Humus was always described as a black amorphous colloidal substance that has gone through decomposition and was the final product. Soil organic matter was described as material that had not fully decomposed.
As you might imagine this causes its own problems of definition.
Where does charcoal fit into this? There are lots of natural sources of charcoal and charcoal is particularly resistant to decomposition.
There seems to be controversy about whether humus is a separate identifiable material found in the soil. By definition humus is not water soluble and has to be extracted using high pH solutions of chemicals like sodium pyrophosphate or sodium hydroxide. These harsh chemicals must have a profound effect on organic molecules in the soil.
So is humus just an artefact of the extraction process? Scientists are wondering if this extraction process is one that produces artefacts – just a result of the extraction process itself.
I find this very plausible. A much better explanation of the decomposition of soil organic carbon is one of breaking down to smaller and smaller molecules with the final product being carbon dioxide or methane rather than a slow process towards large recalcitrant organic molecules. Is this thermodynamically viable? The principle of Occam's razor might lead us to think that a simple breakdown of organic molecules would be the best explanation.
I have often wondered what the biochemistry of mineralisation is especially if the final product of decomposition is recalcitrant large molecule humus that also contains nutrient anions and cations. A decomposition of organic matter into carbon dioxide or methane as final product would allow nutrients to drop out of molecules and be made available to plant roots. While it is understood that a wide range of organisms will utilise this pool of nutrients and immobilise it for a time, the fact that these nutrients are available to soil organisms and plants means that there must be a process of breakdown rather than synthesis occuring. There must also be a recycling of carbon into the bodies of these organisms and the use of organic material for both mass and energy means that carbon can remain sequestered in the soil for some time.
The production of large recalcitrant organic chemicals by microorganisms needs a firm biochemical and evolutionary basis and I cannot find any in the literature. Why would bacteria make these molecules - what are their purpose? If they are produced then they should have some value to whatever produces them.
I find the argument that there are some minerals in the soil (Manganese oxides) which can catalyse the production of complex molecules interesting, and if this can be explained within the restrictions of thermodynamics then fine. There are complex organic chemicals on and in asteroids and meteorites. However, as far as I can see (and that is not very far) entropy always wins out.
The decomposition to carbon dioxide and methane may explain why there is no evidence of a steady build up of humus in the soil. If it is recalcitrant then there should be great thick seams of it in the soil. There is not; even after thousands of years. Where has all this humus gone? By definition it cannot be broken down any more. It can’t be leached out of the soil because it does not dissolve in water and it is suggested that some of it is hydrophobic.
Strong chemicals are used to extract "humus" from the soil because it is tightly bound to soil mineral particles. Yet over the millenium all possible sites for humus attachment to soil particles must have been taken up. Does this mean that humus is washed through the soil in a similar way to water eroded silt? If it is so, why is there no indication of dark staining in the subsoil and for that matter why is it not forming such tight bonds with the subsoil minerals?
If it is not soluble in water how does it move through the soil and coat mineral particles? This could be done by soil animals but there seems to be little evidence of this. If it is a non Newtonian fluid maybe gravity has a hand in the process? However, it is recalcitrant, so why is it only seen at the top of the soil profile? Surely, if the movement of humus through the soil is dependent on gravity, then after thousands of years its black colour should be seen throughout the subsoil.
There is a mounting level of evidence that indicates that fungi and bacteria have the ability to decompose some vey polluting chemicals. Petrol and petrol derived chemicals such as pesticides can be included in this list. These chemicals are said to have a similar structure to that of humus. If these chemicals can be broken down into their constituent parts, why is humus so recalcitrant?
The properties of humus, such as its CEC are so similar to that of ordinary organic matter that it is difficult to separate the two. Except that, if humus is not water soluble and may be hydrophobic, how does it help to retain water in the soil?
Is it in the leachates of compost, manures, comfrey liquids and worm bin liquids? This dark liquid could well contain colloidal particles suspended within it and this may give us an indication of how humus moves through the soil.
Have we got an Emperor’s new suit here? Are we just chasing shadows? Does it really matter to gardeners?
I’m not at all sure. I wonder how the companies selling humus extract it because it does not seem to be an easy procedure. I doubt very much whether it is worth buying. Adding lots of cheap organic matter like compost, manures, comfrey, worm bin liquids and shredded woody material seems to be a much better way of adding humus to the soil if indeed it exsits.