Pine Bark Proves Best for Potting

Agricultural Research Service (ARS) scientists in Alabama have some practical advice about potting media for gardeners and homeowners who plant shrubs or trees.

Their insight comes from a study they conducted with colleagues at Auburn University. The team focused on the effects of pine bark and other types of potting media used to grow many potted shrubs and woody ornamentals that are sold at retail plant nurseries and garden centers.

Pine bark is considered an industry standard as a potting media. But since it is a by-product of logging and timber operations, the supply can fluctuate with the demand for wood.

There are several wood-based alternatives to pine bark media. The researchers decided to compare different types of media for their ability to store carbon in the soil. Storing carbon in soil removes carbon dioxide from the atmosphere and can improve soil health by making it more able to store water and nutrients.

“There’s been considerable research on mitigating greenhouse gases in agricultural and industrial settings, but not so much in residential settings,” says George B. Runion, a plant pathologist with the ARS National Soil Dynamics Laboratory in Auburn, Alabama.

Runion and ARS plant physiologist Stephen A. Prior planted three woody ornamentals in three types of potting media (pine bark, clean chip residue, and “Whole Tree,” a product made from the entire tree). They gave the plants time to take root and then transplanted them, along with their potting media, into outdoor plots. The plants grew for 3 years, receiving only rainfall and the occasional weeding and herbicide spraying.

At 1 and 3 years after the shrubs were transplanted, the researchers measured the amount of carbon stored in soil samples collected near the plants. They also continuously monitored the amount of carbon dioxide released into the atmosphere from the soil with a research tool known as the “Automated Carbon Efflux System.”

The results showed that the pine bark media was much more effective than the other two types of media at storing carbon in the soil. Pine bark stored up to 50 percent more carbon in the first year and roughly 100 percent more by the third year. The amount of carbon stored by the alternatives began to drop after the first year, but it held steady for the pine bark. The ornamentals also grew just as well in the pine bark as in the two alternatives.

Runion says pine bark probably stores more carbon than the alternatives because it contains more lignin, which decomposes slowly. Pine bark should also store carbon just as well in different soils, different parts of the country, and with different types of shrubs and small potted trees. In fact, the researchers found similar results for nine other ornamentals planted at the Auburn site that were not reported in the paper.—By Dennis O’Brien, ARS Office of Communications.