Are ‘dynamic accumulators’ a thing?

‘Dynamic accumulators’ are often promoted as an essential ingredient of organic gardening and forest garden design. I’ve planted them myself, but the longer I grow my forest garden the less I find there to be any point to them, so I’d like to try to answer three questions in this article. First, what is a dynamic accumulator anyway? Second, do they work as advertised? And third, do you need them in a forest garden?

What is a dynamic accumulator anyway?

Let’s start with Wikipedia. According to the entry current at the time of writing, “Dynamic accumulators are plants that gather certain micronutrients, macronutrients, or minerals and store them in their leaves.” The trouble is that there’s a shorter term for this kind of plant. It’s a “plant”. All plants do this so by this definition dynamic accumulators aren’t a thing.

Trying to get back to the source of the term, all roads seem to lead to Robert Kourik’s book Designing and Maintaining Your Edible Landscape—Naturally (1986). Kourik included a list of ‘dynamic accumulators’ – plants that he understood to be high in particular nutrients . The purpose of the list was to give some guidance as to what different plants might be contributing to the compost heap and which might be best for producing high-value compost. The trouble is that Kourik himself seems to have disowned both the term and his original list. For more detail, see The Facts About Dynamic Accumulators, an excellent article from the Permaculture Research Institute tracing the origins of the term.

Dynamic accumulators, then, rather resemble the monster in Mary Shelley’s Frankenstein. Rejected by their creator, they roam the countryside looking for purpose and acceptance. Have they become, as Frankenstein’s creation eventually did, embittered and malignant or have they, as the monster dreamed of doing, found their own way to becoming a useful and accepted member of society? Let’s move on to how those authors who still use the term, almost all of them in the field of permaculture, define it. Taking a survey of these, three key ideas seem to contribute: hyperaccumulators, nitrogen-fixers and deep-rooted plants.

Hyperaccumulators are used in the fairly well established field of biological remediation of polluted soils, in which plants are used to pull pollutants out of the soil, before being cut down and disposed of at a hazardous waste site. As a result there is a reasonable body of research into plants that are particularly good at taking up different nutrients and these have acquired the term ‘hyperaccumulators’. Incidentally, this is the only use of the term ‘dynamic accumulator’ that I can find in the scientific literature.

Nitrogen fixing is quite uncontroversial. Some plants form an association with bacteria that allow them to turn nitrogen gas in from the air into forms of nitrogen that plants can use as fertiliser. Most of these are in the massive legume family (Fabaceae), which runs from weedy climbers to great trees. A few, such as alder (Alnus) belong to other groups collectively known as the actinorhizal plants.

Finally, plants with deep roots are often proposed as dynamic accumulators, with the idea that they will take up nutrients from deep layers of the soil, otherwise at risk of being washed out of the system entirely, and return them to the surface layers where they will become available to other plants, either through leaf fall or through the gardener actively cutting them for mulch or compost.

Some definitions of dynamic accumulators take them to be identical to just one of these groups. Others, including the one on Wikipedia, attempt to ram them together into one concept, ignoring the fact that few plants belong to all three groups and the most popular ‘dynamic accumulators’ almost never do. Is this a case of grouping together ideas that would have much more clarity separately or is there a fuller picture that the all contribute to? Let’s find out in practice.

Stinging_nettle_01_-_Green

Nettles are high in nitrogen, sulphur and magnesium

So do they work?

In some places dynamic accumulators seem to be assumed to have almost magical properties. In one blog I read the author expressed confusion at the fact that her crop plants were struggling despite the large and vigorous dynamic accumulators she had growing all through them. It’s almost like trickle-down theory for plants. Dynamic accumulators are imagined to create nutrients out of nowhere and share them freely with surrounding plants. In reality they are almost by definition plants that excel in grabbing nutrients and keeping them for themselves. Whisper it, but another name for dynamic accumulator might be ‘weed’. If we want to put them to good use it will have to more intelligently than this.

Using plants as a means of moving nutrients around requires us to ask some questions. Which nutrients do we want to move? From where? To where? What for? However you define them, dynamic accumulators are plants that take up space, light and water. Unless we can give good answers to these questions there is no point in using them.

