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Educational hydroponic experiment using Miracle-Gro and artificial lighting... We are going to try growing some tomatoes with Miracle-Gro. We would like to know if you could give us an estimate of how many ounces of Epsom salts we should add per pound of Miracle-Gro for an experiment. We are also going to use an identical setup using a commercially available hydroponic solution. I want to use the results of the parallel experiment in a paper for my son's science class. Can I use a flooded setup like a fish aquarium with lava rock for the medium, incorporating an aquarium bubble stone, air pump, and an under gravel filter to facilitate drainage and aeration? I am going to use 400-watt metal halide lighting as we have several used fixtures that we removed from a job and installed 1,000-watt fixtures to replace them. I can provide from 10-40 watts per square foot. I think that I read that we would need at least 10 watts per square foot. Is this correct? I will be growing the tomatoes in a controlled, 68-72 degree environment. Humidity will vary with the weather but ventilation will be sufficient to prevent formation of mildew. Thank you for your help. Dr. Lynette Morgan replies: Your hydroponic system setup sounds workable. However, tomato plants end up with large root systems, so you want to make sure they won't be overcrowded in the fish tank. A density of two to three plants (for indeterminate tomato cultivars) per square meter is the normal density. Also, tomato plants don't perform that well in a totally flooded system. Using ebb-and-flow (also called flood-and-drain) would work best for this type of crop. The amount of light you would need for tomatoes is at the high end of the radiation scale. Normally, the amount of artificial lighting required is between 20-60 watts per square foot of growing space with low-light plants such as lettuce, shade plants, and herbs at around 20-30 and high-light plants such as those that produce fruit in the 45-60 watts per square foot range. You will also need to add in one or two high pressure sodium (HPS) lamps to supplement the correct light wavelength for the tomatoes to flower well. Metal halide (MH) lights produce mostly light in the blue wavelength, which is excellent for vegetative growth, but light in the red spectrum is also required as the plants start to flower. Getting the correct light wavelengths for plant growth is different from the lighting just used for humans to see by--a combination of lamps is required for flowering and fruiting plants. The temperature range of 68-72 deg. F is suitable for tomatoes. However, it's a little on the low side. The optimm temperature is around 70-75 deg. F for maximum growth rates in hydroponics (provided you can provide good light levels). With the ventilation, you need at least one complete air change in the growing area per hour. This is not just to control and reduce humidity but also to bring in fresh carbon dioxide (CO2) from outside as the plants deplete CO2 rapidly as they photosynthesize when the lights are on. Some fans providing gentle air movement across the plants are also required in the growing area. This criterion can be met by installing extractor fans to take the stale air out of the growing area. Good luck with your hydroponic tomato experiment!
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