PH buffering in pine bark substrates as a function of particle size

Authors: PANCERZ, MAGDALENA - The Ohio State University; Altland, James

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2020
Publication Date: 10/14/2020
Citation: Pancerz, M., Altland, J.E. 2020. PH buffering in pine bark substrates as a function of particle size. HortScience. 55(11):1817-1821. https://doi.org/10.21273/hortsci14969-20.
DOI: https://doi.org/10.21273/hortsci14969-20

Interpretive Summary: Maintaining a consistent and ideal substrate pH is important for nursery and greenhouse crop growers. Stability of substrate pH is referred to as pH buffering capacity. Little is known about the buffering capacity of pine bark substrates used in production of nursery crops. The objective of this research was to determine the buffering capacity of pine bark in comparison to sphagnum peat, and to determine how particle size or texture affects buffering capacity in pine bark substrates. A laboratory method was used to determine how coarse-, medium-, or fine-textured pine bark is buffered to changes in pH by exposing the substrates to increasingly strong acids or bases. A second experiment isolated very narrow ranges of particles from pine bark and sphagnum peat to compare pH buffering among the two substrate types within a given particle size. A third experiment was conducted in a greenhouse using traditional containers and fertilizers to validate the laboratory findings. In all experiments, pine bark was more buffered against acids that would lower pH than sphagnum peat, while sphagnum peat was more buffered against basic solutions that would increase pH. Pine bark pH buffering capacity improved with decreasing particle size, while sphagnum peat pH buffering changed little with changes in particle size. These results will help growers and substrate manufacturers understand how substrate components contribute to pH management during crop production.

Technical Abstract: Stability of substrate pH in container-grown crops is important for proper nutrient management. The objective of this research was to determine the pH buffering capacity of pine bark substrates as a function of particle size and compare those results to sphagnum peat. The weight equivalent of 100 cm3 for fine, medium, and coarse pine bark and sphagnum peat, either as a whole or partitioned into several particle size ranges, was placed in a 250 mL glass jar and filled with 100 mL of an acid or base solution ranging from 0 to 50 meq.L-1 in 10 meq.L-1 increments. After 24 hours, pH was measured. An experiment was also conducted in the greenhouse. The weight equivalent of 500 cm3 of sphagnum peat, fine pine bark, or coarse pine bark was filled into 10 cm plastic pots and irrigated with one of the following: tap water or 10 meq.L-1 of either HCl, NaOH, H2SO4, or KHCO3, and with or without a water soluble fertilizer. Substrate pH was determined 4 and 8 weeks after potting using the pour-through method. In all experiments, sphagnum peat had less buffering capacity than pine bark against pH changes from acidic solutions, while pine bark had less buffering capacity than sphagnum peat to pH changes from basic solutions. Substrate pH buffering in pine bark increased with decreasing particle size, while pH buffering in sphagnum peat was less responsive to particle size. These results will help growers and substrate manufacturers understand how substrate components contribute to pH management during crop production.