## 3 Results and Discussion
According to Figure 4a, the best quantity of fertilizer for lettuce is the maximum considered in this study, namely six aggregates. The model suggests that the use of greater quantities of fertilizer could further increase FW without problems, since no signs of phytotoxicity were detected on the plants. Also, the use of greater quantities of fertilizer could lead to an increase of lettuce yield without any complication, since the slow release should avoid the excessive release of nutrients into the growth substrate. The other two responses related to morphologic properties show a linear behavior. In fact, LA (Figure 4b) shows a stable linear increase with a higher number of aggregates, which is again the only factor that is statistically reliable for this response. In this case, the average leaf area of the lettuce is equal to 13.00 cm\({}^{2}\) when no aggregates are used and is equal to 19.00 cm\({}^{2}\) when six aggregates are used, with a percentage increase of LA equal to 46%. This result highlights again the positive effect of both fertilizers on the growth of lettuce, since lettuce plants with broader leaves are an indicator of greater development. The increase of both LA and FW is an indicator of the health of the lettuce, since the plants obtained using six aggregates (Figure 6c,d) show a higher yield and development compared to plants obtained using no aggregates (Figure 4a,b). A different behavior is shown by Height that is highly dependent on the type of aggregates (in single) and on the type of light recipe (in interaction). The Height increases linearly with the number of aggregates, and this happens for both APV50 (Figure 4c) and APnut (Figure 4d), as the height of plants with six aggregates (red dashed lines) is always higher than the height obtained using no aggregates (black dashed lines). In particular, when APV50 fertilizer is coupled with the LED-2 recipe (Figure 4c), the use of zero aggregates corresponds to a height of 6.00 cm, while the use of six aggregates leads to a height of 8.00 cm, with a percentage increase equal to 33%. The same numbers are obtained using APNUT fertilizer (Figure 4d) but, in this case, the maximum height is reached when the LED-1 light recipe is used. In fact, the comparison in Figure 4c,d shows that the two combinations, LED-2/APV50 and LED-1/APNUT, of the light recipes and fertilizer type, each with six aggregates, allow lettuce to reach its maximum height, equal to 8.00 cm. On the contrary, the model suggests that the two combinations, LED-1/APV50 and LED-2/APnut, when applied with six aggregates result in a lower height of plants, equal in both cases to 7.00 cm. These findings may indicate that the two fertilizers have a different release pattern of nutrients, in particular for nitrogen and phosphorus, as previously highlighted in [39]. Each of the two fertilizers combines better with one of the two light recipes, suggesting that each recipe promotes the exploitation of different nutrients, as also suggested by other authors [51; 52]. Figure 4: Interactions among the input variables for the response variables (**a**) FW, (**b**) LA, (**c**) Height considering the APV50 core, (**d**) Height considering the APnut core. Figure 5: Interactions among the input variables for the response variable (**a**) C, (**b**) SR. The response variable C represents the content of organic carbon of plants, which is also an indicator of the content of soluble sugar in the plants. As shown in Figure 5a, C is mainly influenced by two factors according to Table 8, namely the Aggregates Number and the light recipe. However, it is worth noting that the LED-2 recipe guarantees a stable content of C near 3.7%, independently from the number of aggregates, while the LED-1 recipe tends to lower C with the increase of aggregate amounts. The maximum value of C is 4.5% with no aggregates, while the minimum value is 2.8% with six aggregates.