Research: Biochar as circular economy substitute for peat

Escuer said that fallen leaves can also be left on the ground for a mulching effect.
Escuer said that fallen leaves can also be left on the ground for a mulching effect. Source: Airika Harrik

In Estonia and throughout Europe, ornamental gardening is becoming an increasingly popular pastime and a significant industry. Given the extensive use of peat soils, which upon excavation transform from potent carbon sinks to sources of CO2 emissions, it is crucial to find alternatives for plant growth medium, Olesja Escuer, a researcher at the Estonian University of Life Sciences (EMU) said.

"Peat is a natural growing medium, but its extraction and the resulting disturbance of peatlands have significant detrimental effects on the environment," Olesja Escuer, a researcher at the EMU and gardener at the University of Tartu Botanical Garden, said.

Peat harvesting diminishes natural diversity, disrupts water regime and contributes to carbon dioxide land emissions.

The booming horticulture industry in Europe uses 90 percent peat substrate. As ornamental plant gardening grows in popularity, so does the demand for greenhouse production.

In her recently defended doctorate thesis, Escuer examined sustainable circular economy alternatives to peat.

She looked at how different mulch affects the growth, flowering and potting soil properties of popular flowering plants such as hybrid petunia, Impatiens walleriana, commonly called impatiens or bizzy Lizzy, and Tagetes patula, or French marigold.

In particular, Escuer tested the effects of partial peat replacement with hardwood biochar, essentially horticultural charcoal, on container-grown young marigold plants.

Peat mulch. Source: Airika Harrik

From production residue to wise contributer

Peatlands store water, sequester and store carbon dioxide and form their own ecosystem, Escuer said. "When peat is extracted and drained, it erodes, leading to the emission of greenhouse gases. Everything is interconnected," she continued.

Escuer looked into hardwood biochar, a carbon-rich material resulting from pyrolysation, as an alternative to reduce peat use, specifically in the preparation of growing media for younger container plants.

She experimented with different amounts of wood biochar added to the peat to find the best balance of the two substances. "Although biochar has a high pH, peat is an acidic material, so one cancels out the other," she explained.

Due to the fact that different varieties of hardwood biochar have distinct chemical and physical properties, the neutralizing effect of the particular biochar only became apparent during the experiment. Escuer discovered a formula in which up to fifty percent of the peat in the growing medium can be replaced with charcoal to create the optimal pH (acidity level) for plants.

"In addition to balancing soil acidity, biochar is important because it provides an indirect indication of nutrient content," Escuer explained. Depending on the material burned, however, the charcoal may be too nutrient-rich and increase soil salinity.

In moderate doses, biochar works as a potassium fertilizer, again, depending on the biochar and its original material. "Ashes are used in horticulture in the same way. It is often added to the soil to provide potassium to plants."

Escuer was looking for solution that could contributed to circular economy in the sector, so the hardwood charcoal she used in the experiment is a byproduct of a local company's charcoal produce.

"They ship the larger portions of charcoal to the store, where we purchase them for the grill. Nonetheless, a small amount remains, and they must find a purpose for it." Escuer says. Therefore, substituting even the smallest amount of peat with biochar has environmental benefits.

Gravel can serve as mulch in rock garden. Autor/allikas: Airika Harrik

Cocoa beans and grass clippings

While Escuer experimented with biochar as a peat fertilizer for the young, just emerging plants in the container, for the open fields she used mulching with other organic and inorganic materials. "We used kraft paper, light gravel, grass clippings, pine bark, cocoa bean shells and peat in the experiment," she said.

Of all the media she has tried, Escuer recommended using a mixture of lawn-cutting residues and cocoa shells. "Both of these decompose quickly. The husks provide phosphorus-potassium, while the grass clippings provide more nitrogen and magnesium," she explained.

Cocoa bean shells may be expensive, but they have a place in smaller private gardens, or even in the soil of a plant grown in a container. "Mulch has the same impact in a container as it does in an open field: less watering is required, the soil temperature is better regulated, and the sun dries the soil less," she explained.

"The primary benefit of using mulch is that it increases soil moisture, lowers soil temperature and can reduce weed infestation," she said.

However, she said, the choice of mulch depends on the soil and the plants being grown. For instance, annual summer flowers have to be digged out of the soil in the fall.

"Bark could be used with annual palnts," she said, "but I recommend to remove the bark from the soil surface before taking plants out so that the bark does not mix with the soil."

This is due to the fact that bark supplies excessive carbon to the soil. In the case of perennial flowers, however, bark mulch enhances the soil airation and minimizes the gardener's watering needs.

"I believe that more and more people are considering how to use fewer synthetic fertilizers and pesticides," she added.

Each mulching material works slightly differently: "cocoa bean shells, for example, did not significantly reduce soil temperatures when compared to pine bark and grass clippings," she said. This is due in part to pine bark's low thermal conductivity, but also to the three materials' different albedo, or ability to reflect solar radiation.

"Of course, the carbon-to-nitrogen ratio of the mulch material is a crucial consideration," she continued. When combined with soil, excessively carbon-rich mulch, for example, can produce nitrogen deficit in plants, and vice versa.

The optimal ratio for grass clippings is 25-30:1, although it can be even lower, Escuer said.

In particular, she has private gardeners in mind who can easier manage without the use of chemicals. "Clearly, it is quite challenging to replace them in large-scale production."

However, research community is leaning towards finding sustainable circular economy alternatives, she said.

Olesja Escuer. Source: Airika Harrik


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Editor: Kristina Kersa

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