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Persistence of soil organic matter  RSS feed

 
Dylan Mulder
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Location: North Carolina
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Hello Permies,

I’m a long time lurker and a first time poster. For my first post, I’d like to share some interesting knowledge with y’all, since y’all have shared your knowledge with me for some time.

What I’d like to discuss comes from a scholarly article called, “Persistence of Soil Organic Matter as an Ecosystem Property”. This a good article, which discusses a number of topics related to soil organic matter - I’ll be posting only a few parts that I found especially thought provoking and relevant to agriculture.

Soil humic substances

The prevalence of humic substances in soil has been assumed for decades19. Previous generations of soil chemists relied on alkali and acid extraction methods20 and observations of the extracted (or residual) functional-group chemistry to describe the presence of operationally defined 'humic and fulvic acids' and 'humin'. Humic substances were thought to comprise large, complex macromolecules that were the largest and most stable SOM fraction. However, we now understand that these components represent only a small fraction of total organic matter13,21-23: direct, in situ observations, rather than verifying the existence of these large, complex molecules, in fact find smaller, simpler molecular structures, as visualized in Fig. 2 (refs 13, 22, 23). Some of what is extracted as humic acids may be fire-derived24,25, although these compounds are rare in soil without substantial fire-derived organic matter. In any case, there is not enough evidence to support the hypothesis that the de novo formation of humic polymers is quantitatively relevant for humus formation in soils.


What do you mean it’s not important?

In my area, when one has their soil tested the soil report contains a measure of humic substances. As growers, we desired to increase that percentage because the higher it is, the greater your cationic exchange capacity, and the less amendment you had to put down. One can even purchase humic substances as an amendment.

However, according to the above passage, humic substances only comprise a small part of the bigger picture. In fact, humic substances are simply absent from many soils and their presence is a byproduct of soil testing methods.

What does this change? Not a lot actually, organic matter is still important - and so is humus. However, when one gets their soil tested I’d consider paying more heed to the measure of organic matter than to the measure of humic substances.

Influence of roots

Root-derived carbon is retained in soils much more efficiently than are above-ground inputs of leaves and needles40-42. Isotopic analyses and comparisons of root and shoot biomarkers confirm the dominance of root-derived molecular structures in soil43 and of root-derived carbon in soil microorganisms44. Preferential retention of root-derived carbon has been observed in temperate forests45,46, for example, where below-ground inputs, including fungal mycelia, make up a bigger fraction of new carbon in SOMthan do leaf litter inputs44,47. In addition to many above-ground inputs being mineralized in the litter layer, root and mycorrhizal inputs have more opportunity for physico-chemical interactions with soil particles40. At the same time, fresh root inputs may 'prime' microbial activity, leading to faster decomposition of older organic matter48,49 as well as changing community composition50. Carbon allocation by plants thus plays an important part in soil carbon dynamics, but it is not known how future changes in plant allocation will affect soil carbon stocks51.


When I first read this, it completely changed the way I think about cover crops and building soil organic matter. Now when I select cover crops, and even other crops, I focus more on root over shoot when it comes to building soil organic matter. Some cover crops downright produce more shoot than root, such as buckwheat or flax - and are less valuable to me as soil builders than they were in the past. Large rooted plants like trees, shrubs, comfrey, daikon radish, and greater burdock among others have become my chief soil builders.

This also changed the way I think about mulching. Surface applied mulch, I think, is still an important part of the ecosystem. However, I no longer apply mulch for building soil organic matter; I do apply it for its many other benefits.

Taken together, these eight insights paint a broad picture of carbon cycling in soil that has implications for fundamental research, landmanagement, and climate change prediction and mitigation (Fig. 3). They suggest that themolecular structure of plant inputs and organicmatter has a secondary role in determining carbon residence times over decades to millennia, and that carbon stability instead mainly depends on its biotic and abiotic environment (it is an ecosystem property).Most soil carbon derives from below-ground inputs and is transformed, through oxidation by microorganisms, into the substances found in the soil.


I thought this passage was also thought provoking, as it provides insight into why techniques that involve burying organic matter, such as hugelkultur, build soil organic matter so quickly.

Anyway, I’m interested in others thoughts and opinions on these topics.
 
J W Richardson
Posts: 79
Location: Council, ID
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my immediate thought is that it would be nice to see a list of annual or frost sensitive cover crops by size of root system. Also, in general thoughts about soil structure, the role of earthworms feeding on surface mulch contributes a lot too, but I believe they decrease the amount of organic matter?
I saw Gabe Brown this fall, and am so amazed on what he is getting in terms of organic matter and general soil health with his 9 plus species mix of annual cover crops. He said his general axiom is "no bare ground", and where the cover crop mix is not as feasible, i.e., in the vegetable garden, he uses mulch instead. It would be interesting to see the difference in organic matter and water absorption between the two methods.
 
