As a gardener we all talk about humus. Some of us even buy humus soil, and humic substances like humic acid and fulvic acid. We add compost to gardens to increase the humus level in our soils in the belief that humus is good for soil. Good garden soil is dark because of the high humus content.
If there is one thing all gardeners agree on, it is that humus is good for the garden – right?
Maybe not!
Science now says our beliefs about humus may be wrong. In this blog I will review some earth shattering news – or is that soil shattering news?
What is Humus?
I have answered this question in a previous post, written in 2013, called What is Humus? You can review the previous post for more details, but I will summarize key points here.
200 years ago soil scientists noticed that good agricultural soil was black and in an effort to better understand this black stuff they devised a method to extract it from the mineral components of soil (ie the silt, sand and clay). They treated soil with a strong alkaline solution of pH 13. This procedure pulled the organic component out of soil so they could study them.
Over time, this black substance became known as humus.
Humus has some very unusual properties. It is composed of mostly carbon and some nitrogen – not that unusual. The unusual part is that humus is very stable. In fact it takes 100 years or more for it to decompose. Microbes can’t seem to digest it even though microbes can digest just about every other organic material, including oil.
The treatment with the alkali produces two main components which are called humic acid and fulvic acid.
Decomposition Produces Humus
Lets try to understand the complete process of what happens to organic material in the garden. Organic plant (and animal) material is added to the soil. It might be added directly as leaves fall to the ground, or it might first be composted by a gardener and then added to soil. For the purpose of this discussion both processes are the same. Once in the soil, pieces of plant material are decomposed into large molecules like proteins, and carbohydrates by the action of microbes (bacteria and fungi mostly).
Over time the microbes break the organic material into smaller and smaller molecules and in the process nutrients are released for plants to use. This process takes about 5 years on average. For more details on this see Compost – What is Compost.
At the end of the process, most of the organic material has been used up by the microbes and what remains is the humus. Humus is a substance that microbes can’t seem to digest, so it builds up and remains in soil for many years. It is extremely stable, some claiming it sticks around for 100 years or more.
The black color we see in good soil is due to a mixture of organic matter that is decomposing, and humus. It is mostly humus.
Humus – Does It Exist?
The decomposition process described above has been the accepted truth for many years and was the accepted story in 2013 when I wrote my last post. But scientists have always had some issues with this story. Even 100 years ago some scientists questioned the existence of humus.
Humus could not be characterized in soil. That means that scientists could not analyze humus while it is in soil. They could only work with it after extraction at pH 13. A pH of 13 is very alkaline and at this pH all kinds of weird chemical reactions take place. Nobody could say for sure that the humus in soil was the same humus being studied after extraction.
Why Is Humus So Stable?
The decomposition process happens because microbes break apart plant materiel into smaller and smaller molecules and yet humus is immune to their digestive processes.
What else do we know about humus?
Humus is made up of very large molecules but over many years of study, no one has been able to clearly describe the structure of these molecules. That is odd given that with today’s scientific tools we can establish the molecular structure of just about everything. Humus is still defined as “a large, undefinable, quite variable molecule made up of mostly carbon and hydrogen” – that doesn’t tell us much!
Scientists don’t know what it is, and can’t explain why it is so stable.
Even more interesting is the size of the molecules. Keep in mind that microbes break down organic mater into smaller and smaller molecules. Why is it then that at the end of such a process we have very large molecules? These molecules did not exist in the plants when the process started. This means that they either formed on their own – which would be rare in nature, or they are created by some biological process which is completely unknown.
No one has been able to explain why and how the large molecules are created.
Humus Does Not Exist!
A very interesting paper was published in Nature, December 2015, by Johannes Lehmann & Markus Kleber, called “The Contentious Nature of Soil Organic Matter”, ref 1. For those not familiar with the magazine Nature, it is one of the top scientific magazines that tends to publish the creme-DE-la-creme of new discoveries. It is extremely well respected.
I’ll explain the details below, but what this paper says is that we have been looking at humus the wrong way for 200 years. Humus does not exist in the soil. Humus is created during the pH 13 extraction process. The strong alkali creates humus.
