ARS | Publication request: Effect of pine bark, pine straw, and red oak amendments on pH of potting medium

Submitted to: Journal of Applied Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 22, 2010
Publication Date: July 1, 2010
Citation: Burner, D.M., Pote, D.H. 2010. Effect of pine bark, pine straw, and red oak amendments on pH of potting medium. Journal of Applied Horticulture. 12(2):102-106.

Interpretive Summary: Pine straw has considerable economic value in pine production and landscaping, and has excellent properties when used as a landscape mulch. Our objective was to determine if pine straw mulch affected topsoil acidity (pH). Experiments were conducted in which loblolly pine straw with or without nitrogen fertilizer (field), or two rates of pine straw, pine bark, and red oack wood mulches (in containers), affected topsoil acidity in comparison to an unmulched control during 12 mo. In both experiments, the mulches were mixed into the topsoil to accelerate rotting. Pine straw did not increase topsoil acidity in the field experiment, while nitrogen fertilizer increased acidity. Pine straw and pine bark mulch increased topsoil acidity in the container experiment. Red oak wood generally had no effect on topsoil acidity compared to the control. We concluded that there was little potential for pine straw mulch to acidify soils when a typical rate of pine straw (17 tons/acre) was applied. Nevertheless, we recommend that landscapers using either pine straw or pine bark mulches should periodically monitor topsoil pH and consider lime additions as appropriate for the plant species being grown.

Technical Abstract: Pine (Pinus L. spp.) straw has considerable economic value in pine production and landscaping, and has excellent properties when used as a landscape mulch. Our objective was to determine if pine straw mulch affected topsoil pH. Experiments were conducted in which incorporated applications of loblolly pine (P. taeda L.) straw with or without N fertilizer (field), or two rates of pine straw, pine bark, and red oak (Quercus rubra L.) wood mulches (in containers), affected topsoil pH in comparison to an unmulched control during 12 mo. Data were conflicting for the field and container experiments. Topsoil amended with pine straw and N (pH 5.26) had a lower pH (P = 0.01) than the control (pH 5.47), but this response was mainly due to N, not pine straw mulch. In the container study, pine straw and pine bark mulch decreased topsoil pH by as much as 0.48 units relative to the control, while red oak wood generally had little effect on topsoil pH compared to the control. Results suggested that landscapers using either pine straw or pine bark mulches should periodically monitor soil pH and apply lime as appropriate for the horticultural species of interest.