Biochar to Replace Depleting Peat Moss in Agriculture?

Plant enthusiasts are acquainted with peat moss as the main component of potting mix, but a harvest of the material is turning out to be unsustainable. Not only is peat being depleted faster than it can re-form, but its use in potting mix also adds to the discharge of carbon dioxide into the atmosphere.

Peat bogs naturally store carbon. When peat moss is harvested, there’s a transfer of a global carbon sink into a net source. That’s because, within a couple growing seasons, most of the peat moss from the potting mix is either mineralized by microbes or thrown out and decomposed. Either way, carbon dioxide is released.

Andrew Margenot, Assistant Professor - Department of Crop Sciences

In a new study, Margenot and colleagues from the University of California, Davis explored a material termed as biochar as an alternative to peat moss in potting mix. Similar to charcoal, biochar is created through a process called pyrolysis, or heating to high temperatures without any oxygen present. It can also be derived from almost any organic substance.

In our study, we used one made from softwoods from selective logging. But biochars can be made from corn stover, switchgrass, and lots of other organic waste products. Biochar could even be made from a greenhouse operation’s own waste if there are trimmings from plants or old peat moss.

Andrew Margenot, Assistant Professor - Department of Crop Sciences

Margenot stresses that ‘biochar’ refers to a broad group of materials that can differ significantly in properties based on the pyrolysis temperature and the feedstock used.

When organic material decomposes naturally, the process discharges carbon dioxide. But biochar disintegrates very gradually – possibly on the order of centuries – so when organic material is turned into biochar, the carbon is fundamentally sequestered and cannot leak back into the atmosphere.

But how well does it function in potting mix? To learn this, Margenot and his team grew marigolds from seed to flower in several experimental potting mixtures that replaced peat moss with an increasing proportion of commercially procured softwood biochar.

In the biochar blends, pH soared. “The ones with lots of biochar had a pH up to 10.9, which is ridiculous for trying to grow things,” Margenot says. But this wasn’t unanticipated for the type of biochar the team used.

Marigolds grew and flowered excellently, even when biochar swapped all of the peat moss in the potting mix. However, for plants with high concentrations of biochar, the early growing stages were tough.

You could see that the plants took a hit in the early stages of growth – the first two to three weeks. They were shorter and had less chlorophyll, indicative of a nitrogen deficiency, which you’d expect at such a high pH. But these plants caught up by the end. By flowering stage, there was no negative effect of biochar versus peat moss.

Andrew Margenot, Assistant Professor - Department of Crop Sciences

The plants did not suffer any long-lasting adverse effects of the biochar, and the pH in those pots neutralized by the time the study concluded. Margenot thinks this could have been because of a natural process of ion exchange between potting mix and plant roots, naturally arising carbonates in the irrigation water, or the utilization of industry-standard fertigation – irrigation with low levels of dissolved nutrient ions such as phosphate and nitrate – in the experiment.

Although he only analyzed one type of biochar, Margenot is hopeful about the potential of biochar in nursery applications. “Because we used a softwood biochar known for its high pH, we really tested the worst case scenario. If it could work in this case, it could probably work with others.”

The research paper, “Substitution of peat moss with softwood biochar for soil-free marigold growth,” is published in Industrial Crops and Products. Margenot’s co-authors, all from UC Davis, include Deirdre Griffin, Barbara Alves, Devin Rippner, Chongyang Li, and Sanjai Parikh.