cycle in perennial crops). The farmer must declare this and assume this commitment when processing the CAP application.
How to integrate these plants into rotations, cover crops, and green manures
In practice, the best way to harness the full potential of these species is by properly organizing the crop rotation, cover crop mixtures, and the use of green manures, depending on your climate, soil type and production goals.
A very common strategy is to sow a legume or a mixture of legume and cereal after a nitrogen-demanding crop such as corn or wheat. At the end of the cycle, the cover is mowed and the biomass is left on the ground, or it is incorporated slightly, so that the fixed nitrogen is available for the next crop.
In vineyards, berry orchards, and fruit orchards, clover, vetch, or mixtures with grasses are commonly used as permanent ground cover. These cover crops allow fix nitrogen, protect the soil and facilitate the passage of machineryall at once, while also controlling erosion.
In small home gardens, you can experiment with bands of flowers like calendulas, borage, nasturtiums, and cosmos, interspersed with legumes and leafy vegetables or cucurbits, which are particularly demanding of nitrogen. In this way, a very productive and balanced mosaic is created with fewer problems with pests and diseases.
Another very interesting technique is the use of green manure During fallow periods: instead of leaving the plot bare, mixtures of broad beans, vetch, oats, rye, mustard or turnip are sown, harvested before the seed ripens and left to dry on the ground, then incorporated or kept as mulch.
Optimal conditions for maximizing nitrogen fixation
For soil-fixing and soil-improving plants to work at full capacity, it is not enough to simply sow them: it is necessary respect certain soil, climate and management conditions that facilitate the growth of the bacteria and fungi involved in the process. In general, these species thrive in well-aerated soil with good drainage and a near-neutral pH. Excessively acidic or highly compacted soils should be amended with organic matter and, if necessary, with limestone.
Most root-fixing plants need plenty of sunlight and moderate to warm temperatures to express their full potential. Even so, there are species like broad beans or some clovers that tolerate the cold well and can be used in winter to add nitrogen when other crops aren't even considering growing.
Introduction of bacteria into the soil
In some legumes, especially when introduced into soils where they have never been cultivated, it is advisable to carry out a inoculation with specific bacteria of the genus RhizobiumThis simple step can multiply the amount of nitrogen fixed and ensure the presence of active nodules.
Finally, it is advisable to avoid excessive mineral nitrogen fertilization in plots where high nitrogen fixation is desired, since If there is plenty of nitrogen available in the soil, the plant "relaxes" and stops investing resources. in its microbial partners, reducing nodule formation.
In general, these species thrive in soils with good structure and organic matter; furthermore, facilitating the growth of beneficial microorganisms—such as certain fungi and bacteria—improves soil fixation. To learn more about the role of beneficial fungi in the soil, review information on beneficial soil fungi.
Combining traditional gardening knowledge with what modern science tells us, it becomes clear that Investing in plants that fix or mobilize nitrogen is a sure betThey increase soil fertility, reduce dependence on external inputs, improve biodiversity, and are suitable for both small family gardens and large farms subject to green payments. Integrating them intelligently into crop rotations, cover crops, and agroforestry systems is probably one of the simplest and most powerful ways to care for the land while continuing to obtain good harvests.