Fungal vs Bacterial Compost: Differences & Uses in Garden

Mulching and making compost is crucial for any organic garden.  But your compost could be dominated by bacteria or fungi and they can have different uses in your garden.

What are the differences between fungal and bacterial compost?

Fungal compost is made of carbon-rich materials which are best used for trees and perennials. On the other hand, bacterial compost is made of nitrogen-rich materials and is best for vegetables and annuals.

In this article, we will look at their differences, how to tell the difference, how they are made, and when you should use which.

Differences between fungal and bacterial compost

Fungal and bacterial compost can be differentiated based on how they’re made, what they’re used for, and the decomposing microorganisms.

1. The composting microbe

Fungi and bacteria are unrelated microorganisms and decompose organic matter in different ways.

Fungi and bacteria are both primary decomposers that break down organic matter using enzymes they produce. But the two microorganisms are different. Fungi are (mostly) multicellular and immobile, while bacteria are unicellular and mobile.

The cell wall of fungi is made of chitin and has no nitrogen whereas the bacteria cell wall is made of peptidoglycans. Since chitin has no nitrogen, fungi have a higher carbon-to-nitrogen ratio and require more carbon for growth and development than bacteria. That is why they are more suited to decomposing carbon materials.

2. How it is made

Fungal compost is primarily made of more carbon-rich materials than nitrogen, as fungi require greater amounts of carbon to grow and reproduce compared to bacteria.

On the other hand, bacterial compost is primarily made of more nitrogen-rich materials because bacteria require more nitrogen for growth and reproduction.

Having more carbon in the compost results in a fungi-dominated compost, while more nitrogen will produce a bacteria-dominated system.

Besides the composition, the composting process and time are also different.

Bacterial compost requires a relatively short composting period since the nitrogen materials are soft or fresh and easily degradable, and thus requires regular aeration.

Fungal compost takes a longer time, as the carbon materials are often dry and hard.

3. What it’s used for

Fungal and bacterial compost also differ in the type of plants they are used for. Fungal compost is used for trees and perennials while bacteria compost is used for annual crops and vegetables.

Generally, bacterial-dominated compost is used as a soil amendment for vegetable gardening and for annual cereals (like wheat and maize) as these plants prefer bacterial systems to fungi.

On the other hand, fungal compost is used for perennials and trees, as these plants favor fungal soils.

How to tell the difference between fungal and bacterial compost?

Fungal compost can be differentiated from bacterial compost by the color and appearance, the presence of mold/mushroom, and the pH of the compost

1. Color and appearance

The color of compost can sometimes tell whether it’s fungal or bacterial. Generally, bacterial compost is a lighter brown, while fungal compost appears darker brown to black, resembling the color of humus or forest floor.

The color difference is in part due to the ingredients in the compost. Bacterial compost has a higher amount of “greens” (nitrogen-rich organic matter) than “browns” (carbon-rich matter), giving it a lighter appearance. On the other hand, fungal compost typically contains twice as many browns as greens, as a result, it has a darker look.

Even when the compost isn’t fully degraded, you can differentiate a fungal-dominated compost from a bacterial compost from the visible constituents. An indication of a fungal compost is usually a noticeably greater amount of “browns” such as twigs and wood chips compared to nitrogen-rich “greens” such as fresh grass, fallen leaves.

2. Mold

Fungal compost contains mycelium and mold, but bacterial compost doesn’t.

Finished fungal compost usually has white or light green mold. Or it could also have white or yellowish streaks either on the surface or just beneath the surface. There might also be some mushrooms in the compost pile.

The white streaks are mycelium while the mold and mushrooms are the fruiting body of the fungi in the compost and the presence of this mold, mushroom, or mycelium is an indication of a thriving fungal population.

Bacteria-dominated compost never contains mycelium or mushrooms. There may be some mold, but it appears grey not white. This is a sign of actinomycetes, a fungus-like bacteria, that works like fungi, degrading woody and carbon-rich material like cellulose and chitin. This bacteria also give compost its characteristic earthy smell.