Dylan Mulder
Posts: 51
Location: North Carolina
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I was wondering about worms as well. It may be that when they feed on surface organic matter and take it deeper into the soil, where it is deposited as feces, the carbon and such persists for longer in the soil especially when it is held biologically by other organisms. However, I've also heard that worms can have a damaging effect on ecosystems where worms are an introduced organism.

I reckon that in the ecosystems we design for food production, worms play a largely beneficial role in building soil organic matter. I would be surprised to see otherwise.

I agree that a list would be useful - the only list I have covers common cover crops species but makes no mention of actual root size or depth. I've found that cover crops described as 'subsoil looseners' tend to have substantial root systems, such as sweet clover. In my system I frequently use russian comfrey (Symphytum x uplandicum), and burdock (Arctium lappa) as soil builders because they can punch right through my very heavy clay soil.
 
Matu Collins
Posts: 1976
Location: Southern New England, seaside, avg yearly rainfall 41.91 in, zone 6b
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Very interesting! Thanks fir posting and welcome to permies. I'd love to see more research on the root question, and a list.
 
Bryant RedHawk
gardener
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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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Hau Dylan, welcome to permies. Great Post!

I have been pushing roots as the main builder of soil microbiology for years. If one wants to hurry up the reclamation of land, burying organics is a sure fire way to do so. The easiest way to get organic material into soil without burying something is to grow something, then cut it off so the roots die and begin to deteriorate. If you are going to bury stuff to help soil, Wood, fresh, charred, rotting, will all add carbon to the soils they are buried in. Even fresh downed leaves, compost (finished or not) even cotton clothing, if it is carbon based and buried it will rapidly improve the soil. One of the reasons for this is that carbon based materials support fungi which supports the microbiology of soil.

Earth worms are more for aeration of soils and processing of micro organisms than for processing leaves.
Earthworms do not actually eat leaves, nor do they eat cardboard, rather they feast on the microbes that decompose the organic matter. This can be proven by starting a worm bed and making observations.
Worm castings are found both in the substrates and on the surface of the soils they occupy.
They can go as deep as needed when escaping low temperatures but prefer to live in the top foot of soils.

Most of the really good seed suppliers have a list of deep root and shallow root plants they offer. Deep root varieties will have the largest root systems. For breaking hard ground, long tap root plants are the way to go. Daikon is a huge tap root plant and that is why it is so often mentioned as great to use for opening up soils. Carrots, horseradish, beets, etc. will work fine as well.
 
Peter Ingot
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Dylan Mulder wrote:Hello Permies,

I’m a long time lurker and a first time poster. For my first post, I’d like to share some interesting knowledge with y’all, since y’all have shared your knowledge with me for some time.

What I’d like to discuss comes from a scholarly article called, “Persistence of Soil Organic Matter as an Ecosystem Property”. This a good article, which discusses a number of topics related to soil organic matter - I’ll be posting only a few parts that I found especially thought provoking and relevant to agriculture.

In any case, there is not enough evidence to support the hypothesis that the de novo formation of humic polymers is quantitatively relevant for humus formation in soils.


What do you mean it’s not important?

In my area, when one has their soil tested the soil report contains a measure of humic substances. As growers, we desired to increase that percentage because the higher it is, the greater your cationic exchange capacity, and the less amendment you had to put down. One can even purchase humic substances as an amendment.


Years ago I put a soil extract through a mass spectrometer. The result was one big blur. So many different chemical compounds that we couldn't distinguish any of them. Humic and fulvic acids seem to be important, but that's little more than an educated guess.

A simple test: Weigh a sample of soil. Dry it and weigh it again. Burn it and weigh it a third time. This tells you how much water and how much organic matter (living and dead animals, plant, fungi and microbial material, humus etc) there is in the soil. OK this leaves a lot of unknowns, but soil and humus is basically a big black hole, very hard to study without destroying it. A good farmer will be able to estimate the organic matter content just by rubbing the soil between his fingers.

We know that organic matter is inherently a good thing. Peat contains almost no soluble minerals (unless they have been added artificially), but adding it to soil improves crop yields. It improves structure for one thing. It also seems to act as a reservoir for water and minerals, preventing them being lost from the soil and slowly releasing them to the plant roots, but at this point we are starting to peer into the black hole, because we don't know exactly which components of the soil organic matter/humus break down quickly and which slowly. We know that some of the soil organic matter will not release its nutrients for 100 years.

