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Wood, leaf ashes must be used carefully in garden

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Friday, Jan. 9, 2009
 

Question: Are leaf ashes OK for the garden• We had so many this year that we burned some. Are they good for the soil and should anything else be put with them?

Answer: Both wood and leaf ash has been used for centuries by gardeners to amend soil. This, however, doesn't mean that it's always a good practice. We've learned a lot about soil and plant health over the years, and as a result, the decision to use ashes in the garden has become a hot topic in the gardening world. I'll give you some facts, then you can decide for yourself if it's something you want to do.

First, one of the reasons folks use ash in their gardens is its nutritional content. Ash contains about 1.5 percent phosphorus and 7 percent potassium, two essential nutrients for plant growth. If your soil has a deficiency in potassium, adding a few ashes would help boost the levels.

But, to make an informed decision, you should first take a soil test in the garden to determine both existing nutrient levels and soil pH. Because ashes are alkaline, adding them to the soil also raises the pH, making it less acidic. If the test results show a pH of 6.5 or higher, don't add those ashes. Doing so would raise the pH to non-optimum levels.

In Western Pennsylvania, we tend to have slightly acidic soils so adding a tiny bit of ash to the garden each year usually doesn't throw the pH too out of whack.

That being said, the reason some experts no longer are recommending the use of ash in the garden is because a little goes a long way, and putting even a little too much on the soil can wreak havoc on soil organisms and make the soil so alkaline that most plants cannot thrive. I would also suggest you don't add ash to poorly drained soil because it reduces a soil's porosity.

If you do choose to add the ash to your garden, be sure to spread it very evenly and in minimal amounts. Come spring, mix it thoroughly into the soil to avoid patches of overly alkaline soil.

Just in case you still haven't decided, I'm going to tell you what this "expert" does with our wood ashes (and you can confirm this with my husband who got admonished last week for dumping a pile of wood ashes into a bed of evergreens) -- I compost them. Because I worry that I'll add too much or that I won't get them evenly spread, I cut the risk by adding the ashes to my big compost pile.

They get dumped on top or saved in a metal bucket to be blended into the pile during the growing season. The mixture of organic matter that goes into my pile helps neutralize the ash's pH and, by the time the compost is fully cooked, the pH is not a concern. Obviously, you don't want to add excessive amounts of ash to a small compost bin, but larger piles can handle quite a bit.

And, to avoid all this decision making next year, instead of burning your leaves, chop them up with the lawn mower or a chipper shredder and compost them directly. You'll be avoiding the air pollution that burning leaves produce, and you'll be creating a truly perfect soil amendment -- no strings attached.

Horticulturist Jessica Walliser, co-author of the book "Grow Organic," can be heard from 7 to 8 a.m. Sundays on KDKA Radio's "The Organic Gardeners."

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