Development of an optimal model-based ion-specific nutrition control system for horticultural crops

Closed fertilisation systems improve the water use efficiency of greenhouse crop production, which reduces groundwater pollution considerably. However, the uptake ratio of water and the individual nutrients is not constant over the cultivation period of most crops. This may result in ion imbalances in the nutrient solution if the composition of the supply solution is not adequately adapted to the growth stage of the crop and the growing conditions. To overcome this, it is necessary to develop an ion-specific nutrition control system. The optimal composition of the supply solution can be estimated based on models of the plant's water and nutrient uptake depending on the stage of its phenological development and environmental conditions. Ion-specific sensors can then measure the composition of the drainage solution online and use it as feedback control. In an initial trial, the research team varied the total concentration of nutrients expressed as electrical conductivity (EC) depending on the climate in the greenhouse. This strategy was compared to the supply of nutrient solution of a constant EC of 3 dS m^‑1, as this level is often used in horticultural practice. Tomato cultivar 'Pannovy' was grown in rock wool slabs in a greenhouse with ten closed nutrient cycles at Grossbeeren from April 22 to August 24, 2015. Ion-specific sensors measured the nutrient composition of the drainage solution online. Additionally, laboratory measurements were carried out every 14 days to determine the nutrient composition of the supply and drainage solution. The climate-driven adjustment of the EC in the nutrient solution supplied to the plants resulted in a more stable and on average lower EC in the drainage solution compared to the supply of solution at a constant EC of 3 dS m^‑1. During a harvest period of 10 weeks, the model-based treatment provided a 30% higher yield and a lower fraction of fruit with blossom-end rot compared to a constant EC.