I have two main sources of wood ash: a wood stove in my shop and a soapstone wood heater in my house.
Over 90 percent of the wood burned in the house is Douglas Fir with a little cardboard and paper to get the fire going in contrast to the shop wood stove which I burn wood scraps, newspapers, old correspondence, cardboard food containers, weeds and any combustible material that I think might not be acceptable for recycling.
I always had a problem deciding where to put the ashes from the shop stove because I had heard about toxins from household items. After a little research I found out that ash from most natural wood products is OK as a soil amendment, but we should be concerned about human-altered wood products. The first source is the Canadian Wood Ash Chemistry Database which contains information on wood ashes from 21 Canadian biomass boilers: 10 pulp and paper mills and 11 bioenergy cogeneration facilities. Separate samples of bottom and fly ash were collected at nine of these facilities and all samples were analyzed to determine the chemical composition so that they could be compared to the limits established by the Canadian Council of Ministers of the Environment. The results of the analysis were shown in tables and graphs, including pH, carbon and nutrient concentrations. Trace element concentrations were also measured for arsenic, cadmium, molybdenum, selenium, cobalt, copper, lead, nickel and zinc. Samples were also analyzed for a 2017 wildfire in BC and 13 residential wood stoves from Ontario.
The following is a summary of the results. “Calcium is the most abundant element in wood ash and gives ash properties similar to agricultural lime. Ash is also a good source of potassium, phosphorus, and magnesium. In terms of commercial fertilizer, average wood ash would be about 0-1-3 (N-P-K). In addition to these macro-nutrients, wood ash is a good source of many micronutrients needed in trace amounts for adequate plant growth. Wood ash contains few elements that pose environmental problems. Heavy metal concentrations are typically low.”
The second paper was international in scope with the intention of increasing the use of ash from biomass combustion and co-firing. Nine countries contributed to the research, namely Austria, Canada, Germany, Italy, the Netherlands, Sweden, South Africa, Denmark and the United Kingdom.
In order to encourage the use of wood ashes the authors propose distinguishing source material. For example, clean forest wood, including grown wood, should be separated from used or contaminated wood, straw, solid waste combustion, sewage sludge, paper and animal manure.
The following information is taken from the article IEA Bioenergy Task 32 Deliverable D7 2018, Options for increased use of ash from biomass combustion and co-firing.
“The use of wood ash as a soil amendment in Canada is largely under provincial/territorial control and the process for obtaining regulatory approval to apply ash on forest or agricultural soils differs in each jurisdiction. In many jurisdictions, the approval process is complex and time-consuming and is frequently cited as a critical barrier to the increased recycling of wood ash in Canada.
Alberta is the only Canadian province with guidelines aimed specifically at the use of wood ash as a soil amendment . In some provinces (e.g., British Columbia, Ontario, Québec, New Brunswick), guidelines developed for soil applications of other residual materials (e.g., municipal biosolids, pulp and paper sludge) also apply to wood ash .
Before any approval to use wood ash as a soil amendment can proceed, most Canadian provinces/territories require that it be analyzed to determine the concentrations of 11 trace elements (arsenic, cadmium, chromium, cobalt, copper, mercury, molybdenum, nickel, lead, selenium and zinc; ). Some provinces require additional analyses to determine, e.g., pH, acid neutralizing value, moisture content, or concentrations of potassium, dioxins and furans, and/or polyaromatic hydrocarbons. This information is used to calculate ash dosage rates and to ensure that applied ash does not cause soil and/or water contamination.”
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