In the Canadian province of Quebec, plant coal has been used as a substitute for peat and perlite in reforestation projects for the rearing of tree seedlings for some time. There are two reasons for this. On the one hand, prices for conventional planting substrates have risen, and on the other hand the forestry industry is also striving for more environmentally friendly rearing processes. In the study (see below for details) the use of biochar in the cultivation of white spruce seedlings was investigated for over a year.
The authors concluded that the perlite content can be completely replaced with biochar and up to 25% of the peat volume. The growth of the seedlings was not accelerated by the use of biochar. However, the economic analysis produced a cost advantage of 25 percent, which, according to the authors, is likely to increase further due to further falling prices for biochar on the market, lower transport costs and lower maintenance costs. The authors also identified an ecological advantage: If the entire perlite used in Quebec for gardening and landscaping were replaced by biochar, this would lead to a CO2 saving of up to 4000 t/a.
Problem with conventional growing substrates
The seedlings of white spruce are extremely sensitive to external environmental influences such as the correct pH value and stability of the plant substrate as well as water availability. Most nurseries in Quebec therefore use peat-based substrates containing perlite or vermiculite to maintain the desired aeration, bulk density, water availability and stability. Prices for perlite, vermiculite and peat have risen significantly and are expected to continue to rise. In addition, perlite is usually imported and is considered a finite resource. Peat is produced in Quebec in large quantities and is therefore affordable. However, costs have also risen, and peat is considered a finite resource like perlite. In addition, peat cutting is accompanied by the destruction of important wetlands and high CO2 emissions into the atmosphere.
Effect of biochar on forestry
The use of biochar in forestry would therefore have several positive effects. On the one hand, the forestry industry produces millions of tons of branches, bark, sawdust and other forest residues annually, from which the required biochar can be produced locally by environmentally friendly pyrolysis. On the other hand, biochar has important properties which may vary depending on the production process and input material, but which have in principle been proven to be beneficial for tree growth in various studies: Biochar improves root aeration, increases the pH value and thus prevents root diseases. It regulates the water content and supports the microbial life in the soil, which leads to increased plant productivity. What’s more, biochar offers high water storage capacity and acts as a structuring component in soils and substrates. Furthermore, there is no additional effort for the use of biochar, neither for tree nursery producers nor for reforestation, as the use of biochar requires the same technical equipment as for the processing of peat.
3 different plant carbons in comparison
In the study 3 different plant carbons were tested: Firstly, plant carbons were obtained by pyrolysis (15 minutes at 475 °C) of softwood bark residues/ softwood bark residues (BC1). For the second variant (BC2) spruce sawdust was pyrolysed for a few seconds at 454 °C (BC2), for the third variant hard charcoal residues (> 75% sugar maple) were charred at 500 °C for 24–48 hours (BC3). In the respective test substrates, the biochar replaced either peat or perlite or both. In addition, the proportion of biochar in the total volume varied between 6, 12 and 25 percent by volume. The desired growth effects of perlite and peat could be observed with all biochar variants. Differences to the control group resulted only from greater pH fluctuations in the plant coal substrate variants, but according to the authors this could be avoided by more precise settings/controls.