Now that cold temperatures have returned this winter, recyclers and gardeners who burn wood for heat once again face the issue of disposing of wood ashes. Should they be discarded or used as fertilizer?
Farmers and gardeners have long recycled wood ashes in fields and growing beds, making use of the many nutrients essential for plant growth that remain after wood burns. But today there is discouragement afoot for fear that ashes will cause soil to become too alkaline.
That concern is legitimate if we over-apply them or use ashes on the wrong plants.
While many Western soils are often quite alkaline in nature, our Northern California soils tend to be naturally acidic and need little besides organic matter and a nitrogen boost to bring them into production. But moderate use of wood ashes will not significantly alter this acidity.
How much is too much?
Advice varies. In general, wood ashes are most beneficial in areas that receive in excess of 20 inches of rain each year. A safe amount is less than 5 pounds per 100 square feet along with a 1- to 2-inch layer of compost. But some gardeners have success using two or three times that amount.
Scientists at UC Davis suggest no more than 10 pounds of wood ash per 100 square feet on garden beds, while advice from Oregon State suggests no more than 10 to 15 pounds per 1,000 square feet of lawn and one-half pound annually on a rose bush.
If ashes are over-applied, potassium, calcium and magnesium carbonate can remain in the soil and conditions will become too alkaline, resulting in poor nutrient uptake and plant decline.
Burning eliminates all organic matter in wood, leaving only mineral compounds containing 1-2 percent phosphorus and, on average, about 6 percent potassium, the equivalent of a fertilizer carrying the label 0-2-6, along with valuable micronutrients such as magnesium and trace elements.
Ashes are not organic in the sense that manure, leaves and compost are, but they are an inorganic residue left from burning organics. They do not decompose and are best applied directly on the soil rather than put in the compost pile where they only slow down the process.
Because most of the nutrients in ashes are in a water-soluble form, apply them around plants in spring after the heavy winter rains have abated, and wait until fruit trees leaf out before raking them into the ground over the root zone.
Ashes may be lightly side-dressed alongside mature plants during the growing season, raked shallowly into soil or mulch and watered in, but keep them away from foliage to prevent leaf burn. As a precautionary measure, rinse plants after you rake in ashes to wash off any dust.
Always spread them thinly and evenly — never dumped in piles — and away from sprouts to prevent root damage. In planting beds, it’s best to spread them at least three weeks before sowing seed or transplanting seedlings.
During the dry season, some gardeners have had success using dry ashes as a mulch encircling plants to control slugs and snails.
What to avoid
Many of our favorite edible and ornamental plants thrive in acid conditions and will suffer if soil loses that acidity. Azaleas, rhododendrons and camellias are the most common examples of acid-loving species, but there are others that can be damaged by increased alkalinity from ashes.