Construction of an Automatic Nutrient Solution Management System for Hydroponics-Adjustment of the K+-Concentration and Volume of Water. 
An automatic management system for nutrient solutions was constructed using a programmable logic controller (PLC) and a K+-ion selective electrode (K+-ISE).  The concentration of K+ was monitored by the K+-ISE.  When the concentration of K+ fell to the threshold limit, an appropriate amount of a concentrated K+ solution was added to the hydroponic solution.  The volume was also maintained at a constant level by addition of water.  This system can be constructed simply and inexpensively without any computers and pumps.