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Feeding the forest… with leftovers

By Danielle Gravesande
March 27, 2018

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Scientist hand-applying wood ash to forest soil at the Island Lake Biomass Harvest Experiment

Scientists at Natural Resources Canada are exploring the potential of using wood ash as a forest fertilizer.

This wood ash is a leftover or by-product of biomass boilers that use sawdust, bark, woodchips and other wood waste products as fuel to supply heat and electricity, and can be good for forests in the same way that ash from a wildfire can help the forest to regenerate.

The scientists conducting this research are part of a network called AshNet, which is exploring wood ash application to improve forest soils in field trials across a range of Canadian forest ecosystems.

“We are working with research collaborators, industry, government, and people across the country looking at the impact of ash on forest soils,” says Dr. Paul Hazlett, a Forest Soils Scientist at the Canadian Forest Service of Natural Resources Canada. “[This research] has led to new questions about ash application, new trials, and some joint projects.”

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Soil surface with (right) and without (left) wood ash


The goal is to understand the effects of wood ash on the physical, chemical, and biological properties of soil and to measure its effects on forest growth.

Ash could be very useful to forests that have lost nutrients from harvesting. Conventional forest harvesting removes tree stems, or main trunks, for wood products. When the stems are removed, so are the nutrients within them. More intensive forest harvesting removes more than just stems -- treetops, branches, and smaller trees, which can be used as fuel for biomass energy. On one hand, biomass energy can reduce fossil fuel use. On the other hand, the intensive harvesting of biomass removes even more nutrients from the forest. This greater nutrient removal has the potential to decrease the fertility of forest soil.

Wood ash provides an attractive solution to ensuring soil fertility following harvest. It can replenish soil with nutrients such as calcium, magnesium, potassium and phosphorus, which are essential for plant growth. “By adding wood ash, you are modifying the chemical properties of the soil. And increasing many of the nutrients that are needed by plants,” says Dr. Hazlett.

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A landfill containing wood ash

Researchers are also examining the effects of ash on everything from bugs and spiders to microscopic bacteria. The high pH of wood ash can make it quite caustic, so it is important to ensure that organisms that live in the soil are not harmed. In some cases, adding alkaline-rich wood ash to forest soil that is highly acidic may benefit these communities and improve overall soil health.

Digging Deeper

Despite the benefits of applying ash to forest soil, many obstacles remain before this practice is adopted across Canada.

Most of the ash produced in Canada is currently landfilled. Does the cost of transporting wood ash to forests outweigh the benefits? Or is the disposal of ash in landfills a missed opportunity? By recycling these nutrients back into the soil, scientists can “close the loop” and make forestry more sustainable.


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