Moisture. It’s one of the most important elements of composting – but also one of the most overlooked.
You won’t always need to adjust it, but knowing when – and how – to do so can help you make better, faster compost.
So in this article, you’ll learn:
- why moisture matters
- how to measure moisture (from quick checks to scientific methods)
- how to fix compost when it’s too dry or too wet.
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?
Materials added
Some compost materials are much higher in moisture than others.
For example, grass can have a moisture level of up to 83%, while some brown materials such as newspaper contain very little.
To balance moisture levels, try to ensure a good mixture of green and brown materials when you construct a pile or fill a bin.
Microbial activity
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.
Microorganisms 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 compost
- The amount of air available
- Insulation, which can be provided by the size of the compost heap, straw or through an insulated bin.
Climate/Season
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 the compost, 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.
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.
Look
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.
Squeeze
Secondly, you can take it in your hand. Squeeze it – it should have the consistency of a wrung-out sponge.
If you want to get more advanced, there are other signs you can look for when you squeeze the compost:
- It crumbles: This indicates it is too dry.
- Liquid seeps through your hand: This means it is too wet.
- The compost changes shape, but remains formed: This is a good sign that moisture levels are correct.
(You may wish to wear gloves, especially if you have seen any signs of rats.)
Measure
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
I honestly think this is a bit overkill for most amateur composters, but if you are interested 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 water may 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 add 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, these brown bulking materials will also help provide a better carbon-to-nitrogen ratio.
One of the best bulking materials is shredded leaves. Other options include use wood chips (ideally old and semi-rotted, ripped up cardboard and shredded paper.
Do note there is one disadvantage to adding bulking material to an existing compost heap. As it breaks down more slowly than the green material, the compost will take longer to be ready.
For that reason, I always prefer to turn or aerate a wet compost heap first, and then only add dry material if it is absolutely necessary.
Learn more: Bulking Agents – The Key to Successful Composting
Wrapping up
In summary, moisture is key to getting compost right, but it doesn’t have to be difficult! Simply:
- monitor your compost moisture levels from time to time
- add water if dry, ideally while turning the compost
- if wet, turn. If turning doesn’t work, add dry bulking materials too.