There is organic matter in the soil. When it is treated with a pH 13 solution, it goes through a process which creates the large humus molecules.
The New Decomposition Model
Using the information provided in the above mentioned paper, I’ll re-describe the decomposition process.
Organic plant material is added to the soil. Once it is in the soil, pieces of plant material are broken into large molecules like proteins, and carbohydrates by the action of microbes.
Over time the microbes break the organic material into smaller and smaller molecules and in the process nutrients are released for plants to use.
Organic matter seems to decompose slowly for two reasons;
- molecules interact with soil, in effect hiding from the microbes
- microbes build large molecules making the process start all over again
Large molecules of organic matter interact with soil more than previously thought. In fact the soil tends to hide the organic matter from bacteria to some extent, slowing down the decomposition process.
The second important point is that microbes use small molecules and build them up into large molecules, just like plants do. They take simple sugars, nitrate and phosphate and build proteins, DNA and carbohydrates from them. This is not new information, but the paper suggests that this process is much more significant then we realized. Microbes slow down the decomposition process by making it start all over again.
At any given time, soil contains a wide range of molecule sizes – The Contentious Nature of Soil Organic Matter – the title of the research paper.
There is a steady stream of large molecules entering the system when new plant and animal material is added to soil and by the death of microbes. This organic matter is constantly being decomposed into smaller and smaller molecules eventually turning into simple nutrients like nitrate and phosphate, as well as carbon dioxide.
Soil does not contain humus, as previously defined. There are no large stable humus molecules and there are no humic and fulvic acids. Humus and the associated acids only exist in the test tube after soil is treated with a pH 13 solution.
What Does This Mean For The Gardener?
It does not change a lot for the gardener. We already knew that it is important to add organic matter to soil – that has not changed. We know organic matter feeds the plants over time. We know it improves soil structure by creating aggregates.
As explained in my post What is Humus?, you can’t buy humus even though a lot of people sell it. What they are selling is just organic matter with a fancy label and probably a higher price tag.
Humic Substances
The term humic substance is used as a catch all for a number of products including humic acid and fulvic acid.
We now know that these are created in the extraction process and are therefore man-made chemicals. Contrary to common belief, humic substances are NOT organic (in the sense of organic gardening).
Do they work? Do they add any value for plants? Sounds like a topic for another post – but the short answer is that their may be some value to them – but the scientific evidence supporting a real value in the garden is weak at best.
The Scientific Process
There is a lot of negative press these days about science and the scientific process. Everything is controlled by Monsanto, and all the scientists are paid to lie about their results. Anyone with half an ounce of common sense knows this is bull.
Science is not always right. With humus they were wrong for 200 years. Part of the reason for this is that soil science research is not funded very well – but 200 years is a long time.
The issues about humus were raised as early as 1888 (ref 1), but they were dismissed. They were again raised 50 years later and were again dismissed. Now it seems that there is more support for the idea. Part of the reason for this is that we now have better technology which should be able to detect humus in soil, but it can’t find any.
The important point of all this is that science research is a self correcting process. Over time mistakes are corrected by new studies. The system does work. Over time science does reach the right conclusions. We have just observed for the first time, the gravitational waves predicted by Albert Einstein in 1916. Science may be slow at times, but it is much better than the alternatives.
Is this the end to the humus story? Probably not. The paper I am discussing is very new and not all scientists agree with it. It is quite possible that in another 2 years I will write about humus again and change the story, but I don’t think so. The issues with the old humus story are now clearly explained, and the new theory fits the data we have.
This paper discusses reasons why the new view has not been adopted more.
References:
- The Contentious Nature of Soil Organic Matter: http://www.nature.com/nature/journal/v528/n7580/abs/nature16069.html
- Photo Source: The Contentious Nature of Soil Organic Matter
This one of the best articles I have ever read in this subject, you have done a great job and solved me the unsolved humus question.
