Model-based control of concentration of nutrient solution in glasshouse tomato cultivation

A strategy to control the concentration of the nutrient solution, derived from models of photosynthesis and transpiration, was tested in a greenhouse experiment with tomato grown in rockwool in a closed growing system. The set point for the concentration of the nutrient solution (electrical conductivity) was calculated and adjusted once a day, as a function of the water vapour pressure deficit inside the greenhouse. This strategy was compared with a constant low, and a constant moderate concentration of the nutrient solution in the root environment of 2 dS m(-1) and 4 dS m(-1). Changes in the concentration of the nutrient solution supplied were necessary to achieve these concentrations. Electrical conductivity of the nutrient solution supplied during the growing period ranged, in the low concentration treatment from 1.2 to 1.8 dS m(-1) (mean 1.5 dS m(-1)), in the moderate concentration treatment from 1.9 to 2.8 dS m(-1) (mean 2.2 dS m(-1)) and in model based concentration treatment from 1.1 to 3.4 dS m(-1) (mean 2.2 dS m(-1)). No differences in yield on either a fresh or a dry weight basis were observed between the moderate and model-based treatments. However, the percentage of fruit with blossom-end rot was significantly lower in the model-based treatment. The low concentration treatment resulted in the least incidence of blossom-end rot, and in increased yield related to the decreased dry matter content of the fruit.