Using Fireplace Ashes in the Landscape

Home Grown  by Keith Reed
Fireplace season will soon be on us. This brings to mind a common question: should ashes be used as a fertilizer/soil conditioner in the lawn and garden and if so, how much should be used?

Fireplace ashes do have benefits and can be safely used, but there are some considerations. These include nutrient content, liming value and salt content. If your soil does not fit a certain criteria based on these factors, ashes should not be used. The only way to determine if your soil can benefit from fireplace ashes is to have a soil test performed.

Fireplace ashes contain about 6 % potassium, which is relatively high. Ashes also have acid neutralizing power and can have a salt content of about 22 percent. Either of these factors can have detrimental effect on soil if present in too great a quantity. Ashes should not be applied to soils which have a neutral to alkaline pH (7 or above) because the liming properties of ashes will further increase the pH. High pH soils are associated with micronutrient unavailability with elements such as iron or zinc.

For very acidic soils (low pH) that also need potassium, ashes can be valuable as they are more soluble than lime, neutralizing soil acidity within a few days after incorporation by rain or cultivation. However, care should be taken to avoid excessive application.

If you have determined that ashes will benefit your soil based on its need for potassium and liming, the next step is determining how much to apply. This largely depends on your soil type. The maximum amount that can be applied to sandy soils is only about half that which can be applied to finer textured soils.

For sandy soils, a maximum rate of 10 gallons of ashes per 1000 square feet equates to approximately 3 pounds of potassium and 10 pounds of lime applied. For other soil types, a maximum rate of 20 gallons of ashes per 1000 square feet equates about 6 pounds of potassium and 20 lbs of lime applied.

Keep in mind that excessive applications may damage the soil. Areas treated at the above rates should not be retreated again for 10 years or until a soil test shows a need for lime or potassium again. To avoid repeat applications to the same area, it’s a good idea to make notes in your garden diary of the areas that have received an application.

Remember that if you use fireplace ashes for your potassium needs, other fertilizers applied should not contain potassium. Potassium, like phosphorus, remains in the soil for a long period of time and does not need to be applied very often. Finally, avoid applying ashes to areas where blueberries, azaleas or other acid-loving plants are grown.

For more information on this or any other horticultural topic, you can contact Keith Reed, the Horticulturist in the Payne County Extension office. Keith can be reached via email at keith.reed@okstate.edu, phone at 405-747-8320, or in person at the Payne County Extension office, located at 315 W. 6th in Stillwater.

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