Fig. 40: Profit margin in relation to yield with different heights of the lights in tomato production – calculation scenarios

rather recommended than a combination of HPS top lights (750 W) and LED top lights.

5.2 Yield in dependence of the height of the light source

By lowering the HPS lights in the Hybrid lighting treatment from 1,4 m distance between lights and plant canopy to 1,0 m it was possible to increase the photosynthetic photon flux density from 373 µmol/m2/s to 454 µmol/m2/s by nearly 100 µmol/m2/s. With that was the substrate temperature significantly increased and a positive effect on yield was observed: Total yield, total number of fruits and marketable yield increased significantly. The yield increase was related to a higher first class yield due to a significantly higher average weight of the fruits. However, the number of marketable fruits was independent of the height of the light source, despite of a tendentially higher number of 1. class fruits when lights were lowered. The marketable yield was increased by more than 20% when Hybrid lights were mounted more closer to the plants. Indeed, also Verheul et al. (2022) observed a yield increase with a higher light intensity of HPS top lights or a combination of HPS top lights and LED interlights. However, in contrast to the presented results was the yield increase related to an increase in the number of harvested fruits, whereas the fruit weight was much less affected by a higher light intensity.

Moving the Hybrid lights closer to the plants resulted in a 2.500 ISK/m2 higher profit margin (Fig. 40). The yield was increased by 4,2 kg/m2. Therefore, it is highly recommended to move the lights closer to the lamps, as the µmol level increased and this resulted in an increase in yield and profit margin.