A simple model for salt accumulation in closed-loop hydroponics

The paper presents a simple model for the changes in ion concentration and electrical conductivity (EC) of recirculating nutrient solution in closed-loop soilless culture. The model was developed for closed hydroponic systems in which crop evapotranspiration (E) is compensated by refilling the mixing tank with complete nutrient solution; in these systems, EC gradually increases as a result of the accumulation of macro-elements and, principally, of non-essential ions (Na and Cl) contained in the irrigation water. The model was designed on the basis of balance equation for nutrient uptake by hydroponically-grown plants, then calibrated for tomato using data from the literature and, finally, validated with independent data sets from two experiments conducted during spring and autumn of 2001 using water with different NaCl concentration. The results of validation indicate that the model describes well the variations in Na concentration and EC in closed-loop soilless culture of tomato, although it slightly overestimates the measured quantities. How the model can be used for managing greenhouse soilless culture is briefly discussed.