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The Microorganisms of the Compost Pile

When talking about composting the term "microorganisms" is often used to resemble a very diverse community of different types of microscopic creatures that play very specific roles in the decomposition process. This post aims to give the names and explain the roles of the different microorganisms that we find in our compost piles.


Bacteria are the smallest and most prevalent form of life comprising of 80-90% of the total microorganism population in a compost pile. For some perspective a single teaspoon of compost contains between 100 million and a billion of them! Bacteria are single-celled organisms that can be round, rod, or spiral shaped. One of the most commonly discussed and important types of bacteria in composting is known as Actinomycetes. They behave similarly to fungi because they grow in a filament network that branches off in every direction.



Actinomycetes bacteria are one of the main causes of that staple earth-like smell that compost gives off. They can sometimes even be seen with the eyes because the filament networks form big white cob-web looking colonies on top of compost piles, especially towards the end of the entire composting process. Actinomycetes have specialized biological functions (enzymes) that help break down tough, woody debris like stems, bark, newspaper, and dry leaves. Scientifically this is the same as saying they break down complex organic molecules like cellulose, lignin, chitin, and proteins.


The next type of important microorganism we find in compost piles is fungi. They are both multi and single cellular organisms that come in the form of mold, yeasts, and mushrooms. Fungus live in long extended networks that can even connect underground - in Oregon researchers found a tree-killing fungus extending underground that they believed to be the single largest organism on earth at 3.5 square miles in size. Fungi basically play the same role as bacteria breaking down organic waste both tough and soft. What makes them important is that they often decompose the matter in the compost pile that the bacteria might miss or have trouble with such as materials that are too dry, too acidic, or too low in nitrogen for bacterial activity.



In the microorganism category fungi and bacteria are the two most important players in the decomposition process. They exist in the psychrophilic, mesophilic, and thermophilic stages of the compost pile progression. These are different heat stages of a compost pile which we have written a whole other blog post about here. Although it is important to note that as the pile heats up hotter and hotter it is really only bacteria that work inside the pile while fungi moves and lives on the outside of the pile since they can't sustain the higher temperatures as well.


There are more types of less important microorganisms in compost piles. Protozoa are the simplest form of animal organism that feed on organic material, bacteria, and fungi. Rotifers do the same thing except they also eat protozoa - the hierarchical food chain extends all the way down even to the microscopic level! They start to die off quickly when the compost temperatures start rising. Both organisms dwell in water droplets and water films attached to soil particles, plant parts, and fungal bodies.


There are two more microorganisms that fall between being microscopic or macroscopic (seen by the eye) - they are known as Tardigrades and Nematodes. Both can be very small on the microorganism level but when they are fully grown they can be about 1 millimeter and be seen by the eye. Tardigrades are water dwelling and are called "waterbears" since they are slow and lazy and look sort of bearish. They eat plant matter, bacteria, protozoa, rotifers, and nematodes. They have special qualities that allow them to survive in extreme pressures, dehydration, very high and low temperatures, radiation, and were even identified as the first organisms to survive in outer space! Nematodes/roundworms are closely related to rotifers and eat everything like the others except for Tardigrades. They are often clear and cylindrical and look a lot the earth worms we are used to seeing.



There is a very diverse ecological community in our compost piles. When we construct these piles and introduce them into our soil it restores the ecological environment and allows plants to grow more closely to how nature intended rather than treating everything with manmade synthetic, corrosive fertilizers that has been popularized by industrial agriculture. Our goal at Kaimuki Compost Collective is to use our daily food waste to produce compost and reach this goal!




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