## 4 Conclusions  
In this work, we used the Design of Experiments methodology to combine and evaluate the effects of two types of slow-release fertilizers combined with two different light recipes on the indoor growth of baby leaf lettuce (_Lactuca sativa_ L.)  
To summarize, the following conclusions could be drawn from this study:  
* Among the fourteen properties considered as response variables, five produced statistically validated results. Three of them are morphologic properties, namely FW, LA, and Height of plants. Two of them are chemical properties, namely C and RS.
* The quantity of fertilizer, expressed as number of aggregates and considered in three levels (zero, three, and six aggregates) is the factor that has a greater effect on lettuce growth. Use of the highest fertilizer input guarantees an increase in fresh weight (FW) of lettuce equal to 65% and an increase in average leaf area (LA) equal to 46%.
* The combined effect of both the fertilizer type and quantity with the light recipes leads to an increase in plant height (Height) equal to 33%. This value is reached using the highest amount of fertilizer and two different combinations of fertilizer type and light recipe.
* The two chemical properties of lettuce, namely organic carbon content (C) and solid residue at 105 \({}^{\circ}\)C (SR), depend on the amount of fertilizer and on the light recipe used. C and SR show almost constant values of 3.7% and 7.9%, respectively, when the LED-2 light recipe is used, independently from the amount of fertilizer used. However, when the LED-1 light recipe is applied, C decreases from 4.5% to 2.8% with increasing amounts of fertilizer. SR shows the same behavior, decreasing from 9.5% to 6.8%. This could be because the LED-2 light recipe may allow the better exploitation of nutrients contained in the fertilizer.
* In the future, the use of higher quantities of fertilizer could be investigated to verify additional growth benefits or the existence of a threshold beyond which there are no further effects or negative effects, e.g., phytotoxicity for seedlings. The use of more fertilizer could also lead to the use of the same growth substrate in two subsequent growth cycles, to maximize its exploitation.  
Conceptualization, M.M. and A.B.; methodology, S.B. and F.B.; software and validation, S.B.; investigation, F.B. and A.B.; writing--original draft preparation, F.B. and S.B.; writing--review and editing, M.M. and A.B.; supervision, M.M. All authors have read and agreed to the published version of the manuscript. This research received no external funding. This research received no external funding. Not applicable. Not applicable. Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author.  
The authors thank [PERSON] for the support with growth experiments, [PERSON], [PERSON], and [PERSON] for the laboratory analysis and the fruitful discussions (University of Modena and Reggio Emilia). Europomice S.r.l. is acknowledged for supplying material. Chemicalab S.r.l. (in particular dott. [PERSON] and dott. [PERSON]) is also acknowledged for analytical support.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.