Stability of substrate pH is an important factor in managing nutrition of container-grown crops. Some nursery and greenhouse crops have specific pH requirements. Walden and Epelman (1988) reported increased Japanese boxwood (Buxus microphylla Siebold & Zucc. var. japonica) root and shoot growth with increasing dolomitic lime (DL) rate and concomitant increase in pH. The increase in growth was attributed to a decrease in the ammonium (NH4+)-to-nitrate (NO3–) ratio because higher pH substrates resulted in more rapid nitrification of NH4+ to NO3–. Argo and Fisher (2002) identified three groups of floriculture crops based on pH requirements: a petunia group that is prone to iron deficiency at high pH, a general group recommended to be grown at pH 5.8 to 6.2, and a geranium group prone to iron or manganese toxicity at low pH. Crop preference for pH specificity could be related to the crop’s native environment. For example, Harvey et al. (2004) reported that ‘Aureola’ hakonechloa [Hakonechloa macra (Makino) Honda] grew best in a 3 pine bark: 2 sphagnum peat: 1 sand (by volume) substrate with no DL amendment (pH 4.5). They speculated this favorable response was due to the plant’s adaptation to the low pH soil found in the mesic, forested mountains of its native range in Hakone, Japan.
Substrate pH of nonamended pine bark is generally 4.1 to 5.1 (Altland and Jeong, 2016) and is commonly increased using DL before potting. The two primary factors that increase or decrease pH after potting and during the production cycle are irrigation water alkalinity or ammoniacal fertilizers, respectively. Irrigation water alkalinity is a measure of carbonates, bicarbonates, and hydroxyl ions in solution. It increases substrate pH when these compounds, predominantly bicarbonate ions, react with the substrate solution to consume hydrogen ions and thus increase pH (HCO3– + H+ < = > H2CO3 < = > CO2 + H2O). Irrigation water pH has relatively little effect on substrate pH compared with irrigation water alkalinity. Ramirez and Altland (2018) reported that a 0.0001 mm KOH solution with high pH (8.23) and low alkalinity (10.0 mg·L−1 CaCO3) did not increase substrate pH over 3 months, whereas a 0.005 m KHCO3 solution with similar pH (8.28) but high alkalinity (275 mg·L−1 CaCO3) increased substrate pH by more than 2 units after 3 months.
Many fertilizers acidify container substrates. Johnson et al., (2013) showed that fertilizer calcium carbonate equivalent (CCE) was effective in predicting the overall impact of a fertilizer on substrate pH. Moreover, the authors (Johnson et al., 2013) noticed that a simple estimation of the acidity or basicity of a fertilizer could be determined by nitrogen form and concentration. Fertilizers containing urea (CH4N2O-N) or ammonium (NH4+-N) nitrogen decrease pH by nitrification of NH4+ to NO3–, as well as H+ release during root uptake of NH4+ to maintain charge balance (Lang and Elliott, 1991). Albano et al. (2017) showed that substrate pH of a 9 Florida peat: 9 pine bark: 2 sand substrate fertilized with a controlled release fertilizer (Osmocote 19N–2.6P–9.9K), containing 9% NH4+-N and 10% NO3-N, decreased over time when irrigation alkalinity ranged from 50 to 150 mg·L−1 CaCO3.
The ability of a substrate to resist change in pH over time is termed pH buffering capacity. Buffering capacity can be affected by substrate components. Rippy et al. (2005) measured buffering capacity as the amount of base (mol·kg−1) required to raise the pH of a peat sample from 5.4 to 6.2 and showed that buffering capacity varied among 64 sphagnum peat samples collected from bogs in Alberta, Canada. Despite the importance of substrate pH on crop growth in pine bark substrates (Altland and Jeong, 2016), there is no scientific literature on the ability of pine bark to buffer changes in pH. Therefore, the objective of this research was to compare the pH buffering capacity of pine bark and its particle size fractions to nursery-grade sphagnum peat.
Albano, J.P., Altland, J., Merhaut, D.J., Wilson, S.B. & Wilson, P. 2017 Irrigation water acidification to neutralize alkalinity for nursery crop production: Substrate pH, electrical conductivity, nutrient concentrations, and plant nutrition and growth HortScience 52 1401 1405
Altland, J.E. & Jeong, K.Y. 2016 Dolomitic lime amendment affects pine bark substrate pH, nutrient availability, and plant growth: A review HortTechnology 26 565 573
Altland, J.E., Locke, J.C. & Krause, C.R. 2014 Influence of pine bark particle size and pH on cation exchange capacity HortTechnology 24 554 559
Altland, J.E., Owen, J., Jackson, B. & Fields, J. 2018 Physical and hydraulic properties of commercial pine-bark substrate products used in production of containerized crops HortScience 53 1883 1890
Anonymous. 2019 Ohio Environmental Protection Agency ground water quality characterization program. 17 Oct. 2019. <https://epa.ohio.gov/ddagw/gwqcp>
Argo, W.R. & Biernbaum, J.A. 1996 The effect of lime, irrigation-water source, and water-soluble fertilizer on root-zone pH, electrical conductivity, and macronutrient management of container root media with impatiens J. Amer. Soc. Hort. Sci. 121 442 452
Argo, W.R. & Fisher, P.R. 2002 Understanding pH management for container-grown crops. Meister Pub. Co., Willoughby, OH
Curtin, D. & Rostad, H.P.W. 1997 Cation exchange and buffer potential of Saskatchewan soils estimated from texture, organic matter and pH Can. J. Soil Sci. 77 621 626
Fonteno, W.C. & Bilderback, T.E. 1993 Impact of hydrogel on physical properties of coarse-structured horticultural substrates J. Amer. Soc. Hort. Sci. 118 217 222
Harvey, M.P., Elliott, G.C. & Brand, M.H. 2004 Growth response of Hakonechloa macra (Makino) ‘Aureola’ to fertilizer formulation and concentration, and to dolomitic lime in the potting mix HortScience 39 261 266
Jackson, B.E., Wright, R.D. & Barnes, M.C. 2010 Methods of constructing a pine tree substrate from various wood particle sizes, organic amendments, and sand for desired physical properties and plant growth HortScience 45 103 112
Johnson, C.N., Fisher, P.R., Huang, J., Yeager, T.H., Obreza, T.A., Vetanovetz, R.P., Argo, W.R. & Bishko, A.J. 2013 Effect of fertilizer potential acidity and nitrogen form on the pH response in a peat-based substrate with three floricultural species Scientia Hort. 162 135 143
Lang, H.J. & Elliott, G.C. 1991 Influence of ammonium: Nitrate ratio and nitrogen concentration on nitrification activity in soilless potting media J. Amer. Soc. Hort. Sci. 116 642 645
Parfitt, R.L., Giltrap, D.J. & Whitton, J.S. 1995 Contribution of organic matter and clay minerals to the cation exchange capacity of soils Commun. Soil Sci. Plant Anal. 26 1343 1355
Taylor, M.D., Nelson, P.V. & Frantz, J.M. 2008 Substrate acidification by geranium: Light effects and phosphorus uptake J. Amer. Soc. Hort. Sci. 133 515 520
Walden, R.F. & Epelman, G. 1988 Influence of liming rate on growth of Japanese boxwood in pine bark media. Proc. Southern Nursery Assn. Res. Conf. 33:52–57