The Promising Benefits of Biochar
By Keith Funk & Jane Shellenberger:
Biochar is biomass (wood, leaves, straw, manure, or other biosolids) heated at high temperatures without oxygen. This process, known as pyrolysis, concentrates carbon in a form that is very resistant to biological decomposition. When applied to soil or added to products like concrete, a large fraction of its carbon content is sequestered for centuries. The ancient practice of using charcoal in the soil dates back thousands of years, notably in the Amazon Basin, where indigenous peoples mixed charcoal with organic waste.
Biochar looks like charcoal, is very porous, has a high pH and carbon content. While biochar is not a fertilizer, research indicates it can help retain nutrients in the soil. Its charged surface and high surface area allows it to absorb and hold on to nutrients like nitrogen, phosphorus, and carbon. Studies show Biochar can remove heavy metals from contaminated soil, increase its water-holding capacity, and provide a stable habitat for beneficial soil microorganisms.
Brett KenCairn, the City of Boulder's nature-based climate solutions team leader, describes biochar as a “microbe hotel.” He explains: “Biochar has two features that make it perfect for the garden. First, it has a mild electric attraction to water molecules, drawing water to itself in soil. Second, the original plant structure of whatever the biochar is made out of is preserved during pyrolysis, creating a perfect home for beneficial microbes to move into.”
When Brett’s wife Emily adds biochar to soil she first soaks it in aerated compost tea. It is sometimes recommended to precondition biochar by soaking it in water or mixing it with compost or manure for several weeks to charge it with nutrients and beneficial microorganisms.
When added to soil, biochar can improve plant growth and enhance crop yields. In general, applying between 5% to 20% by volume is considered beneficial; excessive application can lead to undesirable effects on plant growth. On poor soils with low carbon content, many studies have shown it can increase crop yields up to four times. Soil Science Society of America experiments found that biochar supplemented with fertilizer outperformed fertilizer alone by 60%. But it can take up to a year to see results.
Not all soils are well suited for biochar or react to it in the same way. Well-drained sandy soils with added biochar benefit from increased soil moisture retention, but moderately well-drained and/or clayey soils may not always respond as positively. Biochar’s high pH may not be beneficial incorporated into alkaline soils that already have a high pH (8+) common in Colorado. (Best determined by a soil test.) But studies show that adding Biochar reduces salinity in salt-affected soils without significantly impacting soil pH (while also noting that more research is needed to fully understand Biochar’s characteristics).
Though Biochar made from manure has a higher nutrient content than biochar made from wood cuttings, the latter persist over a longer period of time. The two different chars look the same but will behave differently. Especially for large applications, choose high quality biochar and look for certifications or test results indicating pH level, nutrient content, and stability.
My gardener friend Margaret Rogers bought a bag of medium-grind biochar manufactured by BioChar Now (a Berthoud, CO company) last season and had such good results that she says she will buy more this year: “My use of it was not very scientific, however, I can vouch for the fact that even in the quadrant with the poorest soil, the plants produced better than in past years. The biggest beneficiaries were the beans: Romano, yellow wax, borlotti. It was a fairly inexpensive experiment!”
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