Don't recycle wood ashes as a fertilizer

Ashes from wood-burning fireplaces and barbecue pits can be used as fertilizer amendments, but negative consequences may occur. They contain potassium, a plus, but their elevated pH value is a minus. Wood ashes are highly alkaline, with a pH of 9 to 13, depending on the wood source. Add excessive amounts to your garden or flowerbed and plants won’t grow.

From the 1700s through the early 1900s, certain industries burned wood for ash production. Then, the alkaline component (lye) was extracted by water passing through the wood ashes. Many Eagle readers are old enough to remember “lye soap” making. Saved animal fat was mixed with lye to make lye soap.

The second-most important product extracted from wood ashes was potash, or potassium, a plant nutrient. By today’s standards it was a poor fertilizer source. Its analysis was about 0-1-3, meaning that it had no nitrogen, 1 percent phosphorus and 3 percent potash. It also contained modest levels of minor elements, including boron, magnesium, iron and zinc.

From a practical point of view, gardeners and homeowners should not routinely use wood ashes as a fertilizer source. Nutrient analyses are low, and the risk of raising the pH to undesirable levels is enormous. Most garden plants prefer neutral soils to acidic soils. Plants don’t absorb zinc and iron well in high pH soils. If you feel a compulsion to use wood ashes as a fertilizer, do so in moderation. Don’t exceed 5 pounds per 100 square feet and don’t repeat the treatment in subsequent years.

Local soils range at about pH 7.0 in their natural state. After applying water from domestic and well water sources, however, the pH may go up to 8.0 or higher. Adding wood ash as a fertilizer source adds insult to injury.

East Texans who grow crops in acidic sandy soils can use wood ashes to their advantage. They can apply wood ashes instead of lime to change the soil reaction from acid to neutral and gain a nutrient benefit, too.

Geographic areas differ as to how people deal with soil fertility improvement. I was intrigued by suggestions from the Alaska Cooperative Extension Service for gardeners who live in remote areas where commercial fertilizers are unavailable. They recommended using discarded fish parts for nitrogen, collecting and burning bones into ash for phosphorus and using wood ashes for potassium.

Our major concern is how to dispose of wood ashes when routine application to soil isn’t a good idea. City garbage disposal departments for Bryan and College Station agreed that homeowners should double-bag cold ashes from fireplaces and barbecue pits and put them in garbage collection containers.

Using wood ashes for fertilizer would be the perfect recycling plan if doing so didn’t produce an alkaline soil effect. Since we’re interested in growing healthy plants, however, we’re left with the single option of bagging wood ashes for disposal.

• Wendell Horne is a retired Texas A&M University plant pathologist and consultant. His e-mail address is

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