Charcoal Use in Container Growing Media

Tom Miles

Charcoal in Container Growing Media
Tom Miles, January 11, 2009

P Pine Grown in Vermiculite (Left) and Charcoal (Right) MediaP Pine Grown in Vermiculite (Left) and Charcoal (Right) Media

After visiting Richard Haard and Larry Williams in early 2007 I started discussing the use of charcoal with various nursery growers and researchers in the West. A commercial nursery in California became interested in substituting charcoal for vermiculite in a growing media - soilless substrate - for container grown tree seedlings.

There could be both financial and ecological benefits from using charcoal in place of vermiculite. Vermiculite is increasingly expensive, especially in the quantities used by commercial nurseries. Locally made charcoal should be cheaper. Vermiculite has a poor carbon footprint since it is heated to 1000 C (1832 F) in processing and transported long distances, often imported. Charcoal that is made locally as a byproduct of energy production could be used in growing media. Since it would be planted in the forest with the seedling the carbon sequestration would be permanent.

The nursery uses a growing media made of combinations of peat (50%), bark (20%) and vermiculite (30%). Bark is a common material in Northwest nurseries and has been studied extensively. (See Landis, Altland, Buamscha, Scagel). The grower tested seven mixtures substituting charcoal for vermiculite (up to 30% of the mix) and substituting compost for peat, another expensive substrate.

Each blend was placed in two Styrofoam blocks containing 112 plants for a total of 896 plants including the control. Ponderosa pine was grown in all containers.

Charcoal was gathered from mixed conifer burns in a local watershed. It was crushed and screened through a 1/4 inch (6 mm) screen.

The bulk density of the charcoal was 14.6 lbs/ft3 (0.23 g/cm^3 ) compared with vermiculite at 4-10 lb/ft3 (.06-.160 g/cm^3 ); bark at 0.17 to 0.20 g/cm^3 ; and peat at 0.08 g/cm^3. Perlite and pumice are also used in some nursery mixes. They are denser with perlite at 0.32-.4 g/cm^3 and pumice at .38-.66 g/cm^3 .

The density of the 30% charcoal mix 15.4 lb/ft3 (0.25 g/cm3) was similar to the control at 14.6 lb/ft3 (0.23 g/cm3). The other blends were somewhat heavier at 18-29 lb/ft3 (0.29-0.47 g/cm3).

Water availability was similar for the 30% char (67%) to the 30% vermiculite control (68%) and slightly less (48-60%) for the other mixes. 50% is typical. At loading it was noted that the char mix was "very hydrophobic."

Air-filled porosity was similar (14%) for the 30% charcoal to the 30% vermiculite control (16%) and in a similar range (14%-19%) for the other mixes. Typical is 12-15% with a maximum of 25%.

pH was 5.2 in the 30% vermiculite control mix and 6.1 in the 30% charcoal mix. Substitution of compost for peat in the mixes raised the pH to between 7.1 and 7.5.

At the time of my visit last week both plant health and root growth looked the same for the 30% char and 30% vermiculite. Root plugs were firm. The grower is both surprised and satisfied with the success of the charcoal substitution and will be doing further testing after a closer evaluation of the plants.

Tom Miles


Haard, Richard, FourthCorner Nursery, Washington,

Landis, T.,D., 1990. Containers and Growing Media,Vol 2, The Container Tree Nursery Manual, Agricultural Handbook 674, Washington,D. C.: US Department of Agriculture Forest Service 41-5.

Landis, T.D. and Morgan, N. 2008. Growing Media: Overiew and Update Preentation to Western Forest and Conservation Nursery Association, Missoula, MT. (attached)

Altland,J Baumscha G-Nutrient Availability from Douglas Fir Bark in Response to Substrate pH

Gabriela Buamscha and James Altland, Pumice and the Oregon Nursery Industry

Altland, J, Changing Container Substrate pH: What are the affects of peat moss, lime source and lime rate?

Buamscha, G, Container no-brainer, The physical properties of substrates play a big part in crop health and costs, Oregon Associationof Nurseries

Scagel, Carolyn, Container Soilless Substrate Component Fertility for the Northwest Nursery Industry