Wood Ashes for Garden Minerals

This article was previously published in my column "Digging the Dirt" by the Hoopa People News, unfortunately, I am not sure what the date was . . .

Those of us who heat with wood have a free source of fertilizer for our gardens, lawns, and landscapes. As long as you don’t burn anything other than wood and newspaper, what you clean out of your stove is a near perfect plant food. Ashes contain potassium, calcium, magnesium, and phosphorous in adequate amounts to support plant growth!

I first learned that ashes were good for plants, at least in small amounts, in much the same way that some of you may have. Grandmother had an outdoor wood burning cook stove set up in her backyard. When she would clean out the fire box the ashes first went into a galvanized tin bucket to completely cool. Next, if she wasn’t ready to spread them on the garden she would store them in a plastic bucket in the shed. Spring and fall she would sprinkle her garden beds with a fine dusting of ashes. When I helped she always admonished me to keep the ashes off the plants leaves and away from the base of any seedlings. She felt the ashes could burn the plants. Any ashes that drifted onto the plants leaves were hosed off right away. She also kept the ashes out of her shrub borders, because she felt it would burn the skin of her abundant and hard working frogs and toads.

Many years later when I was following one of my mentors, Jim Kaneko, around on his family’s homestead, I would see ashes piled up around the bases of certain trees. These trees had been planted 50 – 60 years earlier by his parents who had emigrated from Japan. At first I was very curious and also concerned. I felt the ashes would burn the trees. He just looked at me funny and said “No, ashes are good for the trees.” Over the few years I was able to help him in his orchards and gardens I frequently saw him pile ashes up around ailing plants. The plants would almost always improve and I never saw any sign of burning. He considered ashes to be the best plant medicine available. Like my grandmother, he would store the ashes in buckets in his shed. However, he saved them for ailing plants instead of sprinkling them sparingly throughout his grounds. (His house had a gas furnace for heat, but in one of his outbuildings there was a traditional wood fired Japanese bath. If he had more ashes available he might have also sprinkled them around.)

In many of our local homes ashes are far from a precious commodity. We shovel them out of our woodstoves in quantities that Gram and Mr. Kaneko would envy. I have been liberally applying them to my garden beds for several years. I really like the granular texture they impart to the soil as they age in the presence of organic matter. Granular soil particles resist compaction and provide spaces for air, water, and roots. Granular, loose soils are also the best for earthworms and soil micro-organisms who help break down organic matter, fix nitrogen, and form symbiotic relationships with plant roots. At first I was worried that the worms might be burned by the alkaline nature of the ashes, but they really don’t seem to be affected. That would probably be a problem in a desert area, but during our wet winters (in the Pacific North West) the alkalinity is quickly leached by the rain.
The nutrients in ashes are considered water soluble. In low rainfall areas it is important to use water soluble nutrients sparingly because they can lead to a toxic build up of mineral salts in the soil. This can be detrimental to earthworms, soil micro-organisms, and plants. Where mineral salts have built up you find a white or pale blue crust forming on the soil’s surface and sometimes on the lower parts of plants. Where we are most likely to see this is in our potted plants that live indoors or on covered porches or patios. The solution when this happens to garden soils is to leach the area with plenty of water, repeatedly, and to add organic matter. The water dissolves the salts and disperses them into deeper soil layers. Organic matter fosters soil micro-organisms who take up the salts and turn them into organic compounds that are slowly released in a more plant and soil friendly form. However, if ashes are only used liberally during the rainy season, and sparingly or not at all during the dry season this is not likely to be a problem in high rainfall areas.

Wood ashes contain almost all the macro and micro nutrients plants need to grow and produce abundant crops, except for two. Wood ashes lack nitrogen and sulfur. Nitrogen can be supplied to our soils by the actions of earthworms and micro-organisms; through the breakdown of green manures such as alfalfa, legumes, cover crops, and grass clippings; from composted animal manure, or from sparing side dressing with chicken manure or bat guano. Be extra careful with manure and guano. They contain soluble nitrogen that can leach into our ground water and end up in our rivers causing dangerous algae blooms.

Sulfur is provided to soils and plants primarily by organic matter. Mixing compost or purchased organic amendments into the soil at planting time and then using water conserving mulch during the heat of summer will supply your plants with all the sulfur they are likely to need. Plants can also extract a certain amount of sulfur from the air. Sulfur is released to the air by burning. Our wood stoves, cars, generators, and other fuel driven engines, as well as local forest fires all increase the amount of sulfur in the air. Rain washes some of this sulfur into our soils making it available to micro-organisms and plant roots. Too much sulfur in the air can cause toxic conditions and devastate natural and man-made landscapes. It is better for the environment, our gardens, and our health to avoid burning trash. Brush, paper, and cardboard can all be shredded and used for compost and mulch; improving our soils and supplying sulfur to our plants while keeping it out of the air.

Because of the alkalinity of ashes they definitely need to be kept away from acid loving plants. Azaleas, camellias, blueberries, and rhododendrons are the most commonly planted acid loving plants in our area. Most acid loving plants originally started out growing wild in conifer forests or peat bogs. Soils with high amounts of organic matter and plenty of rain or standing water are often acidic by nature. Here is a web site with a fairly complete list of acid loving plants found in people’s gardens: http://www.algoflash.com/AcidList.htm Hydrangeas, which aren’t really classified as acid loving, bloom in different colors depending on the pH of the soil. If you have several hydrangeas it might be fun to side dress one of them with ashes and the other with acidifying coffee grounds and/or pine needles and see how the flowers turn out. It might even make an interesting experiment to document for next year’s science fair!

Here’s where we usually talk about sources, but this time you will just have to make your own! Next week we will be talking about Hon tsai tai and some other yummy early greens. Until then, if it is raining, I might be inside starting seeds and only dreaming of Digging the Dirt.