Ashes produced by wood-burning fireplaces or stoves are an excellent example of the idiom “don’t throw out the baby with the bathwater,” University of Missouri Extension horticulturist David Trinklein said.
“When collected and spread on the garden, they are an excellent and free source of calcium and other plant nutrients,” Trinklein said.
Ashes represent the organic and inorganic remains after the combustion of wood. As a rule, hardwood species produce three times more ashes and five times more nutrients than softwood species, he said.
Since carbon, nitrogen and sulfur are the elements primarily oxidized in the combustion process, wood ashes contain most of the other essential elements required for the growth of the tree used as fuel. By weight, wood ash contains 1.5 to 2 percent phosphorous and 5 to 7 percent potassium. If listed as a fertilizer, most wood ash would have the analysis of 0-1-3 (N-P-K). The calcium content of wood ashes ranges from 25 to 50 percent.
Because of their high calcium content, it’s probably best to think of wood ashes as a liming material to adjust soil pH rather than a regular fertilizer to supply a wide array of nutrients, Trinklein said. The ideal pH range for most garden plants is about 6.0 to 6.5. When soil pH falls below this range, certain essential mineral elements become less available to the plant. Since garden soils tend to become more acid as plants take up nutrients, periodic pH adjustment to decrease soil acidity (increase pH) is necessary.
“Most wood ashes have an acid-neutralizing equivalent of about 45-50 percent of calcium carbonate,” Trinklein said.
In other words, it takes about twice as much wood ash as calcium carbonate (limestone) to change the soil acidity by the same amount. For example, if soil test results indicate you need 5 pounds of limestone per 100 square feet of garden area to raise the soil pH to an acceptable level, you’ll need 10 pounds of wood ashes to make the same change.
Apply small amounts of wood ash to the garden yearly to supply other nutrients such as phosphorus and potassium. Trinklein recommends a soil test every two to three years where light applications are made on a regular basis. Excessive application of wood ash can build up pH above the optimum range. This can reduce nutrient availability.
Wood ashes not immediately applied to the garden should be stored under dry conditions. Ashes piled outdoors lose most of their potassium in a year’s time due to leaching from rains. Additionally, weathered wood ash’s ability to act as a liming agent is greatly reduced.
Because of the fine nature of wood ashes, they cannot modify soil structure, so they are not considered a soil conditioning agent. The carbon compounds that act as a soil conditioner in sawdust, leaf mold or compost have for the most part been consumed by the fire.
Wood ashes are highly alkaline. Wear protective glasses, gloves and a dust mask when spreading on the garden. Don’t use ashes that result from burning cardboard, trash, coal or treated wood of any type since they may contain potentially harmful materials.