Moisture in compost is one of several key elements to successful composting. It’s not something you will always need to worry about, but it is worth monitoring in case you do need to take action.
In this article, you’ll learn everything from why moisture levels are important to how to measure and control them. Let’s get started!
Why is moisture important in compost?
In the composting process, microorganisms such as bacteria, fungi and physical decomposers break down organic material.
To do so, they need the right amount of moisture.
A lack of moisture will mean that most microorganisms will not be active or reproduce. As bacteria have no legs, they also need moisture in order to move around the compost heap.
Too much moisture, though, and air will be forced out of the compost heap. This will lead to anaerobic (without air) composting, which is slower, releases more greenhouse gases and can lead to bad odors.
Water also impacts the heat of a compost pile – drier compost heaps heat up more quickly and cool down more slowly than wetter compost heaps.
What is the right amount of moisture?
Compost requires a moisture level of between 45 and 60% – a figure recommended both by the Composting Handbook (p.65) and Cornell University.
What impacts the amount of moisture?
Some compost materials are much higher in moisture than others. For example, grass can be 83% moisture [add title to link], while some brown materials such as newspaper [add title to link] contain very little. Try to ensure a good mixture of green and brown materials when you construct the pile.
As we’ve seen, the microorganisms in the compost need water to thrive. As they become more active, they use up this water, causing the compost to dry. They can also create heat which, if too hot, can burn compost material leaving ash residue behind. This uses more water, further decreasing the amount of moisture in the compost pile.
Bacterial activity is in turn influenced by a number of factors, which include:
- The carbon-to-nitrogen ratio in the soil
- The amount of air available
- Insulation, which can be provided by the size of the compost heap, straw or through an insulated bin
Another factor that impacts moisture levels is the season. Where I live, it is very damp and wet in winter. If I don’t cover the compost, water will soon permeate it. Even if I cover it, the air is very damp and little evaporation takes place.
Cooler temperatures also inhibit the most active microorganisms – thermophilic bacteria. That means there is less activity going on in the compost heap, and less water is being used up.
In the summer the opposite happens. Bacterial activity is faster, using up more water, and more evaporation takes place.
If your compost is covered, moisture levels will be affected. In a wet season or a climate, this can help prevent the compost from getting wet and reduce evaporation However, in hot, dry weather it can mean that your compost can dry out too quickly.
I tend to cover my compost in the winter to prevent it from getting too wet, but take off the cover in the summer.
How can you measure moisture?
I find measuring moisture one of the trickier parts of composting. My compost piles are large, and it’s hard to know what’s going on in the middle! In the past, I’ve turned compost to find some parts moist, and other parts completely dry.
That’s why turning compost helps. It allows you to inspect the inside of the compost, and measure if your compost is too dry or too wet.
There are several ways to do this.
The first is to just look. When I open the base of my compost bin or stir the top, I can often see if it is too dry or wet.
Secondly, you can take it in your hand. Squeeze it – it should have the consistency of a wrung-out sponge. (You may wish to wear gloves, especially if you have seen any signs of rats.)
The most accurate way is to use a moisture meter, which can give an accurate idea of the moisture level of your compost.
You can also measure the moisture levels by:
- taking compost material
- weighing it
- drying it out
- weighing it again
- comparing the difference in weight
See Moisture Content by Cornell University for an in-depth (and highly scientific) guide to how to do this.
How can you adjust moisture?
Increase levels of moisture
As you can imagine, to increase moisture levels you just need to add water. However, that’s not quite as simple as it sounds, as if you just soak a compost pile or bin, the danger is that the water will not be distributed evenly.
It’s easier to add water as you turn your compost. You can inspect each layer, and spray it with water as you do so.
Decrease levels of moisture
Turn the compost
An effective way to decrease moisture levels is to turn the compost. This introduces oxygen into the pile, which speeds up bacterial activity. I’ve found that even quite a wet pile can rapidly gain heat, an indication that aerobic bacteria are breaking down the organic material. As these bacteria increase their activity, they will use up more moisture.
Add dry bulking materials
Another method is to dry bulking material. These help by absorbing some of the moisture. They can also help to create free air spaces in the compost, which will help the bacteria. If your compost is high in nitrogen, it will also help provide a better carbon-to-nitrogen ratio.
Good bulking materials can include dried paper, semi-decomposed wood chips and sawdust. Learn more about bulking materials.
In summary, moisture is key to getting compost right, but it doesn’t have to be difficult! Monitor your compost moisture levels from time to time (I tend to do this twice with my large compost piles, which is the number of times I turn them), add water if dry and turn and add bulking material if wet.
Read more about how to compost…
Home Composting Guide: A Step By Step Guide To Getting Started
What You Can Compost: From Everyday Items To The Bizarre