Can You Compost Ash? – Great Option but Some Limits

After that fire in your fire pit or fireplace, what do you do with the ash?  Many of us throw it out or toss it in a pile.  Can you compost ash?  Is there anything to be concerned about?  Let’s check it out.

Let’s start with a couple of assumptions because that helps us answer the question of whether you can add ash to compost.  First, we assume that you’re burning wood, paper, or other natural materials.  If you’re burning trash, plastic (why?), or something else, then the answer may be completely different. In fact, it probably will be.

You also want to avoid using charcoal ashes. Yes, charcoal is natural, unlike what I just said (sorry).  However, charcoal often has additives to preserve it and/or to make it easier to light, so it’s not “all” natural.  These substances can be harmful to your plants.  Best to avoid this.

Similarly, even though it’s wood, if you’re burning wood like pressure-treated wood, don’t add this to your compost either.  The truth is, though, you probably shouldn’t be burning this at all.  It’s not good for you.

Yes, you can compost ash if you’re burning wood, paper, or something else natural.  After all, they are natural materials, and what’s left over after burning is natural so that alone says it’s probably compostable. 

Why Ash is Good For Your Compost

Ash has a lot of minerals that are good for compost, in addition to the carbon that it adds as brown material.  It has several additional minerals that your plants and flowers will like.  Some of those include

• Calcium
• Chromium
• Copper
• Manganese
• Nickel
• Phosphorous
• Potassium
• Zinc

Calcium alone is about 15% of the total mass of the ash, making it the most common mineral there.  This helps your plants grow stronger plants walls (similar to how calcium is good for our bones.)

The potassium helps to encourage flowering and fruiting. 

Minerals like these will help your plants grow stronger and healthier.  The actual concentration will vary by the type of wood that you burned.  Younger wood tends to have more nutrients.  Hardwoods will usually have more nutrients than softwoods. 

Wood ash also has a neat feature: it tends to repel slugs, snails, and ants.  These slimy little buggers can damage your garden.  You can add it directly to your garden for this and replace it as the rain washes it away, but be very careful about adding too much (don’t want to save your plants from the bugs only to kill them by making the soil too alkaline.)

Can Ash Make My Compost Too Alkaline?

We’ve heard from citrus fruits and a few other materials that they then to lower the pH of the compost, which means they are acidic.  Ash does the opposite.  It’s more alkaline and can raise the pH.  For most of us, this isn’t a problem. It should be okay to add a small amount, like a pound or two, over a season.  It’s similar to what we saw with oranges and other citrus fruit.  Too much of a good thing can be a problem.  In that case, you may not need to add as much or you may need to do more work to keep your compost, and then your soil, from becoming too alkaline.

Plants like slightly acidic soil (just very slightly acidic, so don’t go crazy that way either.)  If you add too much ash, your compost may make your soil too alkaline (also called basic), so you may need to add something more acidic to balance it out.  Short story long, don’t add a crazy amount of ash. For most of us, this isn’t a problem.  A pound or two of ash is a lot of ash, and we’re not likely to have that much unless you’ve started that home incineration business.

If you’re concerned, you can use a soil testing kit to check the pH.  Here are a few kits that you can check out.  They’re pretty easy to use.  Generally, it’s not a problem for most of us.

Where to Avoid Adding Ash to Your Compost

As we saw with mushroom compost, a few plants just don’t like alkaline soil. Just like with mushroom compost, if you plan on using your compost with these plants, it might be best to avoid it. 

Here are some of the plants that you might just want to avoid ash (or mushroom compost) altogether)

• Blueberries
• Holly Shrubs
• Magnolia Trees
• Daffodils
• Marigolds
• Hydrangeas
• Chrysanthemums
• Rhododendrons
• Azaleas

How to Compost Wood Ash

Generally, it’s a good strategy not to dump all the ash in at once.  This varies a bit depending on how you compost.  Let’s take a look at a few options, but the short story is to add it a bit at a time and give it time to be fully composted before you use it in your soil.

Worms tend to like compost with ash in it.  Ash added to your worm bin can actually help to make composting more efficient (but remember that more isn’t necessarily better). 

If you need to store wood ash because you have too much to add to your compost all at once, make sure to store it someplace dry.  The ash is water soluble, and rain will dissolve it.

Single Compost Pile That You Regularly Care For

A good option here is to spread out the ash.  If you regularly turn your compost pile and take good care of it (it’s more of a “hot” compost pile), then once a month or so is a good strategy.  Give some time between when you add the ash and when you use the compost (again, about a month at least.)

Multiple Compost Piles

If you use multiple compost piles to keep things at different stages, you can add small amounts to each pile about once a month until you’ve used up the ash.  Again, it’s a good idea to wait before you use the compost to give the ash time to decompose into the humus.

Rarely Used Compost Piles

If you are more of a fire and forget type, no problem.  These piles are more of a “cold” compost pile and tend not to get as warm.  For these, it’s a good idea to add the compost in the off-season to give it the whole next season to decompose.  For most of us, that’s fall or late summer, depending on how you compost and when you add it to your garden.  It’s best to add the ash after adding the compost to your garden or lawn for the year.

Can I Add Ash to Compost?

For the most part, ash is a great addition to your compost.  There are a few exceptions, like ash that is from charcoal or other non-wood material.  You want to add it in moderation to avoid making your soil too basic.

Frequently Asked Questions about Composting Ash