The standing-aerated nutrient solution technique was devised by researchers in the late 1800s as a method to determine and verify the essentiality of those elements suspected to be required by plants needed for growth and life cycle completion. This method is still in wide use to today for conducting plant nutrition research. The roots of a plant are placed into a container filled with a nutrient solution that is being continuously aerated. This system worked well since researchers were able to remove from the nutrient solution all traces of that element being investigated as possibly essential. They did not have to deal with another source of that element being introduced as a rooting media, such as sand or gravel which were in common use during this time period when growing plants hydroponically. The last element to be determined as essential by using the standing-aerated nutrient solution technique was chlorine (Cl) in 1954, a significant chemical and analytical accomplishment since Cl is ever-present in the environment and the plant requirement is very low. In 1983, using this same hydroponic technique, several researchers have suggested that nickel (Ni) should be included in the list of essential elements, although some have questioned this conclusion (see Jones, 1998).
The standing-aerated nutrient solution technique of hydroponic growing has long been considered not well suited for the commercial production of plants. The nutrient solution changes with time as elements are depleted by root absorption, while the remaining solution quickly becomes a brew for the growth of bacteria and fungi. Therefore, the nutrient solution needs to be frequently replaced with fresh, with the disposal of the “spent” nutrient solution a significant environmental issue.
More recently, however, this technique has found an application is the production of lettuce and herbs (Jones, 2005). A flat shallow water-tight container is filled with nutrient solution, forming a pond, and on that pond, either a Styrofoam or polystyrene sheet is floated. Rockwool cubes, containing either a germinated lettuce or herb seedling, are placed in spaced openings made in the sheet. The nutrient solution is kept aerated by constant movement under the floating sheets. In some instances, air lines are placed in the bottom of the pond and air bubbled up through the nutrient solution. The major factor determining the volume of nutrient solution is its depth, sufficient so that replacement is not required until the crop is harvested. For most lettuces and herbs, the time between setting seedlings into the floating sheets on the pond of nutrient solution and harvest will be between 25 to 30 days. After the crop is harvested, the nutrient solution is either discarded, or prepared for re-use by reconstitution, filtering and sterilization.
Normally, the depth of nutrient solution varies between 4 to 5 inches, but it can be as much as 10 inches, the depth being important because depth adds to both nutrient element and temperature stability. Once the “rafts” are floated onto the nutrient solution, the crop is not easily attended until harvest. Therefore, this system of growing is not suited for the growing of long-term crops, or plants that require daily individual attention.
Lettuce and herbs are being grown by this technique at a facility located in Athens, Georgia. The nutrient solution depth is about 6 inches. The nutrient solution is constantly being pumped from the pool through a filter to remove suspended material as well as serving as a means of aerating.
This method for growing lettuce and herbs is well suited for use by the backyard gardener. Using a large child’s plastic swimming pool, fill it with nutrient solution. Take a sheet of Styrofoam and cut it to just fit the diameter of the swimming pool. Cut small openings in the Styrofoam sheet and place in the openings lettuce or herb seedlings that have been germinated in rockwool cubes (recommended due to their stability). Place the Styrofoam sheet on the surface of the nutrient solution. It is important each day to lift the edge of the floating sheet and stir the nutrient solution to keep it aerated and to add water, if needed, to maintain a constant depth of solution.
J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books including Hydropopnics: A Practical Guide for the Soilless Grower and Tomato Plant Culture. It is available at CrcPress.com.
Jones, Jr., J. Benton. 1998. Plant Nutrition Manual. CRC Press, Inc., Boca Raton, FL.
Jones, Jr., J. Benton. 2005. Hydroponics: A Practical Guide for the Soilless Grower, 2nd Edition. CRC Press, Inc., Boca Raton, FL.