3. pH of compost

Bacterial compost is neutral or slightly alkaline while fungal compost is slightly acidic.

Fungi thrive in low pH or slightly acidic conditions, but bacteria prefer neutral conditions. Thus compost that is acidic will likely have more fungi in it, especially if it contains twigs, tree bark, and branches as these are largely acidic.

In contrast, bacteria prefer alkaline conditions, and as such an alkaline compost will likely have more bacteria than fungi.

4. Temperature

Temperature can be another way to differentiate between the types of compost.

Generally, a bacteria-dominated compost can get as hot as 200 F (93 C).  It is hotter than a fungal-dominant compost because of much bacterial action, leading to increases in temperature.

Fungi can also decompose organic matter at higher temperatures, but this is rarely the case because the fungal compost usually isn’t well aerated, so microbial activity does not get to such high levels as to greatly increase temperature.

But temperature isn’t the most reliable way to differentiate fungal and bacterial compost, as even bacteria-dominated compost will have low temperatures if not well aerated. Furthermore, the temperature can only be used as a differentiator when the compost has not yet matured because fungal and bacterial compost are both cool at the end of composting.

What is fungal compost?

Fungal compost is compost that’s dominated by fungal microbes, with lesser bacterial activity.

Fungal compost is decayed organic matter that contains a high amount of fungi. There may be other microorganisms like bacteria present, but the bulk of the microbes are fungi of different species including Trichoderma, Emericella, Mucor, Fusarium, Penicillium, Acremonium.

All these fungi help to degrade the organic matter in the compost, but Trichoderma is also important to plants as it forms a mycorrhizal relationship with plant roots, acting as a biocontrol agent and inducing growth in exchange for some plant sugars.

Compost may also contain spores of Arbuscular mycorrhizae which improve plant nutrient and water uptake, but the spores only germinate in the presence of plant roots.

– How is fungal compost made?

Fungal compost is made by increasing the number of carbon materials in the compost.

Regular compost is made from a mix of carbon-rich and nitrogen-rich organic matter in roughly equal proportions. To make a fungal-dominated compost, the amount of carbon content in the pile is increased significantly by adding more carbon materials like wood chips, cardboard, and twigs to the pile.

Higher carbon content encourages fungal growth. However, the nitrogen content should not be too low or completely absent as fungi still need nitrogen to survive.

Additionally, the compost is also started with a layer of carbon materials before adding the nitrogen. Starting with carbon materials helps establish fungi colonies early on.

Aeration is necessary, but overturning the compost can destroy any mycelium, mushrooms, and mold, negatively affecting the fungi. To prevent this from happening, larger branches and twigs are spread systematically through the compost to prevent compaction and allow airflow through the system.

Worms can also be added to the heap to speed up the composting and for added aeration.

Fungal compost often takes longer to make than regular compost. This is due to the higher carbon content which takes longer to degrade since the materials are dried, hard, tough, and large.

What is bacterial compost?

Bacterial compost is compost that’s dominated by bacterial microbes, with low fungal activity.

Bacterial compost is decomposed organic matter in which bacteria are the dominant class of microorganisms. There can be several species of bacteria present in the compost, including Lactobacillus, Bacillus, Acetobacter, Actinobacteria.

Actinobacteria and Bacillus in bacterial compost are examples of plant growth-promoting rhizobacteria (PGPR). They improve the availability of nutrients and minerals, induce plant growth, and fight off pathogens.

– How is bacterial compost made?

Bacterial compost is made by increasing the amount of nitrogen-rich organic matter in the compost.

For bacterial compost, the nitrogen content in the compost is considerably greater than the carbon content. Increased nitrogen encourages bacterial growth and activity since bacteria feed more on nitrogen-rich materials like fruit peels, fresh grass clippings, and leaves.

However, there should still be a substantial amount of carbon materials in the compost as bacteria need carbon for energy.

As the organic matter decomposes, the compost begins to heat up and is aerated frequently by stirring it and overturning it to supply fresh air to the microbes. Without aeration, the composting will slow down and the compost will cool, as the bacterial population and activity reduce.

