The suitability of de-inked paper sludge as growing media amendment for ornamental potting plants

The paper industry produces tons of deinked paper sludge (DPS) with high disposal costs and negative impacts for the environment. The recycling of DPS in growing media for ornamental plants could be a sustainable strategy to reduce greenhouse gases and improve soil properties, in a circular economy prospective. For this purpose, Primula obconica Hance, Viola × wittrockiana Gams 'Cool Wave Frost' and Dianthus caryophyllus L. were grown in control (commercial substrate), 30 and 50% S and 30 and 50% P (S = sludge, P = pelletised sludge) growing media. The substrates were analysed in terms of chemical properties (pH, electrical conductivity, and nutrient elements). Plant performance was assessed measuring the effective quantum yield of photosystem II (?PSII), the electron transport rate (ETR) and plant growth (n of flowers, biomass). The substrates containing 30% S, 50% S and 50% P had lower electrical conductivity than control; 30% S had values of exchangeable nutrients (Mn, Ni and Mg) similar to the control. When nutrient deficiency symptoms appeared in Primula and Viola from substrates containing DPS, as suggested by their reduction of ?PSII and ETR photosynthetic properties, a complete fertilization was given. After the fertilization, these plants showed a similar photosynthetic performance in all substrates. At the end of the cultivation cycle, Primula showed a reduced number of flowers and biomass in all DPS substrates, while Viola produced the same number of flowers in 30% S substrate and in the control and showed reduced biomass in DPS substrates. Dianthus was less affected by N deficiency and DPS treatment. Dianthus did not show significant difference as flower numbers and biomass in all substrates. In general, the growing media composed of 30% S resulted the most suitable for plant growth and amongst the tested ornamental plants, Dianthus responded positively to the amendment with DPS. DPS can be utilised as amendment, but the resulted substrate needs fertilization and must have compatible properties with the original growing media (e.g., pH).