First of all, you should rule out the possibility of pests and physical diseases. In addition, what hydroponics method do you use, deep liquid flow or nutrient solution membrane or static? What nutrient solution formula is used? The most common problem in hydroponics is root hypoxia! Observe the root system of lettuce. It should be very white and thick under normal circumstances. If the root tip is yellow or black, there is little root hair on the main root, which is generally hypoxia. After 20 days of planting, the roots should grow very luxuriantly. At this time, the liquid level should drop below 5 cm (deep liquid flow), so that the upper part of the root can be exposed to the air to absorb oxygen. Be sure to lower the liquid level at any time with the growth of roots. In addition, lettuce is an iron-deficient crop. Once the ph value is greater than 7, the iron in the nutrient solution will precipitate out and the new leaves will turn yellow.

You have provided so little information that you can only give a general idea. I am also engaged in hydroponics, so let's talk more when you have time! Qq3035405 15 Soilless Culture Website of South China Agricultural University:/Soilless/PPT Flash/main.asp.