A hydroponic nutrient-feeding control system controlled jointly by an electrolytic sensor and a presettable timer. Spaced electrodes sense the need for nutrient feeding by an increase in interelectrode resistance as the plant bed partially dries out and/or the liquid level drops too low. The sensed condition activates a timer switch to pump nutrient solution for a preselected time. A manual switch is provided to override the automatic subcircuit so that manual feeding can be effected if and when desired. A protective fuse and condition-indicating signal lights are included. The sensor is a pair of spatially adjustable J-shaped carbon or noble metal containing rods.