# HydroLab: A Module for the Investigation of Fertigation Strategies in Hydroponics
## 5 Results and Discussion
### Results  
Through the utilization of HydroLab, the experiment described in Section 4 was implemented. Weekly replenishment of the nutrient solution was carried out by filling tank T320 (Figure 1). A storage tank with a capacity of 60 L was utilized and regular cleaning of the reservoir was performed weekly to prevent calcification. The lettuce height measurements were taken every 2-3 days. The data regarding the amount of consumed solution were recorded and stored in the PLC. Figure 6 illustrates the two rows of plants on day 19, providing a visual representation of the system. Harvesting of the lettuce crops took place on day 30. The experiment was initiated shortly before the Easter holidays (when the university was closed). Despite this, the system operated autonomously, and the authors were able to remotely monitor its proper functioning through the HMI interface that was implemented. Next, the _leading indicators_ defined in Section 5.1 are presented to illustrate the growth process. Figure 7 shows the sigmoid curve of the two rows of plants. The curve represents the average height of each plant sample, and the error bars encompass both measurement uncertainties associated with the instrument and statistical variations. It can be observed that:  
* The crops started at approximately the same height, as lettuce seedlings grown under identical conditions were used;
* The first assessment was conducted on day 0, while the second took place on day 10. This was because the university was closed during the Eastern holiday and the height was manually assessed. In future research, the possibility of utilizing a camera for automatic height assessment could be explored;
* The radiation-based fertigation treatment exhibited greater growth compared to the time-based fertigation one. Figure 8 illustrates the cumulative nutrient solution indicator, representing the total solution consumed by each plant sample over the course of the 30-day experiment. The curve representing the treatment under time-based fertigation shows a consistent slope, indicating regular scheduling and a constant volume of the delivered nutrient solution. Conversely, the curve for the treatment under radiation-based fertigation exhibits variable slopes, reflecting fluctuations caused by the changing radiation and temperature conditions in the city of Bogota. It is worth noting that while height measurements were manually assessed, the consumed nutrient solution was automatically quantified and recorded in the PLC. Therefore, data are available even during the Eastern holidays period. Finally, the computed _lagging indicators_ are presented in Table 5. It is evident that the treatment under radiation-based fertigation exhibited better height and weight compared to the one under time-based fertigation, while also consuming a lower amount of nutrient solution. This superior effectiveness and efficiency of the treatment is reflected in the two higher-order indicators: yield and nutrient solution efficiency. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Indicator** & **Category** & **Leading or lagging** & **Dimension** \\ \hline Height & Morphological & Lagging & [Length] \\ Weight & Morphological & Lagging & [Mass] \\ Nutrient solution consumption & Resource & Lagging & [Volume] \\ Yield & Economic & Lagging & [Mass]/[Area] \\ Nutrient solution efficiency & Economic & Lagging & [Mass]/[Volume] \\ Cumulative height & Morphological & Leading & [Length] \\ Cumulative nutrient solution & Resource & Leading & [Volume] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Selected indicators for the comparison of time-based vs. radiation-based fertigation. Figure 8: Representation of the cumulative nutrient solution consumption over time. Figure 6: Photograph depicting the two rows of plants on day 19. The top row represents the treatment under time-based fertigation, while the bottom row corresponds to the treatment under radiation-based fertigation. Figure 7: The sigmoid curve illustrates the average height of each row of plants.