In order to improve plant factories, an appropriate control system on fertilization is urgently required. An automatic management system to control nutrient concentration was constructed using a programmable logic controller (PLC) and ion selective electrodes (ISEs) of nitrate, phosphate, and potassium ion. The concentration of nutrient components in a culture solution was monitored using these ISEs. When the concentration of the nutrient components diminished to the threshold set as an optimum condition (0.1 - 2.0 mM), an appropriate amount of a concentrated solution of each nutrient component was added to the culture solution using solenoid valves connected with the PLC. The present cultivation system was simply constructed without any computers and pumps. Three kinds of automatic control systems simultaneously worked and did not influence each other.
Keywords: Hydroponics; automatic control system; ion-selective electrodes; nutrient concentration.