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Additional author: Mengmeng Gu, Professor, Colorado State University Department of Horticulture and Landscape Architecture.
Is Biochar Good or Bad for Plant Growth? Mixing biochar into soilless substrates may have negative, zero, or positive effects on plant growth.
Biochar made from green waste mixed with peat at 50% by volume has been shown to increase prayer plants’ total biomass and leaf surface. Adding 10% by volume of sewage sludge biochar with peat-based substrates can increase lettuce biomass by 184%–270%. Mixing pruning-waste biochar with peat-based substrates at 50% or 75% by volume can also increase lettuce biomass. Mixing 20% or 35% (weight per weight) of coir biochar with 0.5% or 0.7% humic acid into a composted green-waste medium showed increased biomass of rattlesnake plants compared to those without biochar and humic acid amendments.
Mixed hardwood biochar (50% by volume) and sugarcane bagasse biochar at 50% or 70% with a bark-based substrate increased basil plants’ average root diameter. Mixed hardwood biochar at 20%–80% by volume increased photosynthesis, shoot fresh weight, and shoot dry weight of chocolate mint, peppermint, Kentucky Colonel mint, spearmint, and orange mint plants. Also, pinewood biochar mixed with pine bark increased chrysanthemum shoot fresh and dry weights.
Biochar may also have adverse effects on plant growth. For example, we tested one type of biochar with high salinity; plants grown in the biochar mixes wilted within 30 min. When plants do not have enough water to dissolve the extra salts, they die.

Additional author: Mengmeng Gu, Professor, Colorado State University Department of Horticulture and Landscape Architecture.
Biochar has been proposed to be beneficial to the environment and plants. However, many people do not know what biochar is, what can biochar do, or how biochar can be used. In Part 1 of this publication, we provide introductory information on biochar used to partially replace peat moss as a container substrate component.
The International Biochar Initiative defines biochar as a solid material obtained from the carbonization of biomass, which may be added to soil to improve soil functions and reduce emissions from biomass that would otherwise naturally degrade to a greenhouse gas. Other researchers define biochar as a multifunctional material related to carbon sequestration, greenhouse gas reduction, soil contaminant immobilization, soil fertilization, and water filtration.
To simplify things, we’ll adopt the most popular definition: biochar is a black, carbon-enriched solid with a porous structure, mainly used in agriculture and environmental industries. Biochar is normally made from the thermal decomposition of biomass materials at high temperatures (570–2200 °F) in a low-oxygen or no-oxygen environment (this process is also known as pyrolysis). Biochar can be produced from pyrolysis of different materials such as pine bark, sugarcane bagasse, rice hull, and straw.