August 04, 2020

Study shows wood ash from power plants a cheap substitute for agricultural lime

CHERRYFIELD — The ash byproduct of Maine’s wood-fired electrical power plants fulfills the same need as lime as a soil amendment and has the added benefit of providing fertilizer in the form of potassium and phosphorus, according to a study prepared by the Soil Conservation Service of the United States Department of Agriculture.

The study, “The Economics of Using Wood Ash as a Source of Lime,” contends that it is cheaper for a farmer to use wood ash than commercial agricultural lime.

Wood ash is less alkaline than an equal volume of lime. One ton of wood ash contains about 746 pounds of calcium carbonate. A hayfield requirement of 3 tons of lime an acre could be satisfied with 7 tons of wood ash, according to the study.

The study assumed that it would cost $36 a ton to buy and spread 3 tons of lime on 1 acre of land. Assuming that the farmer could obtain the wood ash at no cost, the farmer’s only cost would be an estimated $51 an acre to spread 7 tons of ash on the same acre of land.

The study also contends that the farmer would gain a financial advantage from the fertilizer contained in the wood ash. The 7 tons of wood ash would contain 88 pounds of a phosphorus compound and 261 pounds of a potassium compound. The cost of buying and spreading equivalent amounts of phorphorus and potassium fertilizers would be $56 an acre, according to the study.

The yields of crops of hay, corn, alfalfa and some vegetables are said to be improved by the use of wood ash as a soil amendment.

The use of wood ash from industrial plants as a soil amendment is regulated by the Maine Department of Environmental Protection. The ash can be stockpiled for up to 180 days on a concrete or gravel pad. If the storage site meets certain soil and slope criteria, the ash can be stacked directly on the ground.

Wood ash is not applied to soil during the winter season, when snow or frozen ground could create a runoff problem. Each site where industrial wood ash is to be spread must be approved by the state regulatory agency. The appropriate regulatory form is titled “Application for Permit by Rule for Bioash Utilization under the Solid Waste Management Law, May 24, 1989.”

Wood-fired power plants such as the Babcock-Ultrapower plants in Deblois and Enfield and the Down East Peat, Limited Partners, power plant in Deblois produce about 5,000 tons each of wood ash each year.

A table is available at the offices of the Soil Conservation Service that compares the metallic trace-element content of “average Maine soil” with that of wood ash and commercial fertilizer. According to the table, wood ash has about the same amount of cadmium as Maine soil and commercial fertilizer and slightly less chromium than either. Both are toxic metals.

Concentrations of lead, another toxic metal, are greater in wood ash than in commercial fertilizer, but lower than in Maine soil, which contains almost 44 parts per million.

According to the table, wood ash easily meets Maine’s standards for the presence of metallic trace elements in soil. Lead is an example: the Maine standard is 700 parts per million, but wood ash contains only 28 parts per million.

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