The Death of Humus

A centuries-old concept in soil science has recently been thrown out. Yet it remains a key ingredient in everything from climate models to advanced carbon-capture projects.
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The Death of Humus
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by the mid-20th century, the humus paradigm was “the only game in town,” ... Farmers were instructed to adopt practices that were supposed to build humus. Indeed, the existence of humus is probably one of the few soil science facts that many non-scientists could recite.
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our understanding of the nature and genesis of soil humus has advanced greatly since the turn of the century, requiring that some long-accepted concepts be revised or abandoned.
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powerful new microscopes and techniques such as nuclear magnetic resonance and X-ray spectroscopy allowed soil scientists for the first time to peer directly into soil and see what was there
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What they found — or, more specifically, what they didn’t find — was shocking: there were few or no long “recalcitrant” carbon molecules — the kind that don’t break down. Almost everything seemed to be small and, in principle, digestible
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the available evidence does not support the formation of large-molecular-size and persistent ‘humic substances’ in soils
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Old ideas, however, can be very recalcitrant. Few outside the field of soil science have heard of humus’s demise.

— https://www.quantamagazine.org/a-soil-science-revolution-upends-plans-to-fight-climate-change-20210727/

Food for thought.

Very interesting, basically I read it as showing there are no shortcuts, no magic pills, no till isn't going to help and there's no way to put organic matter into the soil and expect it to stay there. You can still increase your organic matter and therefore stored carbon but you have to keep doing it every year, and the "better" your soil gets the faster the carbon comes back out again.

Thanks for posting. My interpretation is the same as Skandi's. If you increase soil organic matter, that takes C out of the atmosphere, even if it is cycling. The story reads almost like it's a politician's view of soil. The thinking that you can just grow some deep-rooted plants to put a bunch of C in the ground and then be done with this climate change mess is a very simplistic way of looking at things.

My take on this is slightly different.

Everything relevant about humus that I have learned I picked up from our own Dr. Redhawk. My takeaway a lot of the time was that humus was such a short-lived soil component that it wouldn't survive the transfer from compost pile to garden bed in any way that was useful. This is perhaps an oversimplification, and if we could get someone who could explain the nuance better than I to do so, that would probably help, but I got the distinct impression that soil humus was so ephemeral that it couldn't be isolated in any quantity for any amount of time without it breaking down.

The conclusions of the article are typically focused on their negative angle. If the climate strategies in question had as their sole vehicle of carbon sequestration soil humus, I would agree that yes, every reasonable climate strategy involving growing things in the ground is flawed beyond belief and should be discarded in favour of technological and industrial solutions.

However, those strategies are multifaceted, instead. So yes, while it might work differently than we'd previously figured, and while some specific mitigation strategies might have to be tweaked with human interventions to, say, collect sequestered carbon in the form of biomass and drop it into subduction zones, every solution that involves tying atmospheric carbon up in carbon-based lifeforms takes it out of the atmosphere for at least as long as those lifeforms live, and for some a long time afterwards.

They addressed one glaring issue in an offhand way, by noting that the suberin researchers were focusing on root zones that grew deeper. This is pretty important, because if you're saying that soil life eats soil carbon and releases it to the atmosphere, we need to look at how deep the soil biosphere goes. Plants like alfalfa can drop their roots six feet down. How far down does soil life live, in absence of light or air? I know that if you bury carcasses too deeply, to a depth of over six feet, decomposition slows much further than if you're decomposing something in the top three to six inches of soil.

Maybe I was under a misapprehension that humus was something like a biofilm, ready to sublimate or otherwise break down into its base components at  the slightest disturbance, and that the geochem points on the biogeochem soil science chart were too heavily weighted. Or maybe I underestimated how widely the ideas had spread.

And maybe soil holds just as much carbon as we previously thought it did, but does so in the form of carbon-based microorganisms in the soil, which live faster, but experience a higher mortality rate, in warmer climates.

Besides, there's also burying biochar. Leave it in larger chunks and bury it. If it can decompose without onboard volatiles, I'd love to see what the microbes make of that.

-CK

Yeah, I always found the argument of highly persistent carbon bio-molecules to be a little hard to believe. It seems to me that if that process actually worked as suggested, we would expect all soil to be just chock full of carbon from the slow build up over eons. Since that is not the case, clearly carbon levels in soils must be more or less in balance. It does sound like there might be some hope of sequestering carbon for a little while; if it gets deep enough down.

I will say that I would be curious to see what this new type of research would reveal about biochar. It would seem the point they were making was that no molecule made by one organism can resist being eaten by other organisms, but humans have created lots of carbon-based molecules that will last for thousands of years. Its a lot more work to make char than to just plant crops, but I think at this point, we need to start using every tool at our disposal to start removing carbon from the air.

Few would deny that the abundance of life above ground in a tropical rainforest (like the Amazon) is enormous — almost overwhelming.  Look below the surface, however, and the story is starkly different.  With the exception of small pockets of Terra Preta, the soil of the Amazon is poor, lacking in nutrients (including carbon).  Heat and humidity provide bacteria (and other micro-organisms) with the conditions they need to rapidly decompose organic matter that ends up on the forest floor.  Nutrient cycling is fast in tropical climates.

In temperate climates, with lower temperatures and fungal dominance, nutrient cycling is slower.  Carbon/nutrients are able to build up in greater amounts and to greater depths in the soil profile.

Is this actually a good thing though?

If your sole concern is taking carbon out of the atmosphere and putting it into the ground, then sure, it is.  But that's not what most gardeners have as their top priority.

'Banking up' nutrients in soil (especially poor soil) is undoubtedly beneficial — at least in the initial stages.  You end up with healthy and tasty veggies because nutrient are readily available to the roots of the plants.  Like most things, however, the Law of Diminishing Returns applies to soils.  In relatively short order you get to a stage where, for every unit of nutrients you (somehow) put into the soil, the increase in 'yield' becomes smaller.

Having carbon and other nutrients immobilised in the soil — beyond the reach of the roots of the plants growing above them — accomplishes nothing for a gardener.  Long-term sequestration of carbon and other nutrients accomplishes nothing for gardening in general.

Money, time and effort spent banking up large amounts of nutrients in the soil makes, in my opinion, as little sense as banking up large amounts of money in an account that attracts an ever-decreasing rate of interest.  You want as little money in the banking system as possible to cover ongoing demands, and as contingency for emergencies, and no more.  Everything above that should be invested in things that provide clear(er) benefits and/or return on investment.

So, even though humus appears not to actually be 'a thing', carbon and nutrients in the soil are, most definitely, 'a thing'.  'Too much of a good thing' is, however, also 'a thing' — so we should try to avoid doing that.  Those who have working 'soil enrichment programs', and already grow healthy food, would probably benefit more if they enriched a larger area.  Diversity is 'a good thing', after all.