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.
weedmat
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?

Got Herbivorous Nematodes? Just add Leaf Litter.

leaf vs root.jpg

This study says that leaf litter was preferred by fungi and root litter by other microbes like bacteria and nematodes, and that herbivorous nematodes were controlled more by leaf litter than root litter.

Probably by the fungi catching the nematodes with lasso! Yeehaw.

It also suggests that cutting plants at the stems and just leaving the roots to feed soil microorganisms may increase nematodes over time, especially when nitrogen or phosphorus is added. Phosphorus is known to negatively impact on fungi populations, while nitrogen amendment can result in plants exuding less carbon from their roots that fungi and other soil microbes feed on, microbes that then fix nitrogen and make other nutrients bioavailable for the plant.

So, if you’re doing a no dig approach to gardening and leaving roots in place, mulch is a must. Whether that’s a carbon source in the form of leaves or a low phosphorus compost mulch or otherwise, basically think of the fungi!

Fungi wake & feed bacteria in dry and nutrient poor habitats.

59 Degrees

Researchers closely examined the transport of water, substrates and nutrients through the microscopically small hyphae of fungi. They grew the fungi on a culture medium of water, glucose and nitrogen-containing nutrients. The fungal hyphae had to pass through a dry, nutrient-free zone in order to grow through into a new area containing the culture medium. The inhospitable transition zone contained spores of the common soil bacterium Bacillus subtilis. Spores are inactive stages of Bacillus that form when there is insufficient water, food and nutrients available for bacterial growth. The bacteria go into a kind of dormant stage, from which they only awake once the environmental conditions are more favourable for living again. As the fungal hyphae grew through the dry zone, the bacterial spores germinated and they noticed clear microbial activity.

The researchers then ‘labelled’ the water, glucose and nitrogen-containing nutrients in the culture medium in advance with stable isotopes. If these substances were transferred from the fungus to the bacteria, they could be detected using the isotopic marker.

They found stable isotopes of the labelled water, glucose and nitrogen-containing nutrients in the cell mass of the bacteria — which could only have come from the fungi.

Fungi awake bacteria from their slumber

Biodiversity can offer protection to weaker species

140565_web

A new Yale-led study of fungi competition illustrates that maintaining a diverse collection of species indeed not only safeguards weaker species but also protects the genetic diversity of the larger community.

For the study, the researchers observed interactions between 37 distinct types of wood-decay fungi, which are any species of fungi that grow on dead wood. Unlike other plants, fungi species grow toward other species and compete for space.

Typically the fungi would meet near the center of the petri dish after about 20 days, after which they would begin an “interference competition” in which each species sought to overtake the other and claim available space.

Often the competitions would end in a stalemate. But in many cases the stronger species would overtake the other, growing on top of and then decomposing the weaker species.

While the most competitive fungal species tended to grow fast, an effective offensive strategy, the researchers found that other species were more adept at playing defense. Some fungal species, for example, tended to remain fixed in one location, developing a dense biomass that became difficult to overcome even by the best offensive competitors. In so doing, these defensive fungi created a buffer between the stronger species and a weaker species.

The study is published today in the journal Nature Ecology and Evolution.

C:Nhoosing Your Mulch? Think of the Fungi.

Ideally we want a diverse range of mulch and C:N ratios such that we get a diverse range of biology. I’d wondered what that range might be so did some digging, or should that be mulching? Here’s one paper.

Effects of carbon concentration and carbon to nitrogen (C:N) ratio on six biocontrol fungal strains are reported in this paper. All fungal strains had extensive growth on the media supplemented with 6–12 g l−1 carbon and C:N ratios from 10:1 to 80:1, and differed in nutrient requirements for sporulation. Except for the two strains of Paecilomyces lilacinus, all selected fungi attained the highest spore yields at a C:N ratio of 160:1

Effects of carbon concentration and carbon to nitrogen ratio on the growth and sporulation of several biocontrol fungi

Seems to confirm that for fungi to reproduce and sporulate, that they need a constant supply of carbon. Note that the study only went to 160:1, and more carbon could be more desirable.

If you look at this chart I made, the curve is steepest at the peak between about 18:1 and 50:1, this range seems likely to be the sweet spot for fungi and for soil carbon priming, however to reproduce fungi, material with a higher C:N is also desirable.

fungi vs c:n.png

mulch-decomposition-rate

Ideal Soil Element Ratios

There’s no such thing as ideal. Or is there?

While calculating the ratios of soil elements I made an interesting observation about carbon. Both plant and soil.

We know from previous studies that about 8% soil organic carbon (SOC) is optimum for yield. It turns out that’s about the same amount of carbon in most plants. Coincidence? I think not.

Optimum soil carbon for yield likely correlates with actual plant carbon.

If I take that and assume an 8% SOC 10cm topsoil and a linear decline over 60cm depth, I get 15.75% Total SOC. You see these levels in some forests and Terra Preta.

I wonder what percentage of fungi and bacteria is carbon?… Just checked… and it turns out fungi are about 8%!

*Mind Blown*

Spirulina, a cyanobacteria? 3.12%. If you divide 8% by 3.12% you get a Fungal to Bacteria ratio of 2.56:1.

Meaning you’d need 2.56 fungi to 1 bacteria to get the equivalent of 8% carbon.

What did David Johnson say was ideal Fungi:Bacteria ratio? Well, will you look at that, 2.56 is pretty darn close to the productivity maxima.

So, depending on what you are growing, it could well dictate the ideal soil organic carbon and the bacteria:fungi ratio.

FWIW, if you’re growing a human, then we’re 18.5% carbon… and a vegetarian would need to consume something like 2.3x the amount of food as a carnivore. This is below the bifurcation growth rate of chaos theory at 3.0.
Fungi:Bacteria Ratio.png

15.875% Total SOC.png

8-soc

1280px-Logistic_Bifurcation_map_High_Resolution.png