Control of concentration of nutrient solution in soilless growing systems, depending on greenhouse climate advantages and limitations

Strategies to control the concentration of nutrient solutions in soilless growing systems were developed and tested in greenhouse experiments using sweet pepper and tomato. In order to synchronise nutrient demand and nutrient supply, the ratio of nutrient to water uptake was estimated depending on greenhouse climate, and then used to derive a daily set point for the concentration of the nutrient solution supplied to the plants. The performance of these feedforward control strategies was satisfactory: feedback control based on the concentration of the drainage water was not necessary. The calculated set point for the concentration of the nutrient solution varied considerably, while the average over the growing period was within the range of the recommendations for horticultural practice in Central Europe. However, changing the concentration of the nutrient solution supplied affected the ion composition in the root environment. The strategies investigated did not affect the total yield of sweet pepper and tomato. They did, however, result in a trend towards reduced incidence of blossom-end rot compared to strategies applied in current horticultural practice that focus on constant concentration of the nutrient solution in the root environment.