Kathryn Fontenot, Koske, Thomas J.
Studies of fireplace wood ashes have found that of the elements analyzed, available potassium-potash (K2O) was the only nutrient element found in significant amounts. The amount of available potash ranged from 3.6 percent for hickory, 4.5 percent for oak to 8.9 percent for a mixture of hard and softwoods. The average for the fireplace ash tested was 6 percent. The analysis also revealed that the ashes have an acid neutralizing (liming) power of at least 20 percent of pure lime. Because ashes are more soluble than lime, they will neutralize soil acidity within a few days after incorporation by rain or cultivation. On the downside, the analysis indicated the ashes contained a salt (Na) content of about 22 percent.
Can I use it as a soil fertilizer?
If your soil has a neutral pH (pH near 7) or has high potassium, then ashes should NOT be applied. This is normally the case for many urban settings with standard fertilization programs. If soil is acidic and moderate to very deficient in potassium, then wood ashes should improve the soil. Wood ash is a liming material that works at sweetening the soil at about half the rate of ag limestone or dolomite. If you need 20 pounds of lime, you can try about 40 to 50 pounds of wood ash. Only a soil nutrient analysis will give a definitive answer as to your soil needs.
How do I use it?
The safe amount of ashes to be applied will depend on the soil type and its current pH. For sandy soils, the maximum rate of 10 gallons of ashes per 1000 square feet will supply 3 pounds of K2O and 10 pounds of lime. For all other soils the maximum rate of 20 gallons per 1,000 square feet will supply 6 pounds of K2O and 20 pounds of lime.
If you should decide to apply ashes to your garden or lawn, please follow these following guidelines as supplied by the LSU AgCenter: