Organic by-product substrate components and biodegradable pots in the production of Pelargonium ×hortorum Bailey and Euphorbia pulcherrima L

The need to reduce peatland exploitation has increased demand for materials that can at least partially replace peat in root substrate mixes. In this study four substrates containing 10% perlite, or 10, 30 and 50% rice hulls (RH) (the rest being white peat) were evaluated alone or amended with 20% of the anaerobic digested residues of fruit and wine distillery stillage wastes (ADR). Substrates were analyzed physically and chemically and used to grow Euphorbia pulcherrima L. 'Red Fox Cosmo Red' and Pelargonium ×hortorum Bailey 'Green Aida Red'. A compostable RH pot was also evaluated as an alternative to the conventional plastic one. Substrates containing 10% RH were very similar to the one containing 10% perlite, while substrates with 50% RH had higher pH, total pore space, air-filled porosity and nitrate nitrogen and potassium concentrations and lower electrical conductivity, water holding capacity, bulk density and ammonium nitrogen concentrations compared to the 10% RH substrate. The addition of ADR reduced organic matter concentration and increased pH, electrical conductivity, bulk density, and almost all nutrient concentrations. For both species growth with 10% RH substrates was similar to that with 10% perlite. However, while in poinsettia, only 50% RH gave worse performances than 10% perlite, in zonal geranium, growth was already poorer with 30% RH. ADR reduced poinsettia growth in general, while root growth of zonal geranium was improved. RH pots affected root dry weight, which significantly decreased in zonal geranium.