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March 6th, 2012

Wood Ash as a Compost Ingredient

I’m not sure what to do with all my excess wood ash from the fireplace. I need to lower my soil pH, so I can’t add the ash to the garden or compost.

blog-dougThe ash from a wood-burning fireplace contains nutrients that plants need, including potassium, calcium, and trace amounts of zinc, magnesium, and phosphorus. But those nutrients come with a warning: Wood ash is strongly alkaline. If added to soil, it will raise the pH, or the measure of a soil’s acidity or alkalinity. Where the soil is too acidic (pH too low) and a higher pH would be beneficial, wood ash can be applied to lawns or gardens in place of agricultural lime to raise the pH. But if you are dealing with a neutral or alkaline (high pH) soil, adding wood ash would be a mistake.

The same goes for adding wood ash to a compost pile. Consider including wood ash as a compost ingredient only if the pile contains significant quantities of acidic materials, such as shredded oak leaves or pine needles. Even then, apply the ash with restraint, because too much can result in a loss of nitrogen from the compost. At a higher pH, the nitrogen present in the compost pile will escape to the air as ammonium hydroxide; if you smell ammonia near your compost pile, it’s losing nitrogen.

If you decide you shouldn’t use wood ash in your garden or compost, and you can’t find a farmer or gardener who can use it, I suggest you send the ash to a landfill with the rest of your household waste.  —Doug Hall

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March 22nd, 2011

The Uncompostables

doug

The other day, an Organic Gardening reader emailed us a question about whether or not he should add shredded newspapers to his compost pile. (My opinion: Shredded newspaper is OK in moderation, especially in piles that are blessed with too much nitrogen and need some extra carbon-rich materials to balance it out. In my garden at home I’m more likely to utilize newspapers in an unshredded layer under the mulch, where they have a compounding influence on the mulch’s beneficial effects.)

The reader’s question inspired me to start a list of items that shouldn’t be added to a compost pile. Weeds that have gone to seed; meat scraps from the kitchen; used cat litter: Some things are obviously unwelcome as compost ingredients.

Others require some research. Wood ash, for example, is often considered a safe compost ingredient—in small quantities. But it is highly alkaline and raises the pH of the decomposing pile, allowing nitrogen to be released into the air as ammonia gas. Not just smelly, but a waste of an essential plant nutrient.

In the end, I came up with 10 items that are best kept out of the compost bin. Look for the list in the Skills & Abilities section of the June/July 2011 issue of Organic Gardening—Doug Hall

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