I was wondering concerning what “paleorthid11” wrote about biochar. The char itself has high PH (around 9-11). Could it be that part of the biochar effect in soil is the process occurring when using an alkaline solution? the process which created the large humus molecules.
Maybe biochar creates the so called humus in soil?
Biochar is alkaline, but I don’t think it has been accepted that it does much to the soil. But maybe?
Robert is an article concerning biochar in the works? Love to read your thoughts on biochar claims. I’ve read a few peer reviewed studies on biochar and water retention and both studies concluded no statistical value except on sandy soils but any organic would help in that case.
Not in the short term, but this will give you some good information on biochar:
https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwjNlK_plILMAhXGtIMKHa5RCFAQFggqMAI&url=http%3A%2F%2Fcru.cahe.wsu.edu%2FCEPublications%2FFS147E%2FFS147E.pdf&usg=AFQjCNHKWe6605uFEJyrmGbhmgsvUStWxA&sig2=e_Si1FqxMGwASa8tdDYU6Q
The discussion here on the subject is interesting, but I have to admit it is not something I have ever thought about using or adding to soils. The subject of Biochar is remotely interesting to me, but probably not for the same reasons as most folks think and it’s not something I’ve ever thought of adding to soil. But I will say that in some mycorrhizal inoculents I use, there is a very small amount of “Humic Acid” which has been derived from Leonardite associated with brown coal mining, because it can stimulate some plant root growth in trace amounts which is what you want for spore germination at the root cap. But I’ve never used the term ‘humus’ with regards to turning soil into humus. The closest I’ve come is adding another ‘M’ to the word humus and using “Hummus” *smile*
There is very little evidence that purchased mycorrhizal inoculents do anything for plants in the garden. If you really feel you need to add some – get some soil from a nearby forest and spread that around. A couple of handfuls will contain lots of spores.
Selling humus and humic substances is big business.
Well, as I’ve stated previously on this subject, I come from the southwestern USA and urban environments. Most of those soils are dry and from a beneficial mycorrhizal standpoint lacking such organisms, especially in their cities. That’s not to say it isn’t there, but many host specific components are clearly lacking. As I stated, I do not purchase product from colourful brochures and websites the Garden Professors slam. The people I purchase from were the original authors of many of the peer-reviewed scientific literature from the 1970s and 80s. Dr Donald Marx was my first teachers and later Mike Amaranthus, both of which went on later in life to develop large successful companies who sell commercially.
I could not successfully grow Manzanita shrubs without the correct host spefici species of fngi. I also use Pisolithus tinctorius which is great for Oak, Pine, etc and done so with 100% success. They colonize very quickly and the improvement becomes clearly noticeable the following season. I’m not taken in by the flowery packaging and promises of magic dust, but I also collect my own mature truffles out in the wild in the same ares for decades. Unfortunately about a year ago, some prime collection areas stopped producing the truffles they had year after year and almost a decade after that trees started dying and I have no idea what has happened. Down in SoCal where I come from this climate change has been noticeably pronounced. Yur area may still be untought, but who knows, maybe one day you’ll even move further north above Hudson Bay to have the same climate you have now. Kidding of course.
But like Sweden, your area has a lot of natural biological material still viable, SoCal does not and it’s getting worse.
If the S. California soil no longer supports a particular type of fungi, then adding it to the soil will result in the added fungi dying as well – unless the local soil has been reverted back to soil that supports it.
Do you have any scientific references to show that Manzanita shrubs or any other shrub grows better in the field with added fungi?
