TITLE:
Spent Coffee Ground Derived Amendments for Horticultural Substrates: Maturity Assessment and Plant Growth Responses
AUTHORS:
William J. Dunford, Alessandro Holzapfel, Claire Bradly, Edward W. Bush, Barbara Dobosz, Adam Ekielski
KEYWORDS:
Spent Coffee Grounds, Organic Amendment, Compost Maturity, Pine Bark Substrate, Electrical Conductivity, Container Substrates, Lettuce, Petunia
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.17 No.11,
November
12,
2026
ABSTRACT: Spent coffee grounds (SCGs) are a widely available organic by-product with potential value as horticultural substrate amendments, but their biological instability, salinity, and possible phytotoxicity can limit direct use. This study evaluated whether accelerated household composting can produce stable SCG-derived amendments suitable for container substrates. SCGs were processed for 1, 5, or 10 days using two electric composting systems, Lomi® and Vego®. The resulting materials were assessed for Solvita® CO2 burst response, pH, electrical conductivity (EC), bulk density, particle size distribution, and macronutrient composition. Lettuce (Lactuca sativa) and Petunia (Petunia × hybrida) were grown in pine bark-based substrates amended with 5% or 10% (v/v) processed SCGs. One-day treatments remained biologically active, whereas 5- and 10-day treatments reached Solvita® values consistent with practical stability. Processing increased bulk density, reduced particle size, and produced acidic, nutrient-rich materials with C/N ratios below 25:1. SCG amendments improved biomass and SPAD values in both species compared with the pine bark control, with the strongest responses at 5% - 10% incorporation. However, increasing EC indicates that higher rates may be limited by salinity. Overall, 5 - 10 days of accelerated processing produced SCG-derived amendments suitable for low-rate use in pine bark-based substrates.