The hyperaccumulator model

This is why the comparison to hyperaccumulators is misleading on several counts. Firstly, the hyperaccummulators that have been researched so far have been identified for their ability to accumulate problematic, toxic elements, not the ones we want in our gardens. There might be equivalents for the more useful nutrients or there might not be, but in any case the hyperaccumulators so far identified aren’t much use for productive gardening.

Secondly, we should remember that hyperaccumulators are used for taking chemicals out of soils. Where there are toxic levels of, say, zinc in a soil it makes sense to use plants that can hoover it up into their tissues in order to remove it from the soil. In vegetable gardening we are trying to put nutrients in to the soil. If we get those nutrients from the soil in the first place then we are merely moving them from soil to plant and back again to no obvious purpose. A futher point here is that you generally want to add the elements that your soil is deficient in – but these are exactly the ones that you won’t be able to accumulate from your soil as they aren’t there. These might seem like rather obvious points but it’s not uncommon to see dynamic accumulators recommended as if they are some sort of fertiliser that simply needs to be added to the soil.

Finally, one more problem with the hyperaccumulator model. Most hyperaccumulators are good for just one or two specific elements, but what your plants need is a balance. Things could therefore get very complicated as you try growing a whole range of dynamic accumulators in the attempt to get a balanced nutrient profile. There might not be much room for crop plants. Mike H, on the blog One Thing Leads To Another has put together a list of plants that at least have higher levels of a broad range of plant nutrients in ther leaves than others. Unfortunately it consists entirely of plants – generally considered as weeds – that either very or relatively shallow rooted, meaning that they will only give you back nutrients that were already in the top layer of your soil.

Green manure and nitrogen fixers

The concept makes a little more sense when we consider ways of using plants to move nutrients from places where our crop plants can’t use them to places where they can. One is deep in the soil. The other is the atmosphere.

There is a long tradition in farming and gardening of growing ‘green manures’, plants that are grown not for eating but to be ploughed in to the soil like manure to feed a crop the next year. Most green manure plants are in the legume family as they can do what few other plants can – pull nitrogen directly out of the atmosphere and use it as food. The nitrogen then becomes available to other plants when the green manure dies or gets turned in.

Green manuring is a well established practice and definitely works. It’s possible to use the term ‘dynamic accumulation’ to describe what’s going on as the plants are definitely accumulating a crucial nutrient in an active way, but does the term actually add anything to the established idea of green manuring? I’d suggest that it simply confuses the matter, conflating nitrogen-fixers with hyperaccumulators and deep-rooted plants.

Deep-rooted plants

The other place that some plants can access nutrients that others can’t is from the deep soil horizons. I first came across this idea during my forestry degree, reading Forestry Commission research papers (see here, here and here) describing how silver birch improves poor soils by bringing up nutrients from deep soil layers and depositing them through leaf fall on the surface.

However, I also thought I’d have a quick look at a site I often find useful as a check on ‘everyone knows’-type facts: Robert Pavlis’s Garden Myths. Pavlis questions the idea that deep-rooted plants get significant amounts of nutrients from deep in the soil, quoting Robert Kourik as saying that “…. some plants are more efficient at absorbing some nutrients compared to others. Is this due, as many gardeners assume, to deep roots or is it due to more efficient accumulation at surface soils. This remains a grossly unresearched dynamic.”

There are two points here. The first is that absence of evidence is not evidence of absence. The area may indeed be grossly under-researched but that doesn’t mean that we can just dismiss it. I have found plant roots below 1.2m in my garden. They must be doing something down there after all and with herbaceous plants it can’t be about stability. The only explanation I can think of apart from nutrient capture is that they are storing nutrients down below the reach of most things that might eat them, ironically in the reverse of what dynamic accumulators are meant to do! The second point is that deep-rooted plants don’t have to be getting the majority of their nutrients (I doubt if they are) from the deep soil in order to be doing a useful service to the system as a whole.

Dynamic accumulators in practice

So, finally, are dynamic accumulators any use in the forest garden? Let’s break that down in to its three aspects.

First, plants that accumulate particularly high levels of some nutrients in their leaves are of no particular use. They simply take up growing space that would be better used productively.

