I first came across references to The Ethylene Cycle in ‘The Earth Care Manual’ by Patrick Whitefield. It was my first ever Permaculture book, and one that I still refer to. The last Permaculture book that I bought was ‘The Permaculture Handbook’, by Peter Bane. It too describes the Ethylene Cycle. Strangely enough I do not recall reading about it anywhere else. In my last post (Balancing Soil Minerals Pt 2) I was reminded of the Ethylene Cycle by a comment from Darius, a fellow Permaculture Practitioner, and decided to look at it more closely.
What is the Ethylene Cycle?
The diagram below gives a quick overview of what the Ethylene Cycle in soil is claimed to be, and what it is supposed to do. It is taken from a Permaculture Site, and we’ll be looking at the accompanying text shortly.
The Ethylene Cycle in Soil
A highly simplified explanation is that Ethylene helps to suppress microbial conversion of organic matter, and helps make available some nutrients to plants.
The Ethylene Cycle and Me
When I first read about the Ethylene Cycle I recognised it, as it was described, as a justification for not digging or ploughing. As I was already convinced that ‘No Dig’ gardening was the right approach, I didn’t bother to investigate it further. When my approach to digging changed, I overlooked the Ethylene Cycle until Darius reminded me of it. A quick bit of internet research, and some help with accessing the original research paper (Thanks Jan), allowed me to dig a bit deeper into this frequently taught piece of Permaculture folk lore.
Permaculture and The Ethylene Cycle
I couldn’t help thinking that there must be an underlying reason for the tight focus on this one piece of soil science by Permaculture Practitioners, and I found it with my first internet search. If you type ‘ethylene cycle in soil’ into your browser and search, you will find the Permaculture College of Australia article fairly close to the top of your list. If you read it it gives you a more detailed explanation of the Ethylene Cycle, and adds a layer of how it justifies or proves the need for no dig/ no plough techniques. The article also hints at why it may be far less appropriate for the UK climate and conditions, but I will save that for later. In the meantime, why not read the article. You can access it directly from the link below.
Permaculture College Australia
The article is a highly edited version of a 1976 Australian paper, and the article states that it was published in issue 7 of the International permaculture Journal in 1981. It was then published again in issue 39 in 1992. My initial thought were that this was a 37 year old paper, published twice in Permaculture Literature, and then spread by other Permaculturalists, with little critical thinking or analysis. The concept seems to have been completely ignored by other scientists, which implies to me that it was either proven to be inaccurate, or that it was perhaps irrelevant in some way. Again it was strange how little was written about this process that didn’t originate from this single paper.
Critique of the Permaculture Article
Relevance for Cool Temperate Climates
In the Introductory passage to this Ethylene Cycle article, the editor writes about soil aeration, and the perceived problems that it brings, principally the loss of soil organic matter through decomposition by microbes. She then writes ‘ The ‘Aeration Theory’ really developed in the northern hemisphere where the extended cold winters prevent microbial decomposition of organic residues in soil. In spring it is advantageous to stimulate the decomposition rate so that plants can obtain nutrients during a relatively short growing season’. This is then contrasted with Australian conditions, concluding that they are very different. So the editor is actually outlining why aeration is advantageous in temperate climates, and I wholeheartedly agree. Sadly, this part of the article seems to have been completely overlooked by the Permaculture network, operating in those cool winter climates. Instead it takes the opposite view, using the Ethylene Cycle as its justification
The rest of the article is written by the author of the research paper, and in it he claims that:
- ‘Ethylene is a critical regulator of the activity of soil micro-organisms and, as such, affects the rate of turnover of organic matter, the recycling of plant nutrients and the incidence of soil-borne plant diseases‘.
- ‘Ethylene does not act by killing soil micro-organisms, but simply by temporarily inactivating them‘
- He also discusses the role of anaerobic conditions and ethylene in the release of unavailable soil minerals
There was not enough of the underlying research and data in the article to justify the claims made in it. So I decided to read the original Research Paper.
ETHYLENE IN SOIL BIOLOGY. A. M. Smith
The research that the permaculture article was based on was titled Ethylene in Soil Biology, by A. M Smith. The paper was published in 1976. Sadly I cannot publish the paper for you to read, so I’ve had to quote directly (in italics) the bits that I feel are relevant. If you have access to academic paper through a University, please look at the original paper, and compare my analysis with the original work
With only a cursory initial reading on this Ethylene Cycle paper, a couple of things really leapt out at me.
The Effects of Ethylene on Plant Growth
The first concern came in the Introduction where the author states that only Cursory attention is paid to the significance of ethylene on plant growth and seed germination. That seemed strange when the whole point seemed to be to show it’s importance to plant growth. The paper then quotes other research papers which ‘showed that it may adversely affect plant growth‘. I was taken back a bit by this as it didn’t seem like a good reason to want to encouarge ethylene production. There was no research undertaken into which plants would be harmed by ethylene, but the paper mentioned that lowland rice was unaffected by quite high concentrations, whereas tomato, tobacco, and barley were ‘sensitive to trace amounts‘. It puzzled me why this would be seen as a good thing.
The author of the paper mentions reports that ethylene inhibits the nodulation of leguminous plants. This is really important, as many of the potential solutions to providing a sustainable agriculture involve nitrogen fixing plants. If ethylene inhibits the fixation of Nitrogen by them, this would be an argument to prevent its formation, not to encourage it. In fairness the author does quote from another, older, paper that Nitrification is enhanced near legumes and that this would restrict ethylene production ‘and may permit unimpaired nodulation under most field conditions‘. My emphasis.