I work for 8 years as a Soils Consultant in a very well know lab in SoCal back in the 90’s. That was the period when soil bacteria and mychorhizza became buzz words. Companies selling those products wanted our endorsement which of course our staff all declined. Now I’m in world of selling soils but I focus mainly on turf and sports. I get approached all the time by some company sell these types of products. It’s all just a waste of money. Test your soil, adjust based cations and fertility and add OM. That’s its. Good compost will add beneficial bacteria and fungi and carbon
Robert, I was strictly referring to the urban landscape environments within cities and growing natives like Manzanita, not the wild. I was a commercial landscape supervisor down in San Diego for some years after moving back from my former wilderness environment in the San Jacinto Mountains. Native plants from SoCal just will not grow well at all with the conventional science-based practices. Mostly, after inoculation with the correct fungi and beneficial bacteria you just have to leave them alone and also community plant them them with other plants of like needs and requirements. But yes, certain ecto-mycorrhizae I have used in the wild when reforesting an area in the wild does move through the soils and attach many of the scrub oaks which do become colonized and there is not only truffle formation a foot or two on the outer dripline, but the foliage reveals a vast improvement as well. If you can get hold of some of Dr Donald Marx literature and scientific papers, or read some of these off the PHC website, you’ll see that Pisolithus tinctorius is the best starter ecto-mycorrhiza you can use in reforestation. And yes some of the surrounding shrubs and other trees “IF” they are also host specific to this fungi will benefit greatly.
I’m sorry for the spelling errors up there, I didn’t notice before and my eye sight is going.
Hi Robert,
There is one more thing I forgot. Saw this today and it dealt with growing plants in artificial Martian soil. Of course there is no such thing, but the researchers put together some moon soil samples with other mineral ingredients they cleaned off the rover’s information and found that crop plants would grow without the biological inputs. Just interesting reading mainly.
http://www.smithsonianmag.com/smart-news/move-over-matt-damon-scientists-grow-ten-crops-faux-martian-soil-180958366/?no-ist
Funny . . . I live in So. Cal. and I just happen to have planted a small one gallon Manzanita in my back yard three years ago in a soil they don’t normally grow in and just 6 miles from the ocean where they don’t grow naturally at an elevation of just 36 ft above sea level and that little shrub has tripled in size and is a very happy little Manzanita shrub on it’s way to being a small tree some day.
Many nurseries use an appropriate mycorrhizal fungus that’s good for specific species so there is no reason to add more to your soil but I did buy some once because it was just few $$ extra over the Grow Power without it and I knew that it would do no harm and I knew I would never have to use it again. Sometimes we take vitamins JUST IN CASE as well. LOL
The ecosystem of Southern Ca was never sufficient to support tens of millions of humans. You bring most of your water from elsewhere. It has always been an arid region. Too many people putting pressure on your resources is the problem, not your fictional “climate change”.
Thought #2. Humification theory helped blind my profession (soil survey) to the role of pyrolytic constituents in the origin of black soils (chernozems) and in the appearance (morphology) and character (soil health, soil quality).
In the Nature article look at the evidence-based side of Figure 3 and note the pyrolytic C and N constituents involved.
Humification theory holds that charcoal is inert. Our blind faith in humification meant we never looked for it, and short of shutting down the occasional undergraduate newbie naively asking “maybe the black in black soil is charcoal” we never even thought about it. The ash from fires, now that’s where the fire effect is coming from. We told ourselves. For 200 years.
We are char blind. As a result soil science knows almost nothing about the role of native charcoal, and charcoal weathering products, in soil. What we do know is
1) Soils have had charcoal for 400 million years, meaning soil biology has evolved in a charcoal context long enough to have evolved very intricate relationships with it. We will find species that cannot prosper in soils without the char component.
2) that there is a professionally embarrassingly (my view) high level of black C (BC) in chernozem soils (Russian for black soil). BC is over 50% of soil organic carbon (SOC) in some chernozems. BC is 5-50% SOC in most soils, depends on fire history and such.
3) that the effects of charcoal are maddeningly complex, changing from ecosystem to ecosystem, from char to char, and over time. If modern soil science’s founder, Vasily Dokuchaev, had picked up on the fire soil origin effects in his work in 1880’s, we would know a lot by now. He should have listened to that newbie undergrad, we all should have. If we had, humification theory wouldn’t be the undead monstrosity we have allowed it to become.
4) we need smarter soil scientists. Parents, Please let your children get a degree in soil science even though you have never heard of it as a career before and you were hoping they were going to be engineers. The world really needs soil scientists. And these are exciting times to be a soil scientist.
