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Wally Peck/Master Gardener: Wood ashes for the garden

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Bemidji Pioneer
(218) 333-9819 customer support
Wally Peck/Master Gardener: Wood ashes for the garden
Bemidji Minnesota P.O. Box 455 56619

One thing we have after a winter like this is a lot of wood ash. Questions Master Gardeners often receive are: Does wood ash help the garden? And how much wood ash can I put on the garden?

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Wood ashes can benefit the soil in vegetable gardens and flower beds; however, they can also do harm that will last for years if improperly applied. It is very important to know your starting point when applying wood ash. Use ashes that come from firewood only, never from treated lumber, plywood or manufactured boards.

When wood burns, nitrogen and sulfur are lost as gas and what remains contains calcium, potassium and many other trace elements necessary to plant growth. Ash contains around 3 percent potassium by weight, more if you burn all hardwood. Potassium, also known as potash, is the K in NPK, the way fertilizers are sold.

When wood ash comes in contact with water, potassium hydroxide is formed. This compound is highly alkaline and has the potential to raise the pH of the soil very quickly. It is the same as putting limestone on your garden or flower bed. The difference is that ash works very quickly, whereas limestone takes much longer. So it is very important to know the pH of the soil on which you are putting ashes.

If the pH of your soil is 6.5 or higher, avoid using wood ashes. In fact, wood ash can push the alkalinity so high that some plants will not grow at all. Some examples are blueberries, azaleas or potatoes. If you like scabby potatoes, put on wood ash. Soils in this area range greatly from sandy, acid soils to alkaline, clay soils.

So, test your soil first or get a soil test done. Bags are available at the Beltrami County Extension Office on the Beltrami Fairgrounds for sampling your garden, flower bed or lawn. The results will let you know the soil’s texture, what the nutrient levels are, the pH, and how much of which nutrient to add to the garden, flower bed or lawn. If your levels of potassium are already high, wood ashes will not be beneficial.

Wood ashes can also be used to repel insects, slugs or snails. Sprinkled around the base of shrubs and plants, ash draws water from their bodies and kills or repels them. Used too often, it will raise the soil pH too high so caution is needed. Different perennials and shrubs require soils with varying pH levels. Again, do not use ash on acid-loving perennials.

Wood ashes will also help the compost bin by maintaining a neutral condition so the heap does not go acidic. Sprinkle a little between layers of compost as your pile builds.

Do not use wood ash directly on gardens that are newly planted. When seedlings contact wood ash, the salts and alkalinity may kill the seedlings. It is better to sprinkle broadly and rake in well before planting. Never exceed 20 lbs. per 1000 square feet of garden. It is easy to use too much. Oh yes — wear a dust mask and gloves.

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