Buffering Capacity of Substrates with Varying Amounts of Compost and Limestone

Peat is the most popular greenhouse substrate component used today. Compost is another component that can be used in greenhouse substrates. However, the chemical properties of peat and compost are extremely different. Peat has an inherent pH range of about 3 to 4, whereas compost has an inherent pH range of 7 to 8.5. Typically, Limestone is added to peat based substrate to adjust pH.  The rates of limestone must be adjusted accordingly when compost becomes a component of these substrates. The objective of this study was to compare pH buffering capacity of substrates that have had pH established by limestone, compost, or a combination of both. The experiment was a factorial design with four compost rates by volume (0%, 10%, 20%, and 30%), four limestone rates (0, 1.19, 2.37, and 3.56 g of limestone per l of substrate) and five replications. Each substrate treatment was titrated through individual incubations with six rates of sulfuric acid (0, 0.1, 0.2, 0.4, 0.7, and 1.0 moles of H⁺ per g of dry substrate)_.  pH was measured at 24, 48, 96, and 168 hours to determine if buffering capacity changed over time. Electrical conductivity (EC) readings were also taken on treatments receiving 0 moles of H⁺ per g of substrate. Generally, substrates that had the pH established by the addition of compost had similar buffering capacities compared to substrates that had pH established by limestone. Additionally, substrates with pH established by a combination compost and limestone also had similar buffering capacities. Limestone rate had a minimal effect on substrate EC. However, increasing compost rates from 0% to 30% caused the EC to increase on average from 0.27 to 0.77 mS. These results indicate compost can be used to establish substrate pH similar to limestone. Furthermore, substrates that have had the pH established by compost rather than limestone will also have a similar pH buffering capacity. However, since the addition of compost can result in higher substrate EC, this practice should avoided with salt sensitive crops.