Very interesting – thanks for taking the time to post. The more I learn about soil – the less I know.
How much humus is there is soil, on a global basis? That seems like a simple question. I what I learned is that we have no idea. Humus quantities are estimated by looking at forest and tree growth, and then extrapolated. Nobody knows how effective this is.
With global warming, CO2 levels are important, and it seems we don’t really know how soil affect this. More guessing!
In gardening there is also a lot of talk about soil microbes – but we don’t have a good idea of species, numbers, interactions etc. We are just starting to understand the life in soils.
My deep gratitude for delving into the meaning (past post) and posting this.
Clearly – as you say – humus does not exist.
Thought #1 – The term humus predates the 19th century origins of humification theory.
“The root here, hum-, seems to have originally referred to earth or dirt but also turns up in humanus “human, kind”. This suggests that our earliest forefathers perceived humans as originating in the soil.” (quote = http://www.alphadictionary.com/goodword/word/exhume)
I like that “forefathers perceived” idea. It resonates with a growing perception today. Long live humus.
This is an extremely important blog Robert. I have been wondering about the use of the words humic and humus for years. Most people use the words “organic matter” and “humus” interchangeably in any case and they were probably right to do so! I think that Johannes Lehmann and Markus Kleber have given earth scientists an interesting challenge. Either they have to show that humus exists using a non extreme analytical procedure or admit that it is probably an artifact of the analysis procedure. Something I had never considered but obviously should have.
We should replace the outdated idea of describing organic matter with a high turnover time as “stable humus” which is created during secondary synthesis with the new model’s definitions. However, the term “stable humus” is not the only way in which the word humus is used in the scientific literature.
Humus forms refers to a completely different thing than “stable humus”. Humus forms are genetically categorized using a taxonomic system of classification. In other words, classification of humus forms is based on identifying all the living species that inhabit the area in between the mineral soil and litter layer, with a focus on the dominant species, or ecosystem engineers. Humus forms are defined by their soil food web.
Here are the different definitions of humus and humus forms given by The Taxonomic Classification of Humus Forms in Ecosystems of British Columbia.
Here is the address: https://www.for.gov.bc.ca/hfd/pubs/docs/mr/Lmr/LMR008.pdf
Humus
Humus, in a broad sense, refers the complex organic
products of the decomposition of plant or animal debris.
These materials are more stable than their precursors,
and are generally dark in colour, colloidal, and commonly
associated with mineral constituents in soil. In soils
in general, and particularly in organic material accumulating
above the mineral soil surface, humus is closely
associated with varying proportions of undecomposed or
semi-decomposed organic materials which cannot properly
be termed humus While “humus” is a valuable conceptual
term, in practical studies it is often difficult( if not
impossible) to achieve a physical separation of humus
from other organic soil constituents. The term “humic
substances” is often applied to organic fractions isolated
from soil in the laboratory, and which are dominated by
materials with humus-like properties, but which may also
include substances which do not meet the strict definition
of humus. Humic substances includes such operationally
defined fractions as humic acid, fulvic acid and humin.
The formation of humus is termed ‘humification’, and
subsequent modification of humus, involving oxidative
or other changes in chemical structure, can be referred to
as “maturation” of the humus or its fractions.
“Forest floor” and “duff” are terms used by many
forest scientists and practitioners in North America to
designate humus forms and a kind of humus form, respectively.
In this context, “forest floor” refers to organic
materials that have formed at and near the mineral soil
surface and can include both humus and unhumified
material. “Duff’, in addition to a reference to Forest floor
in general, has also been used as a formative element in
naming the taxa of Mull humus forms, and is synonymous
with Mor in the systems of Rome11 and Heiberg
( 193 I ) , Heiberg and Chandler (194 I ) and Hoover and
Lunt ( 1952).The terms duff and forest floor are not used
in the proposed classification. The term “forest humus”
has been used in the same sense as “forest floor” or its
constituent layers, but will be used here only to describe
true humus (see above) present in, or formed under a
forest ecosystem.
