A passive hydroponics system will use some sort of mechanism to suspend a plant allowing the roots to hang into the nutrient solution. This method will be moderately successful with the main disadvantage being, ‘how do you support the plant as it grows?’
A simple system would be to fill a 5-gallon cooler with water, mix in an appropriate amount of hydroponic nutrient and float your plant in the cooler in a piece of Styrofoam with a hole for the plant cut into the middle. Just be sure not to sink the plant too low or it will drown. This is a very simple and effective set up except that once a plant grows it will overbalance and sink like a ship in a storm if it is not somehow supported. Still, such a set up may be ideal for growing something like lettuce.
Plants have 5 basic requirements they need to survive: light, water, oxygen, temperature, and mineral salts (or nutrient). Keeping these in mind, I suggest including an aquarium air bubbler in the cooler to add oxygen and, of course, a good light source.
Aquaponics can also be classified as a passive hydroponics system. This method uses a working aquarium, with fish, as the container and nutrient solution into which you suspend a plant’s roots. It already has an aquarium bubbler supplying oxygen and with a good light source this sounds like the perfect way to grow a vegetable plant. The system meets 4 of the basic requirements of plants. The last (nutrients) are provided under the theory that the fish eat and excrete nutrient-rich matter on which a plant will feed and prosper.
At the hobby level you may have some small measure of success, but this is not a viable hydroponics method. Remember that a vegetable plant has evolved to have very specific nutritional requirements – certainly different from the nutrition a water lilly may need. Aquatic plants such as waterlilies have evolved to prosper in such an environment so if you wish to add plants to your aquarium, add aquatic plants, not vegetables. It should be noted that more success is being enjoyed in larger, more regulated aquaponic systems that also double as small-scale fish farms.
Another classic passive system is to fill a jar with nutrient solution, jam the neck with a wad of cotton with a small plant poking through with roots dangling in the water. Then as a finishing touch you can place a small aquarium bubbler into the jar for oxygen.
Larry Maki is an avid, self-taught hydroponics gardener from Connecticut with a passion for alternative types of gardening.