This is the system which uses lava in containers sitting in two to three inches of water, aerated with a couple of aquarium air pumps. It is considered a passive system and is a totally simple system to set up and operate.
As the water is used up, several things happen. First, the nutrients are not used at the same rate by the plants. As a result, the remaining water may contain a concentrate of nutrients, but these nutrients are mostly a mix of leftovers not used by the plants, so it has less food value to the plants. Adding new water to this mix provides some nutrients, which the plants can use, but adds a greater concentrate of unneeded salts.
The solution: Add water/nutrient solution at full strength as needed to maintain the two-to-three inch level of water maintained in the tray. Every three weeks rinse out the containers and change the water. The excess nutrients are drained out and replaced with fresh water/nutrient mix. By the way, this water can be used for other household plants or for your vegetable garden.
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