Humification Theory [Rant]

“Soil organic matter (SOM) derives from dead plant parts in the litter layer”
“A fraction of SOM persists because it can resist decomposition.”
“The process which converts litter into resistant or “recalcitrant” soil organic matter
was called humification”
The product of humification = presumably ‘stable’ organic matter = is called humus

I don’t believe in humification theory. There, I’ve said it. I cringe every time I hear the word humus.

Biochar isn’t even that stable in soil. Unless it’s buried where oxygen doesn’t reach it deep deep in the profile. Something fungi can do with it by embedding it inside soil microaggregates where it won’t be oxidized or access by other organisms. But carbon cycling in soils doesn’t stop there.

I’m not the only one that questions it. Here is a video summary of a paper by Johannes Lehmann and Markus Kleber from 2015, and there are others I’m too lazy to link to.

How is carbon stored in the soil?
Thomas Fester

Here are some slides from a talk about the paper, a talk I wish I could find, and the scientific paper from Nature.
However even Johannes Lehmann the co-author of the paper uses the word humus in the following talk on soil carbon two years ago around the time the paper was released. It’s so engrained in people and our language that I believe it will be around long after the “stable” humus is.


No doubt I’m bound to use the word humus to confuse myself and others too, just a heads up then… 🙂

Should you use plastic weed mat?

Comments and Questions 6 20 2017

Kevin answers questions and comments on his videos about a range of topics and emphasises the importance of soil, water and air.
Here I want to emphasise the importance of plants and organic mulch too.
One of the comments on his Q&A video was the following after seeing Kevin’s use of weed mat “to keep the soil covered”:

My only experience with weed mat, was pulling one up, and finding stinky dead earth beneath

Now it sounds like this person’s plastic weed mat caused the soil below it to turn anaerobic due to either a lack of air penetrating the mat or retaining too much moisture which then excluded the air from entering the soil. When this happens the soil begins to anaerobically digest and produce alcohols, phenols, and gasses such as methane, the scent of sewer gas.

Not ideal plant growing conditions!

However there can be benefits when using plastic weed mat in the right conditions that does let air and water through, as the mat can help keep the soil moist, especially in drier regions. Weed mats can also warm or cool the soil depending on how much sunlight they absorb or reflect. They can also help prevent erosion. But the main reason people use it, is because the mats also reduce weed competition for plants planted into or near the weed mat. Reduced competition for water, sunlight, and for nutrients.

However the biggest problem with plastic weed mat when growing and assuming the correct mat was chosen for the conditions, is that if left on for extended periods, the soil often tends to breed plant pathogens. This happens when soils are not amended with a high carbon source such as plant leaf litter, plant exudates, or an organic mulch.

In soils that are kept moist and aerated and warm the microbiology and fauna become more active and will chomp through organic matter like there is no tomorrow. In doing so there is an increase in soil carbon respiration in the form of carbon dioxide and methane along with other gasses. This increase in respiration can actually help increase plant growth by providing carbon dioxide concentrated around the plant leaves. The increase in microbiological activity also increases nutrient cycling and plant available soil nutrients. However if the plants aren’t putting the carbon back into the soil via their roots, exudates, or plant litter when they die – then over the long term the soil community suffers.

When there is a lack of high carbon input that organic mulch provides to soil organisms, competition for that soil carbon increases. And the less soil carbon, the less complex organisms will survive. This is particularly important for fungi that rely on the carbon because they are made up of more carbon than other microbiology. Worse, the fungi that do survive the hostile conditions are often those that are plant predators able to fight for the carbon needed in order to survive because they now lack competition. As a result those predators infect plants and reduce yields or even kill them, and so gardeners and farmers search for solutions to their fungal problems in the form of fungicides. As a result fungi get a bad name. The same happens to nematodes.

However for short season plants like seasonal crops in Kevin’s example, this may not be much of a problem as the plant may be ready for harvest before the predators have overcome a plants defences, and he’s adding organic matter every year.

To conclude, when organic resources are plentiful, everyone’s happy and works in symbiosis, and when they’re not happy it’s war. Not quite extremophile Star Wars, but certainly localised Planet Wars, and eventually those wars include us higher order carbon beings in Human Wars that result from desertification and a lack of resources.

Plastic mulch is also plastic. Did you know that most sea salt already has microplastics in it after a little over 100 years of plastics use?