The challenge of using hazelnut waste as a pumice stone substitute in lettuce (Lactuca sativa L.) production

Pumice stone is the main inert material used to improve aeration and drainage in different substrates in both conventional and organic nursery production. However, concerns regarding the environmental repercussions of pumice stone have escalated in recent years, coinciding with the rising demand for these non-renewable substrates. As a potential solution to this issue, this study assesses the viability of using ground shells, derived from the waste of hazelnut as alternative inert material to pumice stone in growing substrates. Hazelnuts represent the world’s leading nut crop with a production of about 1,200,000 t in 2022. Most fruits, about 90%, are used within food-processing industry that generates large volumes of wastes, particularly woody pericarps, as a byproduct of the cracking process. Hazelnut woody pericarp contains a large amount of lignin and could be considered an inert material suitable for the preparation of substrate mixes. Three growing substrates contained 10, 20 and 30% (w/w) of ground hazelnut shells (G) compared with a commercial growing substrate containing 20% (w/w) of pumice stone (P 20) were used to evaluate the potted growth of lettuce in greenhouse. The growth parameters such as leaves area, plant height, root length, chlorophyll content, fresh and dry weight were evaluated every week until 28 days. Furthermore, also phenols and flavonoids content as well as antioxidant activity were determined. The results demonstrated that hazelnut shells are a suitable inert material for lettuce growing substrates, effectively promoting root length. Additionally, P20 and P30 showed no significant differences in leaf area compared to P20 after 28 days. Furthermore, phenols and flavonoids content decrease during lettuce growth without significant difference among tested growing substrates. These results provide attractive perspectives in view of low cost and availability of hazelnut shell in Italy and over the world. It seems that pumice stone can be replaced with ground shell in the horticulture sector.