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The Role of Microorganisms in the Soil and in the Composting Process

Updated: Nov 16, 2021

Oftentimes when the topic of composting comes up the term ‘microorganisms’ immediately follows it. There are two reasons for this: the first reason being that microorganisms are essential to breaking down organic waste that eventually becomes compost. The second reason is that once the compost is finished and utilized in the soil, those same microorganisms living in the compost promote healthy soil structure, porosity, and density that is optimal for the growth of plant roots.

But what are microorganisms and what do they actually do in the first place? An organism is defined as some kind of “living system that functions as an individual.” The micro part refers to the unit of measurement the micron (micrometer) and the tool used to observe these organisms in real life - the microscope.

There are many different types of microorganisms that comes in different sizes and all perform different types of roles; we will write a follow up post all about the different types of microorganisms active in a compost pile next!

In general the microorganisms live by producing energy aerobically. Like humans, this means that they “breath” oxygen in order to break down their food. In this case, their food is the waste scraps we give them and they use it along with oxygen to produce energy. As the microbes eat and break down the food waste we want composted while “breathing” the oxygen in the air, they also produce CO2, heat, and water vapor. This is why the compost pile can heat up and sometimes even give off steam - this process is often referred to as hot composting or the Berkeley Method. Eventually the pile cools down giving us the beautiful compost we need for our gardens and farms.

Compost is universally known to improve the properties of soil used to cultivate plants. The microorganisms play a role in this for several reasons. When the microorganisms break down organic waste (our food scraps) it also produces humus (no, not the type you eat with Pita!). Humus essentially acts as a glue that binds soil particles together into clumps. This is why compost can be grabbed by the hand and squeezed into a solid ball that will stick together after it's let go. When the soil particles are glued together it makes it resistant to erosion. “Glued” soil is less likely to run off in every direction when erosion forces like wind and rain pass over time. When the soil particles are glued by humus it will also hold moisture more effectively. A sponge dipped into water soaks up lots of water - once that sponge is shredded up into tiny pieces it becomes unusable and won’t hold any water. The humus essentially forms the soil into coherent clumps, keeping the sponge together allowing it to retain moisture.

Microorganisms also have a role in suppressing pathogens within the soil. Pathogens are also considered to be microorganisms but the difference between them and our friendly composting microorganisms is that they cause diseases that get plants sick, the humans that digest the plant sick, or both.

Good composting, soil health promoting microorganisms are introduced into the soil from compost and suppress pathogens through competition. When the good microbes take the space and live in one area it becomes less likely for harmful pathogenic microbes to fight their way into the space.

Although this is surely not the last thing the good microbes do for the health of the earth's soil, the good microbes can also help break down toxic organic compounds in the soil. Vast amounts of hydrocarbons (petroleum/oil) has been introduced into our environment through everyday industrial production methods (our clothes, food, technology, ect.), big accidents like oil spills, all the way to our everyday transportation in our cars. Every little drop of oil we see drip out of our car or lawn mower makes its way into the soil. Research has found that hydrocarbons leached into our environment causes "reductions in species richness, evenness and phylogenetic diversity, with the resulting community being heavily dominated by a few species." This is academic code for 'oil in the environment kills just about everything.' It's not just oil in the environment either - pharmaceuticals and the waste associated with their production has been found in our soil and water, not to mention the vast amounts of micro-plastics polluting our oceans and soil. The good news is that some microbes that break down contaminants in our water and soil are introduced through a process labeled as compost bioremediation.

If the majority of the world discarded their food scraps into compost piles the earth would become a healthier place. That's likely not going to happen for some time so our goal here at the Kaimuki Compost Collective is to take that burden off of peoples hands for a small fee. We are able to centralize the waste and make it available to local farmers and gardeners who need it.

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