## 2 Materials and Methods  
### Test Materials and Growth Conditions  
The aims of this work included the validation of two slow-release fertilizers which were used in growth tests with lettuce. These two fertilizers are characterized by a core-shell structure, and every aggregate weighs about 1 g. The core of both is made of a porous inorganic material that has a constant formulation shown in Table 1. Among the materials shown in Table 1, redclay obtained from northern Italy (Modena, Italy) and pumice (Europomice s.r.l., Milano, Italy) scraps (a material hard to trade due to too low particle diameter) were used as matrix materials [37,38]. Spent coffee grounds employed for this investigation were a post-consumer by-product obtained from a local coffee bar (Modena, Italy), and they were included into the core formulation as a poring agent. To enrich the cores with nutrients, animal bone meal ash, as a by-product of meat processing, and potassium carbonate (K\({}_{2}\)CO\({}_{3}\), ACS reagent, \(\geq\)99.0%, Sigma Aldrich, Merck, Darmstadt Germany) were respectively used as sources of phosphorus (P\({}_{2}\)O\({}_{5}\) content = 41 wt%) and potassium (K\({}_{2}\)O content = 68 wt%).  
It has to be specified that the two slow-release fertilizers obtained at the end of the manufacturing process differ only in terms of the method for adding nutrients into the matrix. The first type of fertilizer, called APV50, is enriched in nutrients through a fertilizer  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **wt\%** & **Function** \\ \hline Red clay & 28.3 & matrix material \\ Pumice scraps & 41.7 & matrix material \\ Spent coffee grounds & 10.0 & poring agent \\ Animal bone meal ash & 14.0 & source of phosphorus \\ K\({}_{2}\)CO\({}_{3}\) & 6.0 & source of potassium \\ \hline \hline \end{tabular}
\end{table}
Table 1: Core formulation (wt%) of the slow-release fertilizers.  
glass based on pumice scraps, animal bone meal ash, and potassium carbonate. In the second type, called APNUT, the meal ash and the potassium carbonate were simply mixed with the other compounds as they were. A detailed analysis of the two cores' properties and production processes are published elsewhere [39]. However, the main properties of the two cores are shown in Table 2.  
The shell of the fertilizers was made of an organic coating realized using water (11 wt%) and biomass from black soldier fly larvae (BSFL) (89 wt%) reared on a substrate of vegetable waste. BSFL were used as the source of nitrogen, after the removal of the fat fraction, with the double aim of increasing the nitrogen content and enhancing the workability of the material. The coating formulation was defined through an optimization process based on the DoE methodology in a previous study and the content of nitrogen estimated was 7% [40].  
Baby leaf lettuce (_Lactuca sativa_ L.) type cultivar \"Chiara\" (ISI Sementi S.p.A., Fidenza, Italy) seeds were used for the growth tests. The growing substrate was a mix with a 3:1 ratio, respectively, of agriperlite \"Agrilit 3\" (Perlite Italiana s.r.l., Corsico, Italy) and universal peat moss soil \"Potgromd H\" (UNICO, AL.FE s.r.l., Pomponcesco MN, Italy). The agriperlite had a density equal to 90 kg/m\({}^{3}\) and a particle diameter range between 2 mm and 5.6 mm. The peat soil contained 23% of organic carbon, 0.4% of organic nitrogen, and 46% of organic matter. This growing substrate, without the addition of fertilizing aggregates, was also used as a control during the growth tests. Five lettuce seeds were placed on the top of plastic pots. Each pot had a volume of 500 cm\({}^{3}\) that was totally filled with the growing substrate and 50 mL of water for every pot was given at the moment of sowing and then every 3 days until the end of the test, which lasted for 28 days. The tests were carried out in a growth chamber with constant temperature (24 \(\pm\) 1.5 \({}^{\circ}\)C) and relative humidity (64-75%) conditions. Light conditions are described in Section 2.2.