The Effects of Ethylene on Soil Microbes
The permaculture article included references to Ethylene stating that ‘it is a critical regulator of the activity of soil micro-organisms and, as such, affects the rate of turnover of organic matter, the recycling of plant nutrients and the incidence of soil-borne plant diseases. ………… Ethylene does not act by killing soil micro-organisms, but simply by temporarily inactivating them‘. This is directly contradicted within the research paper in a number of areas. The first reported on the premature lysing of fungal hyphae by ethylene. This link on LYSIS explains that it is the breaking down of a cell compromising its integrity. Whilst not strictly killing fungi, it certainly doesn’t appear to be good for it either. With fungi having such a key role in the decomposition of organic matter, and in their mycorrhizal associations with plants, I fail to see how this is a good thing, or something to be encouraged. The paper also reaches the conclusion ‘that bacteria are not directly inhibited by ethylene at concentrations likely to occur in soil‘. Later it states that ‘soil respiration may remain relatively high in the presence of Ethylene because soil bacteria do not appear to be directly inhibited by ethylene‘. Taken together it seems difficult to justify the claims that Ethylene controls soil microbes harmlessly. The two principle components are either damaged (fungi), or unaffected (bacteria). In fact my more detailed reading suggested to me that many of the claims that were made on behalf of Ethylene could be attributed to purely to anaerobic conditions, however they were created.
The Effects of Temperature
The permaculture article touched on the differences between cold winter areas, and those with warmer conditions. This is directly referred to in the research paper, which links a twentyfold reduction in ethylene production from 11 C down to 4 C. A reflection of the reduced microbial activity as temperature falls. Stating that by restricting microbial activity, cold winter temperatures reduce the need for additional regulators such as ethylene. To put that in simple terms, in areas with cold winters, little organic matter is decomposed by microbes, making aerating the soil by ploughing or digging significantly less of a problem that in warmer climates, and allowing for the retention and build up of more organic matter in the soil than under warmer (Australian) conditions.
The Effect of Organic Material on Soil Microbes
The paper notes that the addition of organic matter overrides any suppressing effect that ethylene might have on soil microbes. This is attributed to an excess of nutrients, and is portrayed as a good thing, allowing the rapid processing of surplus nutrients. However this describes the situation in many no dig/mulched gardens. The layer of organic material would seem to override any potential benefits that might have been gained by the production of ethylene in reducing microbial activity. In plain language, even if ethylene did inhibit microbial activity, and that wasn’t the conclusion that I came to in the preceeding paragraph, it wouldn’t work in gardens/soils with high levels of organic matter.
The Mobilisation of Essential Plant Nutrients
One of the cornerstones of the perceived benefits of the ethylene cycle to plants is the claim that it has a role in making some minerals available to plants. To examine this claim takes an understanding of how ethylene is produced and some complicated soil science. Rather than try to explain how that works in a relatively simple way, there are a few things that I can highlight from within the paper and article that make the same point. Firstly, there is no direct evidence linking ethylene to the availability of these nutrients. The author of the paper is making an assumption that as these minerals are released in waterlogged soils, and that when ethylene is produced similar chemical conditions occur, the ‘availability of cations and anions should alter as markedly in these microsites as in flooded soils‘. Note the word ‘should’ which I have highlighted. No evidence is produced to support this, and the sequence that is described after this is a prediction based on what happens in submerged soils. To quote directly from the paper ‘Extrapolating from the chemistry of submerged soil, it is possible to predict the sequential seris of reactions in the anaerobic microsite that will lead ultimately to an increase in avaialability of inorganic nutrients‘. I don’t find this convincing. In fact there are some clear indications that the release of nutrients is not linked to ethylene, but rather to the conditions that produce the ethylene in the first case. This suggests to me that the ethylene is a passenger or bystander in another process, rather than the driver. The first is the diagram at the beginning of this post. To save you from having to scroll up to the top of the page, I’ll put it below.
Ethylene Cycle in Soil
If you look at the bottom left segment you’ll see that both the release of ethylene, and that of soil nutrients, are the result of a change in thechemical form of iron, which in turn is the result of anaerobic conditions. The second is contained in the permaculture article, where the following is written about the release of nutrients. ‘The soil conditions necessary for this mechanism to operate are identical with those required for ethylene production‘.
In actual fact it is anaerobic conditions which create the chemical conditions that release these nutrients, not the ethylene. Those conditions include waterlogging of the soil spaces, compacted soil, and the CO2 given off by the rapid decomposition of organic material by aerobic microbes.
Summary
Many of the positive benefits atributed to the ethylene cycle don’t seem to hold up when looked at critically. Inhibition of soil microbes by ethylene seems to be restricted to the damaging of fungal hyphae, with no direct effect on bacteria. Not that I think that it’s a good idea to inhibit them in the first place, but it’s promoted as a benefit to retain organic matter in the soil. Even were ethylene able to do so, not only is it far less important in temperate climtes, but that effect would be inhibited by the addition of large amounts of organic matter, like those used in mulched gardens.
The negative effects of ethylene on plant growth, the germination of some seeds, and nodulation of leguminous plant roots, would seem to be sufficient reason to discourage its production, and not to encourage it.
The release of insoluble nutrients by ethylene is an unproven prediction, with the anaerobic conditions created by water or microbial respiration likely to be the real agents for this process.
The thirty year love affair of permaculture with the ethylene cycle seems to be based on very shaky foundations.
Comments and Feedback
I’d welcome comments and feedback on this post, but especially from those with access to the research paper, who can compare the post with what is contained in the original.
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