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?
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.
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.