My attitude is that on a large scale over long time periods, the best way to increase soil organic matter is to grow stuff like grass, trees, clover etc. because their roots will ultimately be the best thing for the soil. However if you wish to create a small highly productive garden quickly, its better to bring in lots of organic matter from elsewhere, perhaps cut back vegetation or collect leaf litter from somewhere with lots of grass and trees (making sure you don't take too much over too big an area and trash the place), or, if it is available, rescue some "waste" organic matter which would otherwise go to landfill - spoiled fruit and vegetables, grass clippings
 
Alder Burns
pollinator
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Location: northern California
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All those interested in the issue of persistence of soil organic matter should look into biochar. Charring organic material greatly increases it's resistance to breakdown on and in the soil, while retaining or even improving it's ability to grab soluble nutrients and prevent them from leaching out. In warm humid climates especially with sandy soils, an almost unbelievable amount of organic matter can be applied on an ongoing basis, whether buried or not, and it apparently vanishes. A small percentage does remain, but most of it is simply respired away by the soil microbes. Charring slows this process and enables soil carbon to be built up faster and with less material in the long run, even though at best only a third of the starting material survives the charring process.....
 
Dylan Mulder
Posts: 51
Location: North Carolina
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Bryant RedHawk wrote: ...Earthworms do not actually eat leaves, nor do they eat cardboard, rather they feast on the microbes that decompose the organic matter.


Thank you, I did not know that.

Peter Ingot wrote: ...OK this leaves a lot of unknowns, but soil and humus is basically a big black hole, very hard to study without destroying it.


Interesting stuff, thank you. For something we all depend on, it's funny to know so little about it.

Alder Burns wrote: ...All those interested in the issue of persistence of soil organic matter should look into biochar.


After hearing about biochar from my fellows, I looked into the existing research to get a better understanding of it. I reviewed several scholarly articles, and each claimed a different period for its persistence in soil (from a few years, to decades, to centuries, to millennia). Reckon it's like humus then, in that not a lot is known about it. I'm interested in hearing people's experiences with using biochar as a soil amendment to improve soil organic matter.
 
Bryant RedHawk
gardener
Posts: 2992
Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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Hi Dylan, The interesting thing about bio-char; when underground it provides many benefits including a place for Mycelium to get a foot hold, minerals from the charred wood will leach into the surrounding soil and so on. Bio-char can remain in soils that are heavy in clay content for millennia since these soils don't have a whole lot of bio activity, that's one of the ways we find cook fires in archeology digs. Raw wood will persist for centuries and longer when stuck in anaerobic conditions, that have no bioactive organisms.
 
Dan Kline
Posts: 15
Location: Virginia, USA
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Here is my soil journey in a Richmond, Virginia neighborhood.
After a career and life with little contact with soil, we came to a home with extensive gardens, that I began to love. This led to the love of daylilies, and attempting to breed these for performance led to the recognition that I have poor soil on my site.
First I had to get pH figured out since my soil type "Lenoir" a sandy loam in the class of Ultisols, "tends sharply to acid in the presence of water." Add to this the fact that the land was cleared of virgin forests in 1640 to grow tobacco. County records indicate the Brook Run Plantation raised this one crop here until emancipation in 1864, when the land slowly returned to forest, and its natural pH of 3.6. This home, on .67 acres, was built in 1937, and when we came in 2000 there were only 5 very old apple trees, and large open gardens.
My first amendment was to mulch with the composted yard waste the county gives a way free. This helped reduce watering and control weeds, but the "big bones" of wood in the mulch stayed in the soil a long time and seem to compete with plants for nutrients.
Frustrations with performance, and the obvious blanching of my plants after a rain led me to focus on building compost, hot composting, and the sciences thereof. The study led to vermicompost, so I took up worms and now have millions of red wigglers building soil amendments.
Two years ago I build a new 4 x 20 in ground bed with lots of compost and some vermicompost. The results were astonishing. Seed planted in that bed grew amazing plants. This was a great encouragement. As new breeding stock came I planted it with ample compost. Some of these won "Best of Show" at the Richmond Area Daylily Society show in 2014.
During the spring of 2014 I learned about biochar. It was fall before I could get things together to start burning.
One of the scientific facts that drives biochar is that the Ultisol soils are among those most helped by biochar.
So far I have produced about one cubic yard of biochar, and a barrel burns as I write this. I hope to have 3 yards of charcoal by spring. I am mixing it with 3 yards of vermicompost to inoculate it with nutrients and microbes. This will be partly top dressed, but much will be worked into the soil with the rototiller. My no-till vegetable garden will get tilled this year for the biochar.
I am pretty optimistic that this effort will reduce my water use, reduce or eliminate my use of chemical fertilizers, permanently improve this soil, and sequester carbon.
This thread is named "persistence of soil organic matter" but my experience says the persistence is in the gardener who expends the energy to get the right stuff in the soil.
By fall I will know whether my hard work will really pay off. Some biochar science says the real benefit is not seen until the second year and following, that the char effect builds.


