Use of Compost and Limestone to Establishing Substrate pH and the Impact on pH Buffering Capacity

Peat moss is the primary substrate component used in the greenhouse industry.  The inherent pH of peat moss can range from 3.0 to 4.0 and limestone is typically added to raise pH to a suitable range.  Compost can also be utilized as a substrate component and has an inherently high pH of 6.0 to 8.0.  When using compost as a substrate ingredient, lime rates must be reduced or eliminated.  The first objective of this study was to determine the resulting pH of substrates created with varying amounts of limestone and compost.  The second was to assess the impact of the various amounts of limestone and compost on pH buffering capacity. The compost was created from a 1:1:1 weight ratio of a mixture 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. The base substrate consisted of 25% pine bark, 5% calcine clay, 15% vermiculite, 15% perlite with the remaining 40% consisting of peat and/or compost based on the treatments. 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.  These data indicate 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 experiment one, 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 moles of H⁺ per g of dry substrate)_. Buffering capacity was determined by taking the negative reciprocal of the linear regression slope of pH vs. mM H⁺. 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 pH buffering capacity.