Lettuce Leaf Curl

In one tray of transplanted seedlings all of the lettuce (three varieties) have begun to curl their leaves downward. How can I tell if this is from a nutrient deficiency or from over watering? The radishes and kale are not doing this and look fine. The only fertilizing I've done is a little bit of superthrive in the water they were planted with and with the water from the day I transplanted them.