Æ : Action Ecology

Top mistakes people make using N fixing plants

and why you arguably don't need nitrogen fixers in your property design (most of the time). 



Like swales, Nitrogen fixing (and other support) species are something that I see as an over-used element in many Permaculture property designs. Of course they absolutely have their uses, but sometimes you need to take step back and appreciate their specific role and purpose in an ecological framework before rushing ahead and planting them everywhere thinking your supercharging your fertility (something I have been guilty of doing in the past). Let me explain.

'Support species’ are one of the core elements / concepts that people are taught in permaculture PDC courses to include when designing fertility into their designs. The idea of course being that having plants and trees that can fix atmospheric nitrogen into the soil for you is a great way to support the nutrient requirements of productive species as well as the overall health of your land without the need for additional inputs (synthetic or otherwise) - it should be essentially ‘self supporting’.

This was certainly the way I used to approach food forest design a few years ago. The rub came however, when I decided (in a noble effort to maximise diversity in my design) to include a mix of these nitrogen fixing species when planting things out, and I started to have questions - questions that we’re not easily answered in the usual reference books and materials I could find to hand / easily search for.

My first point is a reminder of something that many people already know - technically - but seems to get a bit forgotten about, or slip to the back of people’s minds.
It is not the tree or plant that is fixing nitrogen. We all know that right? When we talk about ‘nitrogen fixing species’, we are (usually) talking about a variety of plants and trees etc that form a symbiotic association with specialist bacteria who form nodules on the roots of said plant or tree and fix all that yummy nitrogen for us. 

HOWEVER, in my questioning and search for answers I spoke to severe of the world’s leading research scientists who specialise in Soil Microbiology, Rhizobia and Frankia research among other things and discovered that there are a few key points that people using these plants and trees need to be aware of.

  1. You can’t sick a ’nitrogen fixing’ tree in the ground and expect it to ‘just work’.
    This is the number one thing. It sounds silly even as I write it, but seriously, I have seen this a LOT. People read that a plant or tree ‘fixes nitrogen’ so they get one, they plant it and dust their hands off - job done. Hmmm, no that’s not going to work. In order for nitrogen fixation to occur, you must have not only the correct species of bacteria present around the root zone of your plants and trees (rhizobia for leguminous plants and frankia for actinorhizal plants), but also the correct strain (especially for rhizobia - which are the more commonly used ones). This is because research has demonstrated that different strains may not nodulate on some plants, and some may indeed nodulate but not effectively fix any nitrogen. Some native strains will nodulate exotic plants but only some natives and vice versa and then there’s competitive pressures between how well adapted certain species are to different sites and the whole thing is rather complex and not really very straightforward - but you get the idea hopefully.

  2. If the soil isn't nitrogen poor, you wont fix any.
    If there's sufficient nitrogen in your system already, the plant won't put out the required exudates to attract Rhizobia / Frankia and form symbiosis. It might be worth reading that first line again because this is a pretty important point. When you consider that most soils that people are working with are relatively good temperate climate soils, it should give you pause before planting that tree. Whats more, if you want to grow other productive species in a healthy soil food web, then you could just put your focus on that (getting the biology healthy and nutrient cycling operating well) and the need for nitrogen fixation not only disappears (because your microbes are extracting all they need from the sand/silt/clay/organic matter) but also becomes unavailable because now the nitrogen fixation wont be happening in any meaningful way. 

This is why I started this piece by saying ‘You don’t need nitrogen fixers in your permaculture design” (most of the time).

So when DO I need them? 
Well, if you are working with degraded land with poor soil that for whatever reason you are not able to apply good biology to (because you don’t have the time/resources/know how to create or apply a properly formulated aerobic compost/tea/extract etc), then these pioneer species are perfectly suited to rapidly regenerate land and build biomass. So - as a rule of thumb - I’d say that there’s certainly a use in arid or heavily degraded sites for hardy, fast-growing pioneer species to fix nitrogen and move ecological succession forwards, but perhaps less of a use for them in established agroforestry, orchard or food forest perennial systems where (hopefully) you’ve already got growing/productive trees and thus at least reasonable nitrogen levels in the soil already. 

But how do I make sure they work?
Now here is the crucial bit of information that we’ve probably all been told but seems to slip through the cracks sometimes -
inoculate your plants and trees with the appropriate strain of bacteria. This is mentioned in many of the great forest gardening and permaculture reference books on the subject, but it’s usually one paragraph somewhere on page three hundred and something and - while critically important- doesn’t usually spell out the complexity and importance that underlies this simple step. Exactly how you might do that depends wildly on where you are in the world, but suffice to say that it’s not always easy to find a commercial inoculant for more than common agricultural plants like clover or vetch etc. Going out into the wild and finding a successful specimen that shows clear nodulation (when cut open look red like blood) growing in an otherwise inauspicious location is probably a good bet. Grab a bit of soil from the root zone of that plant/tree and you may get lucky. In this scenario, I'd suggest applying this to a seedling first and seeing if it nodulated successfully, before planting out (which then means you now have a good source of what's hopefully a successful symbioses for future. NOTE: a useful fallback for me (in my location at the time) was sowing inoculated clover seed around Tree lucerne / Tagasaste (Chamaecytisus palmensis) - if that will grow well in your area - as they both are effectively nodulated by the same bacterial strain: R. leguminosarum. 

So while the devil is always in the detail, I hope that this quick overview of the issue will prove useful when it comes time to decide when and if to use biological nitrogen fixation in your designs.

While some of you may find this post a bit controversial - as for a very long time the dominant view was that nutrients in the soil needed to be replenished regularly though inputs by humans, and that nitrogen was a key one of those for productive plants - it becomes less so when you take into account how far our knowledge / the state of the science has moved on over the last thirty plus years in regards to soil biology (specifically nutrient cycling in nature, the soil food web, nutrient stores in the total and exchangeable pool of parent material etc), and the research regarding soil nitrogen levels on nodulation.

For those of you interested in this subject and keen to learn more, I intend to post a longer and more in-depth post on all this in future - so stay tuned.

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A really good interview with world famous author and professor Michael Pollan on how we talk about climate change, agriculture and food - but also how to simplify the solutions.