Using Wood Ash in the Vegetable Garden

18 November 2011, written by Helen Gazeley

Bonfires are a contentious issue, what with smoke and global warming. Personally, I love them, though I go for the fast and furious approach of an incinerator, piling everything in and (a personal best) achieving a spurt of flame two-foot high from the chimney. Reservations aside, a fire gives you the great satisfaction of getting rid of perennial weeds, branches too thick for the shredder, and diseased material while, at the same time, producing a valuable by-product: ash.

Wood ash (as opposed to coal ash) can be a great addition to the garden. It contains potassium or potash (they’re not identical but - scientists look away now - the terms are often used interchangeably), and potassium is a vital nutrient for crops.

Just as it does in humans, potassium regulates plants’ water balance (so tissue is firm and juicy), and has a part in transporting food within the plant and creating sugars and starches. Without enough, vegetables are more vulnerable to drought, frost, pests and diseases.

By now, I hope you’re reaching for the matches. However, bonfires aren’t the only source of ash and the increasing popularity of wood-burning stoves means that more people have to dispose of a lot more ash. In fact, a cord of wood (the standard unit of firewood in Canada and USA, measuring 4 x 4 x 8 feet (120 x 120 x 240 cm) is likely to produce around 25 lbs (over 11 kg) of ashes. Ideal for the garden? Well, yes, except you can have too much of a good thing, and you do need to think about where you put it.

Add Ash to the Compost Heap

Wood ashes make a great addition to the compost heap, where they’ll aid fertility (most of the nutrients needed by plants are contained in them to some degree). If you have a lot, don’t add them all at once as they are alkaline and raising the pH too much will affect the bacteria and worms at work. It’s better to keep the ash in a nearby container and sprinkle on a layer every so often.

If you tend to compost a lot of acidic material, such as fruit waste, the ashes will help to keep the compost at a lower pH and reduce the need to lime the vegetable plots at a later date.

Wood Ash as a Substitute for Lime

Speaking of liming, because ashes are alkaline, it is possible to substitute them for the usual ground limestone. However, home-produced ash isn’t a standardised product, which means its content will vary.

Hardwoods, for example, generally produce more ash and contain more nutrients than softwood. According to the very thorough information from Oregon State University Extension Service, ash from a cord of oak will provide enough potassium for a garden 60 x 70 feet, whereas a cord of Douglas Fir will be sufficient for a garden 30 x 30 feet, while both will raise the soil pH slightly. Bonfire ash is even more variable, because of the mix of plant tissue.

Like the potash content, the calcium carbonate content will also vary (although it’s unlikely to contain more than half that of ground limestone), so it’s a good idea to test the pH of your soil before adding the ash and three to six months after, to check on its effect. It wouldn’t hurt to check up on the potassium content while you’re at it. There’s no point in adding potash to a soil that’s already high in potassium, as too much can affect the plants’ take-up of other nutrients.

Wood ash for fruit crops — Wood ash provides potassium which is essential for fruit crops

Where Not to Use Wood Ash in the Garden

Being alkaline, wood ash obviously isn’t an ideal addition if your soil already has a pH of 7.5 or greater. There’s no point in spreading it around acid-loving plants such as blueberries. Nor is it recommended for areas where you intend to grow potatoes (much though they enjoy potassium) as increased alkalinity can encourage the fungus, potato scab.

It’s also worth remembering that potash is extremely soluble, so keep it absolutely dry before you use it (this includes before adding it to the compost heap). Leave your ashes out in the rain and all the potash will wash out and you’ll be left with a sticky and fairly useless sludge. If you pile a large amount of ash in one area, you also risk over-liming that area and damaging nearby plants.

Adding Ash Direct to the Soil

All this sounds rather alarming, but I don’t mean it to. Those of us who have the occasional bonfire won’t be damaging the soil with the small amount of resulting ash but rather adding a little of one of the nutrients that plants use most.

Sprinkling ash straight onto the soil also deters slugs and snails (the moment it gets wet, this effect unfortunately vanishes). I haven’t tried it myself, but some recommend sprinkling ash in the drills when you sow carrots, and dusting it on turnips to keep carrot and turnip fly away.

I generally add ash to the soil in spring and autumn, but it can be spread it around at other times whenever it’s available and you might as well if you know you can’t keep it bone-dry. Root vegetables such as carrots, parsnips, peas and beans (pods are a better weight and colour) and fruit all appreciate potash.

Regarding fruit, if you have only a little potash, it should go to dessert apples, redcurrants and gooseberries first, then to cooking apples, pears, raspberries, blackberries and strawberries. Plums, apricots, cherries and blackcurrants appreciate a regular sprinkle, but don’t need it so much.

In summary:

Keep ash dry before use.
Test your soil before spreading large amounts around.
Use it in particular around root vegetables, peas and beans, apple trees and soft fruit bushes.

By Helen Gazeley