March 20, 2010 Wood Ashes Change Soil Chemistry Originally published in the Contra Costa Times by Chantal Guillemin, UC Master Gardener Adding wood ashes to Contra Costa soil or
compost can negatively affect plants.
SUMMARY If you are thinking about using those phosphorus, iron, boron, manganese, wood ashes you have accumulated over copper, zinc and potassium become Gardening books and the winter in your garden or compost chemically bound to the soil and are not web sites may lead you pile, proceed with caution. Though available for plant use. In time, due to astray with advice on many gardening books and websites this change in the soil chemistry, plants using wood ashes as a encourage homeowners to add wood will exhibit mineral deficiencies by soil amendment. Wood ashes to garden soil or compost, in producing abnormal leaves, stems and ashes can make our Contra Costa County, there are several flowers. A common symptom of plants already alkaline soils good reasons why doing this may not be growing in alkaline soil is interveinal even more so, and can advisable. Too many ashes can cause an chlorosis, a yellowing of normally green also increase salinity. excess of alkalinity and salinity. tissue. Both have detrimental effects on plants. Ashes May Increase Alkalinity: Ashes May Increase Salinity: Contra Costa County has over 50 named About 80 to 90 percent of the minerals soil series ranging from loamy sands to in wood ash are water soluble. This mucky clays, nearly all of which have a means that when wetted these minerals pH of 7.0 or greater. This is considered wash out of the ash and into the soil in neutral or alkaline. Adding wood ashes, the form of salts, which are harmful to which contain 25% calcium carbonate plants. This is especially true if ash is and as a result are very alkaline with a left in a lump as the leached salts are pH of 10 to 12, increases soil alkalinity concentrated in one area. These salts that creates an adverse condition for contain less than 10% potash, 1% growing plants. Many plants prefer a phosphate and trace amounts of micro- slightly acidic environment to absorb nutrients such as iron, manganese, nutrients from the soil. boron, copper and zinc, but they also contain heavy metals such as lead, When soil alkalinity increases and the cadmium, nickel and chromium, pH rises, necessary minerals such as depending on the material burned.
Reissued 2020-03-01 1 of 2
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ccmg.ucanr.edu March 20, 2010: Wood Ashes Change Soil Chemistry, page 2
“Too many ashes can Seedlings in particular are very sensitive Disposing of Wood Ashes: to salt injury and their growth can be Before disposing of wood ashes, wait cause an excess of stunted and their foliage turn yellow. In until they and the hot coals buried broadleaf plants, necrosis (death and under them are completely out and cold. alkalinity and discoloration of tissue) and defoliation This may take several days. You may can occur. Excess salts can also cause need to pour cold water over the ashes. salinity… A common medium to fine soils to lose their Do not transport or store ashes in plastic aggregated structure, becoming or paper bags. While working slowly symptom of plants impervious to air and water. Also, if and carefully, use a metal tool to scoop fertilizers such as ammonium sulfate, the ashes and a covered metal pail to growing in alkaline urea, or ammonium nitrate are remove them from a wood-burning combined with wood ash, they produce stove or fireplace and store them away soil is interveinal ammonia gas, a severe respiratory from flammable materials. Do not place irritant detrimental to the health of the cold ashes in the green re-cycling bin. chlorosis, a yellowing gardener. Completely extinguished ashes can be disposed of in the trash bin and placed of normally green at curbside on garbage collection day.
tissue.”
Reissued 2020-03-01 2 of 2