Compost As a Limestone Replacement for Substrate pH Adjustment

Peat moss is the primary substrate component used in the greenhouse industry.  The inherent pH of peatmoss can range from 3.0 to 4.0 and is typically increased to a suitable pH with the addition of limestone.  Compost is a product that can also be used as substrate component and has a high inherent pH of 6.0 to 8.0.  When using compost as a substrate component lime rates must be reduced or eliminated.  The objective was to determine the resulting pH and electrical conductivity (EC) of substrates with varying amounts of limestone and compost.  Two different compost were tested and one was created from a 2:2:1 ratio by weight of horse manure : wood chips : fine wood chips (particle size < 1 cm) and the another was created from a 1:1:1 ratio of a mixture green plant material and restaurant food waste : horse manure : wood chips.  The 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.  Three batches of each compost type were tested with this experimental design giving a total of 6 experiments.  The substrate consisted of 25% pinebark, 5% calcine clay, 15% vermiculite, 15% perlite with the remaining 40% consisting of peat and/or compost based on the treatments.  Both types of compost responded very similarly for both substrate pH and EC.  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.  At all lime rates, substrate EC increased with compost rate.  Other than with 0% compost, substrate EC decreased over time for all treatments.  The effect of lime on substrate EC was minimal or not significant at all compost rates.     These data indicate substrate pH was significantly affected by both compost and lime treatments.  Growers who use composts in their substrate mix will have to adjust lime rates accordingly to achieve the target pH.  When using 20% or more compost, initial substrate EC ranged from 1.1 to 1.9 mS.  Growers may need reduce fertilization practices offset the nutrient load associated with compost.