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The Plant Medic


Wood ash may not be good for gardens in Midwest

Q. We have wood ashes that have accumulated from our fireplace this year. I have heard they contain many nutrients for plants. Can we throw those on the vegetable garden now?

A. The Romans first used wood ashes from their fires in gardens as a fertilizer. Early settlers in the U.S. added ashes and water in a large pot or kettle and boiled the mixture to make soap. They also would add the ashes to their gardens and noticed that plants grew better after the ashes were added.

It is now known that ashes contain about 2 percent to 3 percent potassium, which is an essential plant nutrient. Since the ashes were used in these large pots in the soap-making process – the settlers named the nutrient contained in the large kettle as “potash”– short for potassium. There is also a very small amount of phosphorous in wood ash. Phosphorous is also an important nutrient.

Wood ashes are great to use in gardens on the East Coast where the soils are usually very acidic; but not so great to use in large quantities in our area. Garden soils in our area are generally quite alkaline (pH values of 7.0 and above) because our soils are created from limestone bedrock, an alkaline material. Adding wood ashes to an already alkaline soil makes a soil sometimes so alkaline that plants won’t grow properly.

Wood ash is about 25 percent to 45 percent calcium carbonate; which actually changes the soil pH quickly and dramatically. In our area, never add wood ashes to an area without first testing the soil to see whether your soil is acidic or alkaline.

There are exceptions. Some folks have muck soils, which tend to be very acidic. Some materials used in raised beds can be acidic especially if they contain large amounts of peat moss. Some land that was farmed for long periods might also be more acidic because of the type of fertilizer that is used by farmers.

If you happen to be one of the lucky few with an acidic soil, then generally one can add about a 5 gallon pail of wood ashes to a 1,000 square foot area. One can also spread about an inch deep layer of wood ash around plants that like wood ashes. If you grow tomatoes in a raised bed with acidic soil, then the tomatoes will like the potassium from the ashes.

Small amounts of wood ash can be used around plants to ward off flea beetles, slugs or other crawling insects. One should never use ashes that are from treated lumber or wood that contains contaminates that might be harmful to plants and people. Just a few ashes won’t be a problem in the garden. Avoid using large quantities of ashes in our area – at least until you test the soil to see whether the ashes can help – or hinder – the growth of your garden plants.

The Plant Medic, written by Ricky Kemery, appears every other Sunday. Kemery is the extension educator for horticulture at the Allen County branch of the Purdue Extension Service.