 
Bryant RedHawk
gardener
Posts: 2992
Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
243
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hau, Dan. Sounds like you have things going your way, congratulations. Bio-char does indeed get better with length of time in the soil, the longer it is there, the better the soil microbe population, it also will be home to many mycelium, which will help your plants take up nutrients.

We have a county run place here that you can get free "compost" from too. I have taken to re-composting the stuff as it is very roughly made by the county. I put it in the chicken yard, they free range but we also have a fenced yard around the coop, which allows us to offer them protection when we are off the farm. The chicken's yard is supplied with many little piles of various vegetative material for them to scratch through to find bugs and other tasty treats while they are "confined".

May I suggest that after you use the rototiller to incorporate the bio-char that you make up a batch of vermicompost tea to water your plantings with. If you have some mushrooms growing wild, chopping or grinding some of them up and adding to the tea barrel will help get some mycelium going straight off.

I totally agree with your observation of the real persistence is the gardener expending the energy to constantly improve the soil they care for.
 
Dan Kline
Posts: 15
Location: Virginia, USA
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Thank you for your encouragement, Bryant.
I do not know of another person in my entire area doing biochar. Everyone I speak with about it needs a definition and explanation.
Most of the daylily growers are high chemical users, spraying for weeds, insects, and a deep south leaf rust. Many have state inspections that require chemical treatments. I only know of one person in my daylily society who builds compost. At meetings they give leads for this fertilizer product or that.
Late in February I will do a presentation to a garden club. They think it is about beautiful blooms, but I think it is about soil. They are in for an education.
About mycelia: I have lots of that going on in a few piles of compost. Last summer a small piece of oak the size of a finger had a mushroom growing. I am rather new to mushroom thought, so thank you for the heads up. I had not thought of putting mushrooms in the tea.
The compost from the county gets run through a two screen trommel I made, giving me 1/2" fines, 1" mediums, and big bones out the end. I put the fines on for mulch and weed control. The medium goes on paths. The bones get dried for biochar. Some of my paths are full of fungi and some mushrooms.
I change out trommels to 1/8" and 1/4" for worm sorting and to take nails out of the biochar, most of which comes from pallet material.
Today I took the carb off the shredder since I need to pulverize char. Cleaning alone did not do the trick. Now for the carburetor rebuild kit.
So there I go... to get it.
 
Bryant RedHawk
gardener
Posts: 2992
Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
243
chicken dog forest garden hugelkultur hunting toxin-ectomy
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When I process bio-char I use a burlap bag and a sledge hammer, fill the bag and zip tie it closed, bounce the sledge head over the bag, flip the bag and repeat the bouncing. This way I end up with all sorts of sizes, which is really good for the soil. I wish I had a shredder, but that will come eventually for a few items that I would rather chop up before putting in to a compost heap or growing mound.

I find it somehow troubling that so many flower growers (I'm a Rosearian) don't rely on compost, manure, mulch as much as they should. Being a show rose grower, I initially had some rust and mildew problems but, once I spread my bushes out further apart and added mycelium to my beds along with some green sand and powdered granite for a top dressing, almost all my need to use chemicals went away within a year. My bushes were so healthy that there was no need for any type of spray after that, I alternated the type of stone powder every six months after that and before I retired from competition I was invited to not show anymore so others could win the classes I entered. My Rose Society tried and tried to get me to tell my secrets. I always explained that I didn't keep any secrets, I gave lectures and demonstrations to all the garden clubs and fairly laid out exactly how I produced show winner blooms year after year. Funny how some folks just don't listen or bother trying what they are told works. If you don't have the soil, you don't get the eye stopping blooms, pure and simple.

Small quantities of dried, spent coffee grounds, used as a top dressing have many benefits such as nitrogen leaching into soil, attracting more earthworms, proliferation of "good" microbes and mycelia. You can also use this as a component of vermiculture tea or manure tea, it seems to have a buffering effect on these tinctures of awesomeness.
 
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