What are the Effects of Moisture in the Decomposition of Compost?




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Is your compost pile’s moisture level too dry (or too wet)? Find out how moisture affects the decomposition of compost, and you will know exactly what to do to ensure proper moisture levels.

Millions of microorganisms are hard at work breaking down the organic matter in your compost pile. Those microorganisms require water to survive and perform their decomposition duties properly. We know water is crucial, but what are the effects of moisture in the decomposition of compost?

An aerobic compost pile ideally has a moisture content of 40-60%. This level usually means a thin film of water will cover the compost material.

When your compost pile’s moisture level falls below 40%, microorganisms go dormant or die, stopping biological activity from happening.

Compost piles that are too wet tend to become anaerobic, which means they lack oxygen and are foul-smelling.

In this article, we will take a closer look at the role water plays in decomposition. We have also provided an in-depth guide on keeping moisture levels where they should be for optimal composting activity.

So whether your compost pile is too wet, too dry, or you are just getting started and want to get it right, make sure to continue reading.

The Role of Moisture in Decomposition

Billions of pore spaces surround the microorganisms in your compost pile. Pores are what allow essential oxygen to circulate through the compost materials.

Pores also provide an escape for a byproduct of decomposition: carbon dioxide.

Without adequate moisture levels, the microorganisms cannot function properly and die off or become dormant. If there is too much water, there is no airflow, and the organisms drown. 

You typically want your compost pile to have a “wrung sponge” consistency. When you squeeze a handful of compost material, there should be water dripping out of your hand as a wet sponge would.

It’s of utmost importance to balance air and moisture levels to create the organisms’ ideal atmosphere. Microorganisms thrive when a compost pile’s moisture content and oxygen levels work in perfect harmony. 

It will pay off for you in the end, with the result being rich, nutrient-dense compost for your garden.

Anaerobic vs. Aerobic Composting

There are two types of composting: anaerobic and aerobic. We’ve mentioned these terms previously, and it’s essential to understand the difference as moisture level requirements are not the same for both.

Aerobic Composting

Aerobic composting is performed using heat, oxygen, and moisture to break down organic materials. Aerobic compost piles are typically turned once every week and are ready in four to six weeks. Aerobic compost kills weeds and pathogens.

Anaerobic Composting

Anaerobic composting does not use oxygen to break down the materials; therefore, it lacks heat. The result is a process that takes much longer, up to six months.

Bacteria and moisture fuel this method of decomposition. Because anaerobic compost piles do not require oxygen, moisture levels can be much higher than aerobic composting.  

Anaerobic composting can spread weeds and pathogens. It also releases large amounts of methane into the environment.

Just as a reminder, we are discussing ideal moisture levels for aerobic composting in this article. 

Let’s move on to how moisture levels affect a compost pile, and how to fix piles that are too wet or too dry.

A Compost Pile That is Too Wet

If your compost pile reaches moisture levels exceeding 60%, there is not enough oxygen to keep your hard-working microorganisms alive. They are likely going to drown, which in turn causes decomposition to come to a screeching halt. 

Your compost pile is now anaerobic. It probably smells like ammonia or rotten eggs. Maybe even worse than that – it probably isn’t decomposing as it should anymore. It needs oxygen.

Luckily, it’s relatively easy to tell if your compost pile is too wet. As mentioned, it probably stinks. You may be able to look and tell that it’s soaking, or feel it and tell that it is extremely watered down.

To help get your too-wet compost pile aerated and back to an ideal moisture level, you can try doing the following:

  • Turn your pile 
  • Create a drain for excess water with an aeration tool or pole
  • Add newspaper, leaves, or straw (make sure to fluff the straw if compacted)
  • Add sawdust, cardboard, or wood chips

The goal is to release some of the water and add air back into your compost pile.

Live in a rainy climate? Try covering your pile with a tarp or plastic. This method helps keeps moisture levels where they should be and keeps the pile warm during cooler months. Heat and moisture are two of the main drivers of composting.h

A Compost Pile That is Too Dry

If your compost is too dry, dipping below a 40% moisture level, add water until you reach the desired moisture level. If you give it a good turn, it will make sure the water is evenly distributed.

You can quickly tell if your compost pile is getting dry. It will not produce any water when squeezed. Dry compost will emit little to no heat. And of course, it will just look like dried-up old soil and organic material.

If your compost moisture levels are too low, here are some ways to getting it back to where it should be (full of moisture and happily decomposing):

  • add water and turn to distribute water evenly
  • cover your compost pile with plastic, tarp, or a natural material such as straw to prevent evaporation, keeping moisture in
  • design your compost pile so that it catches rainfall 

In Summary

Moisture levels in critical so that the microorganisms responsible for breaking down material can do their job. If a compost pile is too wet or too dry, the process of decomposition becomes stagnant and will eventually not produce any usable compost.

By making sure your compost pile starts with the right balance of organic materials, air, and water, you significantly reduce the chance of moisture-level issues.

Make sure to aerate your compost pile. Oxygen is key.

Using a tumbler or compost bin can help make maintaining moisture levels easier. 

We hope this guide provided you with practical, useful ways to ensure that your compost pile is at just the right moisture level. Decomposition depends on it!

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