Wood Ash for the Garden: What You Need to Know
By Steven Biggs
Fireplace Ashes for the Garden?
I have happy memories of cooking holiday roasts over charcoal with my dad. He taught me how to cook over charcoal—and he drilled in the importance of NEVER leaving a barbecue full of ash out in the rain.
Wet ash in a barbecue is like road salt on a car…corrosion just waiting to happen.
Yet many people spread wood ash on the garden.
Actually, sprinkling ash around the yard isn’t anything new. And in an era when people pay attention to how much waste they generate, it seems to make sense not to send wood ash to landfill.
My neighbour Al always sprinkled wood ash around his garden. I’d see the ash as the wind blew it over the fresh snow, and ash specks peppered the sidewalk.
Whether wood ash is good for your vegetable garden depends on your soil and how you’re using the ash.
Thinking further back, I remember as a child, playing on my grandmother’s gravel driveway and eagerly collecting the nuggets of coal clinker from decades before, when ash from the coal furnace was sprinkled around the yard.
But is ash good for the garden? The answer depends on what type of ash it is, how you’re using it, and your soil.
If you’re wondering about using ash in your yard, this post tells you more about wood ash, ways to use it—or when not to use it.
What’s in Wood Ash
Wood ash contains a lot of plant nutrients, including potassium, phosphorus, calcium and magnesium. There are other minor nutrients too.
The amount of the various nutrients in your wood ash depend on the type of wood you’re burning.
The other thing to know is that wood ash is alkaline. So in the same way that lime is used to make soil less acidic, so can wood ash. (More wood ash is needed than lime, though wood ash is faster acting.)
Using Wood Ash in the Garden
Ash has two uses in the garden:
Fertilizing
Reducing the acidity of some types of soil
Whether wood ash is beneficial in your garden depends on soil nutrient levels and soil pH (which is the measure of acidity or alkalinity.)
When soil is too acidic or alkaline, some nutrients get “locked up.” The nutrients might be in the soil—but unavailable to plants. That means:
Ash applied to acidic soils can make more nutrients available by making the soil less acidic (more alkaline)
Ash applied to a soil that’s already alkaline can sometimes make the soil more alkaline to the point that nutrients become unavailable
That’s why you might come across the advice not to use wood ashes on soils with a pH over 6.5.
Don’t Know Your Soil pH?
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Some sources say not to use wood ash in the garden without testing your soil first.
What’s a gardener to do if you don’t want to have the soil tested…but don’t want to unnecessarily send ash to landfill?
I’m not a proponent of soil-testing hypochondria, so I understand. Garden soil is often highly disturbed and highly variable, so one garden bed can be quite different from an adjacent one. (If you guessed that I don’t test my soil, you’re correct.)
Keep reading and get a few ideas.
What to Know About Wood Ash if You Don’t Plan to Test the Soil
Like many things in life, moderation is key.
Soils with high levels of organic matter and clay can resist pH change—so the occasional sprinkle of ash isn’t a problem. I sprinkle some wood ash the slightly alkaline clay soil in my vegetable garden; it’s got lots of organic matter, and I don’t put on a lot of wood ash.
You can also sprinkle in ash as you add new layers to the compost heap. The organic matter in the compost absorbs the nutrients so they’re not washed away. Organic matter also lessens the change to pH.
Using Wood Ash in the Garden
If you’re using wood ash in the garden, remember that it’s not a complete fertilizer. It doesn’t have any nitrogen, for example.
Here are tips for using wood ash in the garden:
On bare soil, sprinkle on wood ash at least a couple of weeks before planting, and then rake or dig it into the soil.
If applying wood ash to the soil and you don’t want coarse bits, sift out big unburnt chunks (hardwire cloth works well, though you can skip this step if you’re putting your wood ash in the compost)
On bare soil, sprinkle on wood ash at least a couple of weeks before planting—but not too far ahead because it’s soluble and nutrients will wash away
Rake or dig in the wood ash—you don’t want to seed into ash, and mixing with soil at least a couple of weeks beforehand gives it time to start breaking down
Don’t apply wood ash near germinating seeds, because they can be easily damaged by the high salt levels.
Don’t overdo it…keep it to once per year because too much wood ash (or too much of many fertilizers) can make some nutrients unavailable to plants
Use only wood ash. Don’t use ash from cardboard, briquettes, pressure-treated timber, or painted woods.
If you heat with only wood, you might find you have more ash than you have garden to spread it around. Don’t make heavy, repeated applications in the same part of the garden. Consider spreading some over the lawn, or sprinkling in some with new additions to the compost pile.
Your acid-loving plants such as blueberries, raspberries, azaleas, and rhododendrons won’t appreciate ash treatments. Skip these plants, even if you have an acidic soil. (And potatoes too, because alkaline conditions encourage the disease potato scab.)
Types of Wood Ash
Ash from wood burned in stoves, fireplaces, and outdoor fire pits is suitable. So is ash from hardwood lump charcoal.
Stick to wood ash: Ash produced from cardboard, coal, briquettes, pressure-treated wood, and painted woods can contain stuff we don’t want in the garden, including heavy metals.
Storing Wood Ash
Keep wood ash dry until you use it.
The potassium in wood ash is soluble, meaning it leaches out with rain. So keep the wood ash dry until you’re ready to use it in the garden.
Other Wood Ash Uses
Wood ash has long been used to make lye for soap. I don’t make my own soap, but I do use wood ash to clean the windows of my wood stove. I dip scrunched up newspaper in water, and then into wood ash before rubbing it on the glass. It’s an excellent glass cleaner.
Wood ash is caustic, meaning don’t breathe it in, and mind your skin and eyes.
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