For an efficient closed hydroponic system for recycling nutrient solution, it is necessary to precisely measure the ion concentration of the macronutrients in the discharged nutrient solution. Nutrient imbalance in the solution can cause plant growth failure, substantial fertilizer waste, and environmental pollution. To optimally recycle the discharged nutrient solution, it is necessary to measure the individual ion concentration. This study was conducted to develop a computer-controlled on-line monitoring system using commercial ion selective electrodes (ISEs); these electrodes measure the ion concentration of macronutrients (K, Ca, NO3) in discharged nutrient solutions. The developed system was applied to a Proefstation voor tuinbouw onder glas te naaldwijk (PTG) nutrient solution for lettuce. The system mainly consists of processing units including a pumping device, a sensor, an agitator, a rinsing device, and control units including the controller, control program, and interfacing device. Particularly, a separate measuring container was used for each ISE in this study. The selected ISEs were tested for time response characteristics and the drift effect to analyze the electromotive force (EMF). The sensitivities of the three electrodes (K, Ca, NO3) were evaluated using concentration controlled standard solutions. The EMF values exhibited high linearity (R2 = 0.99) with the log values of the ion concentration in the regression analysis of all the Ca, and NO3 ion ISEs. A monitoring algorithm was programmed using LabVIEW to control the entire process, which involved the discharged solution sampling, standard solution and ionic strength adjuster (ISA) control, ion concentration measurement, and rinsing & keeping of the electrodes. A two-point calibration method was used in every measurement to compensate for the drift, bias, or long-term change in the electrode response. Various diluted solutions with the PTG nutrient solution for lettuce were used to test the measuring performance of the developed system. The ion concentration of the diluted solution was determined based on the real concentrations in the discharged nutrient solution measured during the lettuce cultivation experiment. The concentrations measured by the developed system and real ion concentrations exhibited high linear correlation. Overall, the measuring errors of all the ISEs were below 4.0% within the concentration range.