Second, nitrogen-fixing green-manures are useful, but there is no good reason to call them dynamic accumulators. In the forest garden there is the opportunity to plant perennial green manures that are active the whole season round rather than just the usual annual ones. When I visited Graham Bell’s forest garden in the Borders he was growing Laburnum, one of the few nitrogen fixing trees to flourish in Scotland, for composting material.

Laburnum

Laburnum

Third, deep-rooted plants may well have a role to play in preventing nutrient loss from the system as a whole, but I no longer see much point in planting anything specifically for this property as so many of the crop plants that you can use do it anyway. This has been my experience many times over with the forest garden. I began by planting some plants for crops and others as hoverfly attractors, wildlife plants and ornamentals. As the garden matured and the range of crops expanded I realised that the crop plants were fufilling all these other roles for free. It is the same with deep rooted plants. Crops like sweet cicely, udo, horseradish, monk’s rhubarb and no doubt many more have very deep roots and are useful for harvesting biomass as well as the desired crop. In fact I have come to think of these species as dual-use plants, providing both an edible crop and compost for the hungrier species in the garden.

I do still have a few comfrey plants around, including a clump planted next to my ‘toxics’ compost bin. This bin takes the tattie haulms, brassica roots and any diseased materials. Rather than returning directly them to the soil I just let them break down and be absorbed by the comfrey, which grows at a phenomenal rate and can be cut several times a year. In the future, however, I might replace even this with a crop plant.

SONY DSC

comfrey-triffid

SONY DSC

bee enjoying the comfrey flowers

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13 thoughts on “Are ‘dynamic accumulators’ a thing?

  1. Hi Alan. Great article. Very informative and well-researched. The only comment/question I have is in relation to your mention of perennial nitrogen-fixers such as laburnum. My understanding is that fixed nitrogen is only in the soil temporarily until the plant is setting seed, at which point it is rapidly drawn up from the nodules to generate the large and numerous seeds. That’s why legumes are such a good source of protein. The nitrogen comes free in the sense that the plants fix from atmospheric nitrogen but as gardeners we can only benefit from it once, either in the soil by digging in plants before flowering, or by eating the peas or beans. Eating seeds is not an option for laburnum of course and neither, it being a tree, is turning in before flowering. The only benefit for your friend that I can see (apart from the visual pleasure of laburnum when flowering) is for him to compost laburnum seeds. The nitrogen is otherwise inaccessible or undigestible!
    Just musing!

    • Hi Ian. Good question, to which I don’t have an immediate answer! Graham was pruning the trees quite heavily so they weren’t getting a chance to set seed. What happens to the nitrogen then? I don’t really know.

      • Hi both. I’ve been following some research from France that argues that soils feed on carbon. The more carbon you feed them, the more nitrogen they pull from the atmosphere (through free-living nitrogen fixers and other mechanisms). This is backed up in practice. See their extensive collection of lectures on youtube, unfortunately in French:

        I’d like to do subtitles of their work when I get some free time (don’t hold your breath!)
        In this paradigm, nitrogen-fixers’ main advantage is that they can grow (produce carbon or biomass) on nitrogen-deficient soils. You can then feed this to your soil and it and fix more nitrogen.
        It turns conventional thinking about legumes on its head, but it seems to work for them.

        • Interesting. Unfortunately I doubt if my French is up to following the videos but I’d be interested to read about it if anything comes out in English.

  2. This is a great, thought provoking post Alan. I’ve been reading similar questions about dynamic accumulators elsewhere, with similar reasoning to yours. I’ve also been reading stuff about the soil life web which points to using perennial nitrogen fixers as apparently the nutrients are transferred best to other plants via the mycelium web whilst the plant is alive, and not when it is “dug in”. (This is the case for all nutrients, not just nitrogen fixers, so presumably this is how comfrey etc would work). Apparently the nitrogen in the nodules of annual nitrogen fixers often gets released as a gas again and lost from the soil during the digging in phase…….everyday is definitely a school day!

    For info, the soil life web and mycelium info has been from Elaine Ingram and Christine Jones – lots of their videos are available on YouTube.

    • Yes, I kept my focus on plant roots to stop things getting too complicated, but fungal mycelia and for that matter soil bacteria are a hugely important part of the story of how a living soil holds onto and transports available nutrients too.

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