Nutrient delivery system

A plant nutrient delivery system, comprising:

• a sensor and a riser, the sensor being coupled directly to the distal end of the riser, positionable in plant nutrient liquid in a plant nutrient liquid space and operable to sense a low level of the plant nutrient liquid indicating a condition of plant growth media located above and covering the plant nutrient liquid space;
• an electronically actuated valve with a connection to a source of the plant nutrient liquid, the electronically actuated valve being coupled to the riser and spaced apart from the sensor, wherein the electronically actuated valve and riser are positionable in an operating position with the riser extending downwardly from the electronically-actuated valve and the sensor in the plant nutrient liquid space;
• and
• an electronic controller linked to the sensor and to the electronically actuated valve, the controller being programmed to carry out automatic demand-based nutrient delivery by controlling the electronically actuated valve to turn on and off based on signals received from the sensor regarding the level of the plant nutrient liquid, thereby causing a flow of the plant nutrient liquid into the plant nutrient liquid space to start and to stop, wherein in response to the sensed low level, the controller initiates a stress cycle for a predetermined duration.