Establishing Growing Substrate pH with Compost and Limestone and the Impact on pH Buffering Capacity

in HortScience

The pH of peatmoss generally ranges from 3.0 to 4.0 and limestone is typically added to raise pH to a suitable range. Compost is also used as a substrate component and typically has a high pH of 6.0 to 8.0. When using compost, lime rates must be reduced or eliminated. The two objectives of this study were to determine the resulting pH of substrates created with varying amounts of limestone and compost and assess the impact of the various amounts of limestone and compost on pH buffering capacity. Compost was created from a 1:1:1 weight ratio of a mixture of green plant material and restaurant food waste:horse manure:wood chips. The first experiment was a factorial design with five compost rates (0%, 10%, 20%, 30%, and 40% by volume), four limestone rates (0, 1.2, 2.4, and 3.6 g·L−1 substrate) with five replications. The experiment was conducted three times, each with a different batch of compost. With 0 lime, initial substrate pH increased from 4.5 to 6.7 as compost rate increased. This trend occurred at all other lime rates, which had pH ranges of 5.2–6.9, 5.6–7.0, and 6.1–7.1 for rates of 1.2, 2.4, and 3.6 g·L−1 substrate, respectively. Substrate pH increased significantly as either compost or lime rates increased. The second experiment was a factorial design with four compost rates by volume (0%, 10%, 20%, and 30%), the same four limestone rates as Expt. 1, and five replications. Each substrate treatment was titrated through incubations with six sulfuric acid rates (0, 0.1, 0.2, 0.4, or 0.7 mol of H+ per gram of dry substrate). Substrates with a similar initial pH had very similar buffering capacities regardless of the compost or limestone rate. These results indicate compost can be used to establish growing substrate pH similar to limestone, and this change will have little to no effect on pH buffering capacity.

Contributor Notes

We gratefully acknowledge for technical assistance; Longwood Gardens Students Nicole Blevins, London Brown, Lindsay Byrne, Kerry Ann McLean, Kenneth Silveira, Naoko Seko, and Gavin Witmeyer; Longwood Gardens Volunteer Beth Yount; and University of Delaware graduate students Ma Yuecong and Liu Hsiaochang.

Corresponding author. E-mail: mtaylor@longwoodgardens.org.

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    pH of substrates created with 0%, 10%, 20,% 30%, or 40% compost in combination with 0, 1.2, 2.4, or 3.6 g or lime per L of substrate over 22 d in Expt. 1. Error bars represent se (n = 5).

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    Electrical conductivity of substrates created with 0%, 10%, 20%, 30%, or 40% compost in combination with 0, 1.2, 2.4, or 3.6 g or lime per L of substrate over 22 d in Expt. 1. Error bars represent se (n = 5).

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    Titration curves of substrates created with 0%, 10%, 20%, or 30% compost in combination with 0, 1.2, 2.4, or 3.6 g or lime per L of substrate using 0, 0.1, 0.2, 0.4, or 0.7 mm H+ per gram of substrate in Expt. 2. Error bars represent se (n = 5).

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    The main effect of acid concentration (0, 0.1, 0.2, 0.4, or 0.7 mm H+ per gram of substrate) from 24 to 168 h on pH of substrates created with 0%, 10%, 20%, 30%, or 40% compost in combination with 0, 1.2, 2.4, or 3.6 g or lime per L of substrate in Expt. 2. Error bars represent se (n = 5).

  • View in gallery

    Four graphs showing select titration curves of substrates created with 0%, 10%, 20%, or 30% compost in combination with 0, 1.2, 2.4, or 3.6 g or lime per L of substrate in Expt. 2. In each of the four graphs, the pH of selected substrates at 0 mm H+ is within 0.34 units. Calculated buffering capacity (mm·g−1·pH−1) for each curve is also presented. Error bars represent se (n = 5).

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