A comparative study on some locally available organic materials for their potential utilization in sustainable growing media blends

There is an important need of cost effective and good quality growing media from locally available alternatives for the hobby markets and for commercial nursery and containerized ornamental plants production. A study on the availabilities and supply of renewable organic materials was conducted in northern Italy in 2022. The research highlights the leading role of compost and digestate (solid fraction) as best candidates to partially replace peat in growing media. Other material locally available in large amount has been studied and the potential availability of rice hulls, hazelnut and almond shells (2,000,000 and 250,000 m³ y^‑1, respectively) was estimated, even if there is an intense competition for their use of the energy and green building sectors. Spent mushroom substrate is available in significant amount (800,000 m³ y^‑1) in Northern Italy. Locally produced wood fibers do not fully meet the demand of growing media producers. Eleven materials were then selected, based on current and future availabilities and agronomic properties: chest nut wood fibers, pine wood fibers; rice hulls, spent mushroom substrate, solid part of digestates, green compost, hemp core, almond shells, hazelnut shells. Representative samples were chemically and physically characterized; their short-term stability, nitrogen immobilization potential and plant responses were determined. In addition, binary mixtures were prepared from wood fibers and nitrogen-rich materials (compost, digestate and spent mushroom substrate) in 3:1 volume ratios. After 21 days incubation, they were evaluated by measuring pH, EC, germination test and OUR. Deriving from a broad variety of sources, the materials show very different physical and chemical characteristics. Blending leads to plant responses like peat due to an improvement of physical properties and the amelioration of pH and salinity. The results indicate that it is possible to design sustainable growing media, paying great attention to peculiarities of each component and to the interaction among them. An additional aspect is the need to pre-process the labile materials for their stabilization.