Ashes in garden soil

Many people who burn wood save the ashes to throw on the garden. Wood ash does have some nutrients in it, primarily potassium and phosphorous. However, the amount of nutrients depends on what species you’ve burned. Hardwoods such as oaks and maples will have more nutrients than softwoods such as pines. But, you need to be cautious.

Ward Upham is an extension horticulturist at Kansas State University. He recommends taking a soil test because your soil pH will dictate whether you should put ashes on a garden.

"If you have a high pH to start with, let’s say anything over 7, then it would not be a good idea to use wood ashes because that’s going to raise the pH and make that soil less available to provide nutrients those plants need," says Upham. "Now, if you’re below a pH of 7, then it can be beneficial because it does provide those nutrients. Not nitrogen, but potassium and to a smaller extent phosphorous."

Azaleas, rhododendrons, blueberries and other plants that require a very acid pH are examples of plants that should not be exposed to wood ash. He says potatoes also tend to have more potato scab if you use wood ashes.

Ashes can be worked into the garden any time. They do blow around easily so choose a day that’s calm and spread them out by hand.

"Normally what you’re after is maybe 5-10 lbs. per 100 sqft. So, if you measure that out and measure the amount of area you’re applying them to, then it’s a good idea to go over that area in different patterns in order to make sure you get an equal amount of wood ash on each part of the area," he says. "Once you’ve applied that, work it into the soil."

If you’re going to be seeding, it’s a good idea to work the ash in three-to-four weeks before you seed.

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