Study highlights biochar’s potential in horticulture
November 26, 2014
Interest has never been higher in adding charcoal, or “biochar,” to agricultural systems, where it’s touted as a way to boost the soil’s water-holding capacity, reduce the need for fertilizer, and counter climate change. But so far, biochar has gotten relatively little attention in the horticultural and growth media industries.
That could change. Research led by Reza Nemati of the growth media company, Fafard & Frères, in Quebec, Canada, suggests that biochar could be a good replacement for perlite and to a lesser extent, peat moss. However, several questions still need answering before biochar enjoys widespread adoption in horticulture.
The study appears in the November 2014 issue of the Vadose Zone Journal.
Growth media are solid materials other than soil, which alone or in mixtures can provide superior growing conditions for plants compared with agricultural soils. For decades, the horticultural industry has relied on substrates such as peat moss and the aggregates, perlite and vermiculite. But several factors have recently driven companies to look for alternatives, Nemati says.
Horticultural substrates are becoming less available, for example, and those made from aggregates, especially, are rising in cost. As part of the sustainability movement, companies also hope to reduce their mining of peatlands, and cut the energy costs associated with making perlite and vermiculite.
Finding substitutes isn’t easy, though. Successful growth media must be homogeneous, free of weeds and toxins, well balanced in physical and chemical properties, and capable of physically supporting plants and providing them with nutrients, air, and water. Ideally, their impact on the environment should also be low. As a first step toward developing biochar as an alternative substrate, an experiment by Nemati and his colleagues characterized three kinds of biochar and growth media made from them.
Charcoal, or biochar, is generated by burning organic material in the absence of oxygen, and part of its appeal is that it resists microbial decomposition and so can potentially store carbon long-term. One type examined by the researchers was produced from sugar maple and yellow birch logs (biochar A); a second was made from balsam fir, white spruce, and black spruce (biochar B); and the third came from hardwood waste byproducts (C).
These feedstocks were chosen because they are locally available, Nemati explains, helping eliminate the need for long-distance transport and thus reducing the environmental and economic costs of manufacturing substrates. Moreover, the three biochars differed in particle size distribution, imparting a range of physical properties to growth media made with them. Biochar A was coarsest in texture, C contained the finest particles, and biochar B was intermediate.
The researchers then created five different growth media: One composed entirely of peat moss; a second made of 70% peat moss and 30% perlite; and three more in which biochar A, B, or C was added in place of perlite. When the team subjected the growth media to a battery of tests, they found the biochar-containing media performed as well or better than the traditional substrates.
For example, biochar improved nutrient retention in the media significantly, resulting in an 11% drop in nutrient leaching, on average. Adding biochar also raised the pH, suggesting it could neutralize the natural acidity of peat and thereby reduce the need for lime. However, biochar can’t make up more than 30% of a growth mixture, as amounts above this will push the pH too high, and the substrate’s water retention and aeration properties may become unbalanced, Nemati cautions.
The coarsely textured biochars (A and B) also increased aeration and drainage in the growth media in a manner very much like perlite. Meanwhile, fine-textured biochar C helped retain water—a property that’s especially useful in hanging basket substrates. Based on these results, biochar could be feasible substitute for perlite, the authors conclude, especially since the cost of perlite and vermiculite is so high today. Its suitability to replace peat moss is less clear; in Canada, peat moss is still much cheaper to acquire than biochar, Nemati says.
There are other hurdles, as well. Upon handling, biochar releases black dust—a contributor to global warming—although the dust can be controlled by pelleting biochar or increasing its initial water content. An even bigger obstacle is that biochar is by no means a standard product: Its properties differ widely depending on the feedstock and production method. So some kind of certification program is needed to ensure that biochar meets industry standards, the authors say.
Before that happens, though, biochar needs to undergo another big test. How will it affect the growth of plants? Stay tuned.
Read the full study: https://www.soils.org/publications/vzj/articles/0/0/vzj2014.06.0074. The paper will be open access for 30 days from the date of this news story.