You’ve probably heard the term ‘nitrogen-fixing’ – it means extracting nitrogen from the air. Which doesn’t seem like it should be too difficult, since nitrogen makes up 78% of air^ref, but in reality plants can’t use gaseous nitrogen.

In nature (ie. where nitrogen is not added artificially as fertiliser) plants mostly rely on microorganisms to help them get nitrogen – they access it in dissolved inorganic forms as ammonium (NH₄⁺) and nitrate (NO_3-). This is the nitrogen cycle, where organic nitrogen from dead organic matter is converted back to inorganic nitrogen as ammonia (NH₃), then ammonium, then nitrate.^ref Although this is performed by a range of different bacteria and fungi, this is NOT nitrogen-fixing, it’s ammonification followed by nitrification.

Nitrogen-fixing is the specific act of extracting nitrogen from the air, and it’s also performed by a range of different bacteria, known as diazotrophic bacteria. Certain plants create symbiotic relationships with these bacteria, with the most effective being root nodule symbiosis. These plants have evolved to provide a safe home for nitrogen-fixing bacteria in their roots, in small nodules where the bacteria live. The bacteria get food from the plant and protection from the outside world, and in return the plant gets nitrogen. Plants which can do this all belong to the ‘FaFaCuRo’ group – Fabales, Fagales, Curcubitales, and Rosales – they are all flowering plants (angiosperms).^ref You can download a database of all 825 known species with root symbiotic nitrogen fixation here – they include green manure such as clover and legumes, as well as some trees – Acacia (wattle), Casuarina (sheoak), Albizia (Persian silk tree), Robinia (locust), Wisteria, Alnus (alder), Elaeagnus (oleaster) and Hippophae (sea buckthorn).

The initial question behind this article was me wondering whether planting clover or similar nitrogen-fixing plants in my bonsai pots would achieve anything – like somehow supplying my tree with a free source of nitrogen. After looking into it further I concluded that the answer is no! Nitrogen-fixing plants have a great system – for themselves. The reason why they are used as green manure, or as rotational crops, is because they don’t require (or require less) supplemental nitrogen, so the land where they are planted gets a break from fertilizer. When they are harvested they can be ploughed back into the ground for bacteria to break down via ammonification/nitrification, so the next crop can benefit from a nitrogen source which hasn’t come from fertiliser. Basically it’s a way of making natural fertiliser – effectively compost – which hasn’t had added fertiliser as an input.

You could benefit from nitrogen-fixing plants such as clover for your bonsai practice – if you composted it and used it as organic matter in your soil mix. In fact it has been found that nitrogen-fixing trees in a tropical forest inhibit their neighbours (possibly due to their stronger growth rates), so you definitely don’t want your trees to share a pot with these species while they are alive.^ref

There is also what’s known as ‘associative nitrogen fixation’ – this is when a nitrogen-fixing bacteria ‘associates’ with a species of plant without actually taking up home in root nodules. They are found on the roots, in the rhizosphere, and sometimes within plant tissues as endophytes.^ref It has been suggested that up to 24% of nitrogen supply to cereal crops such as maize, rice and wheat is actually supplied in this way and that ‘mucilage’ (sugar exudates from roots) may be responsible for attracting the responsible bacteria.^ref Although interestingly it may not actually be that the bacteria provide nitrogen directly, but instead they influence the plant to be able to access more nitrogen in the soil, for example by increasing root hair surface area.^ref This is the mechanism by which biochar improves nutrient acquisition as well – by increasing the plant’s Nitrogen Use Efficiency or ‘NUE’.^ref1,ref2

Which unfortunately brings us back to needing a source of nitrogen in the soil in the first place. What I have concluded is that unless a bonsai tree is a nitrogen-fixing species itself, the only way for it to obtain nitrogen is from the soil via the nitrification of dead organic matter, or by adding chemical fertiliser. And from a sustainability point of view, using at least some dead nitrogen-fixing organic matter (such as legume plants) for composting may be best as this is net-positive for nitrogen, bringing previously inaccessible air-borne nitrogen into the soil (so – go forth and compost your legumes!)

The main impact you can have as a bonsai tree custodian (aside from providing a nitrogen source) is to improve your tree’s nitrogen use efficiency so it can gain the most from the nitrogen which is present. There are a few ways to do this. Adding beneficial bacteria to the soil provides the associative nitrogen fixing effects explained above, and keeping the pot at the requisite temperature, pH, aeration and soil water level that is attractive to these microbes is also a factor – although it’s hard to know exactly what these conditions are! Avoiding extremes is probably the best approach. Adding biochar to the soil is known to improve nitrogen use efficiency.^ref Encouraging a high root surface area through root pruning and encouraging root ramification is another contributor. Finally, do not overfertilise, as this has the opposite effect on root ramification since nutrients are easy to find and roots do not need to increase their surface area.^ref