Compost Magazine

Composting tips, advice and science.

Ash falls from a hand against a backdrop of trees.

Is Ash a Good Addition to Compost? An Evidence-Based Answer

I love using (the right kind of!) ash in the garden.

I use it as a barrier around young tender plants to deter slugs, as part of a mix to make highly effective homemade fertilizer and to adjust the pH of the soil before planting brassicas.

(I’ve even started to ask friends to keep leftover ash from their wood burners for me.)

But what about composting – could you use it there too if you had enough of it?

Let’s take a look.

What is ash?

Ash is a byproduct of burned organic material, such as wood. It is composed of minerals, metals, and other trace elements from the burned material.

It can be beneficial to soil – when used in the right quantities.

Although it contains little or no nitrogen, some elements may encourage beneficial microbial growth whilst others can help to retain moisture in the soil.

Still, care must be taken when using ash – this alkaline material can contain metals or toxins which can end up in the soil if handled incorrectly or if large amounts are added.

There are certain keys to dealing with this which we’ll cover later on in this article. 

What nutrients does wood ash have?

While the nutrients in ash can vary, the following table provides a useful guide to average nutrient levels in wood ash. Do note that other ash types may vary significantly.

NutrientApproximate levels
CalciumAlmost 20%
Phosphorus, Magnesium, Aluminum, Sodium>2%
Phosphorus, Magnesium, Aluminum, Sodium>2%
Nitrogen 0%
Source: University of Maine

Can you compost with ash?


In fact, certain types of ashes have been used as an effective form of fertilizer for centuries. There are a couple of cons – but as we’ll see, there are also plenty of pros.

Six Benefits of composting with ash

One study by Koivula et al tested adding different amounts of ash (10% and 20%) to composting material that were then processed in a drum composter. They found surprisingly strong benefits:

Increases oxygen levels

The scientists found that oxygen levels at the feeding area of the drum were higher in the mix that had ash.

Oxygen is essential for composting bacteria, which need it to reproduce and break down compost material.

Increased heat

In both the mixes with ash, the heat was higher than the control. The mixture with 20% ash got as high as 80C.

As we discussed in our article on pathogen elimination, that’s not necessarily a good thing, as too high a temperature during the composting process can reduce microbial diversity.

Reduced nitrogen loss

Some nitrogen is always lost in the composting process – but Koivula et al found that adding ash reduced the amount lost over a two-year period.

Reduces Hydrogen sulfide

Hydrogen sulfide (H2S), aka manure gas, is a poisonous gas that smells of rotten eggs.

It can be fatal in large quantities, although it’s not likely to be of concern to the average garden composter.

Koivula et al concluded that the additional oxygen in the compost meant that H2S was prevented from forming.

Increased decomposition speed

Using a number of different methods, which included measuring humic matter and mineralization, the researchers found that the mixes with ash broke down organic material more quickly than those without.

Better plant growth in ash-amended compost

A separate study by Kuba et al tested adding different composts to soil.

Three composts were tested, one with 8% ash and one with 16% ash.

They found that plants grew better on soil with ash compost than it did with regular compost.

They did note that only compost with low heavy metal content should be used.

Potential cons of composting with ash

Ash can contain heavy metals.

However, the study by Koivula et al noted that metal content was at acceptable levels for organic gardening. As mentioned below, though, it is advisable to limit the types of ash you use in compost.

Wood ash can be very alkaline, and can increase the levels of salts and pH in compost.

This may be worth bearing in mind if you have alkaline soil, or grow acid-loving plants. On the other hand, if you have acid soil, that alkalinity could help balance the pH.

Ideally, you should keep the level of your soil below 7. However, the RHS notes that raising alkaline levels further can be useful if your soil harbors club root disease.

Indeed, Demery et al found that ash compost was most suitable for tropical acid soil and woods.

(I have also used ash on my neutral soil, and haven’t found any problems, although I use it most heavily when planting alkaline-loving plants like Brassicas.)

If you’re not sure of the pH of your soil, it may be worth purchasing an inexpensive pH meter – these can also be useful for monitoring your compost.

The best type of ashes to use for composting

Flames lick logs of wood in this woodburner.
Ash from a wood burner is ideal.

It is important to think about the source of the ash and what might have been burned.

Ash from some sources are likely to contain heavy metals at levels that cause health problems.

Ash from untreated wood is generally considered one of the safest forms of ash, and I get most of my ash from my wood burner (and my friends’!) and garden incinerator.

You can also use ash from garden bonfires.

Biochar is also a good option, especially if inoculated with microbes.

Coal is best avoided, while charcoal raises some debate. Briquettes, as well as charcoal that has been mixed with lighter fluid, should definitely be avoided.

How much ash should you add to your compost pile?

As we have seen, adding as much as 20% ash to your compost could make it too hot, and reduce the number of microbes in the finished product. It may also make the mixture too alkaline.

A good rule of thumb is to add a maximum of 1 part ash to 10 other compost materials and to mix it in well with the compost material so it is not concentrated in one part of the compost.

Wrapping up

Ash has many uses in the garden, and improving your compost is yet another way to use it!

Just be sure to take care with the type of ash you use, the soil and plants you use it with and the amount you use.

Read more

How Biochar Can Improve Compost Speed and Quality
Can you compost hair? Here’s what the science says
Scientists grow cabbage in compost from human poo
What can you compost? From everyday items to the bizarre

External links

Using wood ash in the garden (RHS)