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Wood Ash Update 2018


You Bet Your Garden
Question of the Week © 2018 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Wood Ash Update 2018: How Much Ash Should a Gardener Use if a Gardener Should Use Ash?

Q. I've got a lot of wood ashes and wonder if I can use them in the garden; and if so, where?

---Bob in Wardensville, West Virginia

A. Well, that question sure was 'short and sweet', which is highly appropriate, as our topic this week is also 'sweet'. Hardwood ashes from a wood-burning stove are very alkaline; and old-time farmers would tell you to use ashes to 'sweeten' a 'sour' soil, as alkaline soil actually tastes a little sweet (while acidic soil tastes sour).

This is a great time of year to run this story (we are posting this article on St. Patrick's Day 2018), as it it's the time of year when people who burn wood for heat realize that they have a lot of wood ash sitting around—and they're really hoping that there's a good use for it in the landscape.

Luckily, the people who burn the most wood are also the ones who can probably use the largest amounts of ash safely in their landscape—and that's people in cold-weather regions. In general, people who have cold winters and/or a lot of rain tend to have soils that are on the acidic side, which is great for growing plants that require an acidic soil, like blueberries, azaleas, rhododendrons, and camellias. In fact, depending on how acidic your soil is, those and similar plants may need to have their pH lowered a bit more with a mulch of peat moss, some added sulfur, or a packaged product that acidifies the soil while providing a natural feeding.

But wood ashes are the opposite of acidic—and it is vitally important for people to remember that alkaline wood ashes should never go NEAR the plants we just named, or others that require an acidic soil. It always surprises me how many people get it backwards and think that wood ashes make soil more acidic. But hey--dyslexia is its own reward! Or "Lex Luthor is his own dew point." Or…

Anyway, this is where a soil test that includes a reading of your soil's pH can be incredibly useful—especially if you have a lawn, because that's where you might be able to get rid of a lot of your ash. Most lawn grasses grow best in a slightly acidic to neutral soil—a pH of 6.5 to 7. (High numbers—greater than 7—are alkaline. Numbers lower than 7 are acidic.)

If your lawn soil tests lower than 6.5, you can—and should—raise the pH to the optimum number using wood ash instead of the oft-recommended agricultural lime, as ash contains good amounts of phosphorous and other nutrients in addition to lime's calcium. And the whole point of this article is to help people who got 55-gallon drums of the stuff get rid of some of it!

The 'calcium carbonate equivalency' of wood ash is slightly lower than lime, so you would use 1.3 times as many pounds of ash per thousand square feet of turf than the recommended amount of lime. A nice, easy calculation.

But what about in the garden?

Some crops, like peas and beans, prefer a 'sweet soil'. I always dust a little wood ash into the soil of these crops at planting time. Boxwoods, despite being evergreen, also require an alkaline soil and struggle in acidic conditions. So they get a dusting as well.

Folks who burn a lot of wood should search legitimate State Extension and University websites for lists of other plants that like it sweet. Don't depend on posts from fellow gardeners; you could easily wind up with the dyslexic blues. (And don't add wood ash to your compost piles, no matter who tells you to.)

Now: We often recommend using small amounts of rock phosphate to improve blooming in under-performing plants. Can a gardener use wood ash to add extra phosphorus to their soil instead?

Absolutely. But as we stress with rock phosphate, less is more. We're talking a quarter to half a cup of ashes per good-sized plant. And because minerals like phosphorus are released slowly over time, you should only add this amendment every three to four years. (As with any soil amendment, cover the material with some compost or garden soil to help it become more quickly available to the plants in question.)

And finally, you can also add wood ashes to nitrogen-rich composted manures to balance out their nutrient content. Cow manure is already balanced, so don't add ashes to that one. But you can use a good amount of ash to amend composted horse manure, and an even larger amount with chicken manure (the most Nitrogen-rich of the barnyard manures). Mix the ash well into the manure and let the mixture sit for a week or two before using near plants.

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