Modelling Na and Cl concentrations in the recycling nutrient solution of a closed-cycle pepper cultivation

A mass balance model relating the concentrations of Na or Cl in the recycled nutrient solution to the cumulative uptake of water by the plants was calibrated and validated in greenhouse sweet pepper crops grown in closed-cycle cultivation systems. The results indicated that the model allows for simulation of the courses of Na and Cl accumulation in closed-cycle cultivations of pepper, provided that the fluctuations in the volume of drainage solution and the moisture status of the substrate during the cropping period are small. Furthermore, the model predicts a delay in the process of salt accumulation in closed-cycle hydroponic systems when the drainage percentage is relatively high due to enhanced irrigation frequency, in compassion with standard irrigation frequency. The proposed model may be incorporated into intelligent automation systems to automatically adjust the target total salt concentration in the outgoing irrigation solution to levels allowing for a constant nutrient supply, despite the Na and Cl accumulation in the recycled leachate. As a result, the entire amount of drainage solution can be recycled without risking depletion of essential nutrients in the root zone.