Certified GKH Gardening Expert
This can have a few different causes. When the soil is too dry, if it gets too cold in the area, or if it is suffering from a bacterial/fungal infection.
If the area receives enough water, then you can rule out this possibility. If there has been a recent cold snap then this is a very likely culprit. If it hasn't been chilly, then you can peg this as an infection of some sort. Iron sulfate will, both, help keep the pH low enough to its liking, and kill off any infection in the soil.
Here is an article with more information that will help: https://www.gardeningknowhow.com/edible/fruits/citrus/curling-citrus-leaves.htm