## 3 Results and Discussion
### Applicability of Bio-Based Fertilisers in a Full-Scale Field Trial  
At the start of the growing season, a soil scan was carried out to provide insight in the spatial variation in the soil electric conductivity (EC) within the experimental field. The average EC per plot at 1 m depth ranged between 10.0 and 13.2 mS m\({}^{-1}\), with highest values in the north and west and lowest values in the south and east (Figure 6B). A gas pipe line was identified as a straight line in the upper part of the field. At three moments during the growing season thermal and NIR images of the field were collected. The weighted (near-infrared-red) difference vegetation index (WDVI) image taken at the beginning of the growing season showed little difference in the treatments that received manure (left side of the field), and the control plots are clearly visible (Figure 6C). The other NIR images showed  
Figure 5: The yield response curves of mineral fertiliser N (**A**) and K (**B**) together with the performance of the liquid fraction of digestate (LFD), the ammonium sulphate solution (AS), and the K-concentrate (KC). Y\({}_{0}\) is the yield obtained from the yield curve when no mineral fertiliser is applied, Y\({}_{1}\) is the yield obtained from the yield curve when a fixed amount of mineral fertiliser is applied, and Y\({}_{2}\) is the yield obtained from the treatment that received BBF. similar results. The thermal map taken at the end of the growing season showed clear indications of heat stress (Figure 6D). The plots treated with AS had a lower temperature compared with the other plots. Again, no clear pattern was visible in the plots that received manure. The control plots showed clearly lower yields (60 \(\pm\) 13.6 t ha\({}^{-1}\)) compared with the other plots (Figure 6E). Highest yields were obtained in the plots that were treated with AS (73 \(\pm\) 9.1 t ha\({}^{-1}\)) followed by the plots treated with manure and mineral fertiliser (68 \(\pm\) 5.9 t/ha). The other treatments showed quite similar results (65 \(\pm\) 7.8 t ha\({}^{-1}\) for LFD, 65 \(\pm\) 4.1 t ha\({}^{-1}\) for the manure and AS treatment, and 63 \(\pm\) 1.7 t ha\({}^{-1}\) for the manure and LFD treatment). For the market, consumption potatoes need to have an underwater weight between 70 g and 80 g kg\({}^{-1}\). The treatments with only LFD are at the lower part of this range (72.2 g kg\({}^{-1}\)), the treatments with mineral N fertiliser are at the upper part of this range (80 g kg\({}^{-1}\)), and the control treatment (no mineral N fertiliser) is far above (92 g kg\({}^{-1}\)). Looking at the size distribution of the potato tubers, the control treatment that received no N fertiliser and the treatment with manure and LFD had a large number of small potato tubers compared with the other treatments. In these treatments, 70%, in the case where no N fertiliser applied, and 59%, in the case where manure and LFD was applied, had a size smaller than 5 cm of the total numbers of potato tubers counted during test probing. In the mineral N fertiliser-treated plots only 39% of the counted potato tubers had a size smaller  
Figure 6: Summary of the field performance during the growing season. The different treatments and repetitions (**A**), the electric conductivity (mS m\({}^{-1}\)) (**B**), the weighted (near-infrared-red) difference vegetation index (-) measured on 21 June 2020 (**C**), the thermal heat map (\({}^{\circ}\)C) measured on 7 August 2020 (**D**), and the average potato yield per plot (t ha\({}^{-1}\)) (**E**). than 5 cm during the test probing. For the other treatments, the size of the potatoes were comparable with the treatment with mineral N fertiliser. The number of tubers that were counted during test probing were lowest in the plot that received LDF only (45 tubers). The number of tubers for the other treatments were very similar and ranged between 57 for the treatment with LFD and manure, and 66 for the treatments with AS and manure. These results illustrate the slightly poorer performance of LFD compared with AS.