Lettuce is one of the easiest and most common hydroponic crops, but there is a drawback to growing lettuce hydroponically. Nitrates can build up in the leaves, and if not carefully managed can reach levels that are harmful to humans. In a traditional outdoor soil garden, nitrates are converted to harmless nitrogen via photosynthesis. In an indoor hydroponic garden, photosynthesis happens at a much lower rate, leading to high nitrate concentrations. This can be managed by altering the hydroponic environment.
Adjust Nitrate Levels in Hydroponics
Use a home nitrate test kit to determine the level of nitrates present in your hydroponic nutrient solution. Lettuce ready for consumption should have nitrates less than 4,000 parts per million.
If the nitrate level is too high, completely flush the nutrient solution and replace it with fresh, de-chlorinated water. Mix in a low-nitrate concentrated nutrient solution.
Add enclosed UV grow lights to your hydroponic garden. These will stimulate photosynthesis, which will naturally remove nitrates from the lettuce.
Re-test the nutrient solution for nitrate levels once per week. Continue to adjust nutrient solution and UV lighting until you maintain ideal levels.