With proper aeration, bacterial compost takes 5 to 6 months for all the organic matter to be degraded and no longer identifiable. If there is no air in the system it takes longer (up to one year) as the anaerobic bacteria take over the process, producing foul odors in the process

Is compost a bacteria or fungus?

Compost can be either bacterial or fungal, depending on which types of microbes have a greater presence in it.

Compost can have a high percentage of bacteria or more fungi but in either case, the compost can never be completely bacterial or completely fungal.

Whether the compost is dominated by fungi or bacteria depends largely on the organic matter used to make the compost, and to a lesser extent, the composting time and process.

If there’s more carbon material, the resulting compost will have more fungi species. If there is more nitrogen organic matter and more frequent and thorough aeration, then the resulting compost will have more bacteria in it.

Which is better: fungal or bacterial compost?

Fungal and bacterial compost work best for different situations. Bacterial compost is best for vegetables, legumes, and annual plants, while fungal compost is best for perennials and trees.

–  For vegetables and annual grains

Bacterial compost and soils recycle nutrients rapidly to support the short life cycles of annual plants.

Bacterial compost is more suited for vegetables and grain crops like wheat, rice, maize, and other annual crops with rapid growth and short life cycles.

This is because bacterial compost encourages growth-promoting rhizobacteria that are mobile and recycles nutrients rapidly to meet up with the faster growth cycles of annual plants (Hoorman, 2016).

Additionally, brassicas like cabbage, cauliflower, rapeseed, kale, and Brussels sprout are one of the few plant groups whose roots do not associate with any type of mycorrhizal fungi. Their roots contain high amounts of glucosinolates which give the characteristic slightly bitter taste. In the soil, glucosinolates transform into isothiocyanates, an antifungal chemical that prevents root colonization by fungi (Glenn et al, 1988). This leaves only bacteria as the beneficial microbes available to these plants.

–  For legumes

Bacterial compost is best for legumes, as bacteria is needed for nitrogen fixation.

Legumes like beans, soybeans, lentils, alfalfa, clover, and cowpea are Nitrogen fixing plants. Rather than absorbing nitrogen from the soil, they form symbiotic relationships with different bacterial species to absorb nitrogen from the air. As such, they need soils with more bacteria than fungi.

–  For perennials, and trees

Fungal compost benefits perennials and trees by supporting the development of mycorrhizal relationships with plants.

Perennial crops and trees have much longer life spans than grains and vegetables. And they take some time before bearing fruit. As a result, they aren’t suited for the rapid nutrient recycling of bacteria.

Instead, they favor fungal soils and compost that support the germination of mycorrhizal fungi which, unlike bacteria, aren’t mobile. Fungal-dominated compost also supplies nutrients at a slower pace that matches the growth of the plants.

Happy gardening!

Related

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Top 8 Soil Inoculants for Plants (Editor’s Picks)

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References

Di Lonardo, D. P., van der Wal, A., Harkes, P., & de Boer, W. (2020). Effect of nitrogen on fungal growth efficiency. Plant Biosystems – an International Journal Dealing With All Aspects of Plant Biology, 154(4), 433–437. https://doi.org/10.1080/11263504.2020.1779849

Fungi Vs Bacteria. (n.d.). Soil Health. http://www.soilhealth.com/soil-health/organisms/fungi-bact/

Hoorman, J. (2010, September 7). Role of Soil Fungus. Ohioline. https://ohioline.osu.edu/factsheet/anr-37

Gao, L., & Liu, X. (2010). Effects of Carbon Concentrations and Carbon to Nitrogen ratios on Sporulation of Two Biological Control Fungi as Determined by Different Culture Methods. Mycopathologia, 169(6), 475–481. https://doi.org/10.1007/s11046-010-9282-9

Glenn, M. G., Chew, F. S., & Williams, P. H. (1988). Influence of glucosinolate content of Brassica (Cruciferae) roots on growth of vesicular-arbuscular mycorrhizal fungi. New Phytologist, 110(2), 217–225. https://doi.org/10.1111/j.1469-8137.1988.tb00255.x