Wood ashes from the fireplace or wood stove may be used to supply both calcium and potassium to soil. The pH level of soil is measured on a scale of 0 to 14, with 7.0 being neutral. Target pH for growing a garden is 6.5 (slightly acidic). Wood ash works best if the soil pH level is somewhat acidic, below 6.5. Wood ash is high in calcium content, with the effect of raising soil pH. When wood ash is used at pH levels above 6.5, interference with plant growth may occur as the alkalinity level of the soil increases.

To determine if a garden is a good prospect for wood ash, get a soil test. Details for taking a soil test are available from the Michigan State University Soil and Plant Nutrient Laboratory. It is recommended that garden soil be tested every two to three years to determine current nutrient levels and to make nutrient adjustments accordingly.

Wood ash is composed of many major and minor elements needed by the tree for plant growth. Since most of these elements are extracted from the soil and atmosphere during the tree’s growth cycle, they are elements that are common in our environment and are also essential elements in the production of crops and forages. Calcium is the most abundant element in wood ash and gives the ash properties that are similar to agricultural lime. Ash is also a good source of potassium, phosphorus, magnesium and aluminum. In terms of commercial fertilizer, average wood ash would probably be about 0-1-3 (N-P-K). In addition to these macronutrients, wood ash is a good source of many micronutrients that are needed in trace amounts for adequate plant growth. Wood ash contains few elements that pose environmental problems. Heavy metal concentrations are typically low and not in a highly extractable or available form.

Field and greenhouse research have confirmed the safety and practicality of recycling wood ash on agricultural lands. It has shown that wood ash has a liming effect of between 8 and 90 percent of the total neutralizing power of lime and can increase plant growth up to 45 percent over traditional limestone. The major constraints to land application of wood ash are transportation costs, low fertilizer analysis and handling constraints. With ever-increasing disposal costs, land application of wood ash will probably be the disposal method of choice in the coming century, resulting in savings for the industry, an opportunity for agriculture and conservation of our resources.

The previous two paragraphs are an excerpt from Soil Acidity and Liming Internet Inservice Training, Best Management Practices for Wood Ash Used as an Agricultural Soil Amendment, by Mark Risse, Extension Engineering, 307 Hoke Smith Building, Athens, GA 30602, and Glen Harris, Extension Agronomist, P.O. Box 1209, Tifton, GA 31793.

For further information on using wood ash in the garden, contact Dr. Hal Hudson, Michigan State University Extension Horticulture Educator for commercial and consumer vegetable production at 989-672-3870 or hudsonh@anr.msu.edu.