The continual interaction of all processes involved in
humus and soil formation leads to the differentiation of
more or less distinct horizontal layers, termed horizons,
which collectively constitute the pedon. Both organic and
mineral horizons are recognized when describing a
pedon. For the most part, organic horizons of mineral
soils or, in the upper solum of organic soils can be designated
as humus horizons, only if substantial transformation
of original compounds to has taken place.
Humus Form‘
The humus form, introduced by Muller (1878), is defined
as a group of soil horizons located at or near the
surface of a pedon, which have formed from organic
residues, either separate from, or intermixed with mineral
materials. Thus a humus form may be comprised of
entirely organic, or both mineral and organic horizons.
The mineral horizons which are included in humus
forms are restricted to melanized A horizons characterized
by a significant accumulation of organic matter from
residues of root systems or the activity of soil fauna, both
which may be associated with infiltration. Although organic
matter may be found in B and C horizons, these are
not considered components of the humus form (Babel
1975, Barratt 1964). Humus forms are considered
natural bodies, like the soil with which they are associated.
In relation to the rest of the pedon they are characterized
by the most intensive biological activity.
Another great paper from a great author on this subject, Jean-François Ponge
Humus forms in Terrestrial Ecosystems: a framework to biodiversity
https://hal.archives-ouvertes.fr/hal-00498465/document
Very interesting – thanks for posting. It seems that ‘humus forms’ is a way to describe the whole soil layer as opposed to a component of a soil layer – it includes both organic and mineral material. add that they would not have called this something like ‘soil forms’, but I guess the main interest is in the organic component.
Great post. Interesting that the extraction process makes humic and fulvic acid not organic. That should rattle a few people. People and products throw around the terms humate, humus, and so forth and have little idea what they mean but they will back them like zealots.
Re: “the extraction process makes humic and fulvic acid not organic”. That is what i said in the post, but it is important to understand this. to a chemist they are still organic because they are organic molecules. However, to an organic gardener they can’t be considered organic since man has to add man-made chemicals to soil to make them.
I am sure the ‘organic people’ will disagree – but I don’t see how they can logically make their point.
Well…wow!
Personally I have never given much credence that anything subjected to pH 13 would bare any resemblance to what came out of the soil and have thought of the idea of fulvic and humic acids rather passé.
It does seem that a portion of the organic matter is very slow to decay and I thought carbon dating had shown some of the organic matter to be very old and the new research does not necessarily refute this.
Thank you Robert for bringing this to our attention and explaining the science in a very clear way.
The current study does not refute that some of the carbon in soil is old. However the explanation as to why old carbon exists in the soil is quite different. In the old humus model it is there because humus is very stable. In the new model it is there because it is constantly being recycled. Today it is a large molecule – tomorrow a small molecule eaten by bacteria and the next day a large molecule again.
It appears to me that this is a basically definitional item. If one is looking to improve soil structure using decayed organic matter the presence or lack of “humus” (as redefined) is not that important. If one is looking at the role of the item historically called “humus”, then minds need to be changed. The “news”, as I read it, is that humus is a human-produced (in an analytical laboratory setting) array of slowly-decomposing chemicals, produced at a pH unlikely to be found in natural environments, that have little utility to a plant. However the benefits of adding organic matter (decayed or not) have not changed. Soil structure modifications and slowly-released nutrients from the decay process are still valid concepts. Have I got this right Robert Pavlis
That is basically correct. But I think there are some important new concepts here. We think of the organic matter and nutrients coming from the plant material we put onto the soil ie adding compost and manure. What this change of thinking says is that:
1) The microbes rob the soil of a lot of the nutrients we think we are giving to the plants.
2) As the microbes die, they are adding material equivalent to the compost.
The microbes are a huge storage container holding complex organic molecules. On a chemical level microbes = compost.
It also means that many of the so called humic products on the market are probably nothing more than a form of manure.
Good point!!
I wonder how this relates to the ‘humic’ material in brown, ‘dystrophic’ water? So much of what we know is a byproduct of the way we study it.
No idea – sorry.