The 2011 ASHS Annual Conference

Non-circulating or passive hydroponic methods are closed systems that utilize water and fertilizer very efficiently. These systems avoid the additional production costs and complexities associated with mechanical aeration and circulation, including the need for electrical power and pumps that are required in many conventional hydroponic systems. The most common non-circulating hydroponic methods involve suspended pot, floating top cover, float-support, and sub-irrigation techniques.  The entire crop of short-term vegetables (such as lettuce) may be grown by the suspended pot method with only an initial application of water and nutrients. Plants are grown in net pots that are initially in contact with nutrient solution.  Plant growth causes the nutrient solution level to decrease, and this creates an expanding moist air space between the tank cover and the nutrient solution.  When the liquid level drops below the net pots, the expanding root system is capable of absorbing nutrient solution from the tank.  The nutrient solution level should not be raised, because this will ‘drown’ the plants by submerging roots that were previously exposed to moist air.  Therefore, tanks should be sheltered from rain.  The floating top cover method is very effective for growing watercress.  Watercress is direct seeded into net pots, which are held by extruded polystyrene sheets that continuously float on nutrient solution.  The lower portions of the net pots are in constant contact with nutrient solution. The tank may be refilled with nutrient solution after harvesting to produce a ratoon crop.  Lettuce has been successfully grown with the float-support method, which is a modified version of the suspended pot method. Lettuce seedlings are transplanted into net pots supported by extruded polystyrene sheets that initially float on the nutrient solution and come to rest on 10-cm-diameter plastic pipes as nutrient solution is lost by evaporation and transpiration, thus creating an expanding moist air space.  Cucumbers, tomatoes, and other long-term vegetables are typically grown with some version of a sub-irrigation method where additional nutrient solution is added throughout the life of the crop. Nutrient solution flows by gravity from a storage tank to a float valve assembly that is set to maintain a 5-cm depth of nutrient solution in the tank.