Reservoir research

I am growing some plants in a “Liz’s Garden.” This is a passive reservoir system. I am using pea-sized lava as the medium. Should the nutrient/water solution in Liz’s Garden be replenished when the tray dries out or should it be kept at the same level by topping it off?
Second, would adding an aquarium air pump to the tray help in oxygenating the water? The total cost would be around $20.

Hilo, Hawaii

The reservoir set-up is one of the easiest methods of growing plants hydroponically. Containers are filled with lava or clay pellets and placed in a tray. Water/nutrient solution is added to about one quarter the height of the container. For instance, a tray with 8-inch-tall containers would have 2 inches of water added, and one with 12-inch-tall containers would have a 3-inch water level. Hydroponic nutrients are added to the water as recommended by the manufacturer.

The water should be topped off as needed to keep the level constant. The new water should be pH-adjusted but not supplemented with fertilizer. After two weeks, the reservoir fertilizer is used up so the reservoir should be drained and new water/nutrient solution added.

A modified version of this system can be made using a wetter medium such as a bark or peat-based planting medium. Hydroponic solution is added to the 2-inch level, but no new water is added until one day after the reservoir is dry. This allows the planting medium, which becomes fairly saturated with water, to be replenished with oxygen.

A small air pump or submersible pump that circulates the water, so that CO2 is released from the water as oxygen is absorbed, provides the roots with more oxygen. Roots release CO2 and are big oxygen users, so oxygenating the water will stimulate them.

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