## 4 Conclusions  
In this study, a new formulation of porous inorganic materials as a controlled-release fertilizer was characterized and preliminarily tested in the soil. The use of pumice scraps effectively increased the porosity and lightness of the PIMs, reducing the consumption of natural raw materials. The application of another industrial waste in the PIMs formulation, in addition to the spent coffee grounds and bovine bone ash, already tested in previous studies, increases the sustainability of this type of controlled-release fertilizers, according to the principles of circular economy promoted by European Union. Nutrients were added to the formulation both as separated materials and in a fertilizer glass. First results suggest that the two fertilizers showed similar properties, although the application of the vitrification process affected porosity and absorption capacity. The characterization of the PIMs also demonstrated the possibility of using them in the agricultural field for pH and EC values, measured according to the regulatory limits, while antibacterial analysis confirmed the possibility of safely handling the granules, which showed inert behavior even for the BSF-based coating. This aspect opens the possibility of extending the use of porous materials in a domestic context. Comparing lettuce growing in unfertilized soil and with the coated PIMs gave a preliminary indication that fertilizers could have a significant impact on biomass growth, measured as fresh and dry mass. The two formulations of PIMs, APNUT_BSF, and APV50_BSF appeared to contribute similarly, although APV50_BSF may have slower nutrient release due to fewer open pores. Finally, the first approach to verify the behavior of porous materials in terms of nutrient release was carried out with a chemical analysis of the nutrients contained in APNUT_BSF and APV50_BSF before and after 21 days of lettuce growth. The measured amount of nutrients in general decreased, suggesting that they were released into the soil and were made available to the plants, thus contributing to the growth of biomass, as confirmed by the morphological data and the weight of plants. The content of hazardous elements such as Pb and Al, on the other hand, did not change, and they were measured in small quantities in the PIMs, suggesting that they were not hazardous for agricultural application. This multidisciplinary preliminary study is therefore a positive start that encourages future research on fertilizers based on pumice scraps for crop growth in confined spaces. However, further confirmation is needed, and statistical validation of these preliminary results will be the object of future and more extended works. Conceptualization, F.N.A., E.S. and L.B. ; investigation, all authors; resources, L.B. ; writing--original draft preparation, C.R., E.S. and L.B. ; writing--review and editing, all authors; supervision, C.R. and L.B. ; funding acquisition, L.B. All authors have read and agreed to the published version of the manuscript. The experimental development of this research was funded by FAR Mission Oriented 2020 UNIMORE project (budget FOMO), project title \"GREW (Garden from Recycling & Wastes)--New integrated system for house and vertical gardens cultures by synergic application of innovative fertilizer and LED lighting: A circular economy strategy giving to waste materials a new second life\" CUP E99C20001100007 (January 2021-July 2022). Not applicable. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to founding regulation. **Acknowledgments:** This work was presented at the workshop \"Engineering and circular economy: the road to sustainability\" funded as a part of the ECO-MET-AL Project (PID2019-109520 RB-I00), \"Can industrial and mining metalliferous wastes produce green lightweight aggregates?