Growing lettuce hydroponically Lettuce can be grown using commercial hydroponic fertilizers. Three fertilizers are commonly used: Chem-Gro, magnesium sulfate, and calcium nitrate. When the trays we constructed were filled with tap water up to the bottom of the net pots, they held about 61 gallons of water. To do hydroponic cultivation, while filling each tray dissolve 180 g of Chem-Gro, 108 g of magnesium sulfate, and 180 g of calcium nitrate in the water. No additional water or nutrient additions are necessary for the full grow-out period. After each harvest, nutrient-depleted tray water may be used to water terrestial plants. The trays may then be refilled with water and fertilizer and new lettuce plants stocked. After three harvests, tray water should be replaced with fresh water. The cost of the fertilizers was less than 2 cents per head of lettuce produced.