My lemon seedling is starting to turn yellow, the plant was once a rich green but it has started to fade now, the 2 top leaves are the most effected as can be seen in the picture. It has also stopped developing and has been at the same size for like 2 weeks.
From some research I learned that this could be iron deficiency but also other things like overwatering and such. How can I be certain? As adding additional fertilizer without being sure that it's the cause might harm it even more.
Yes, the phenomenon is called chlorosis, and is very common for Citrus species when you use calcareous soil or soil with a high pH or you are watering using calcareous tap water. Such conditions leads quickly to iron deficiency. Investigate this hypotheses first. It is far the most likely one.
Some Citrus species like micrantha and aurantium tolerates such conditions. If you just want a citrus tree, then this option is most optimal.
Assuming you are in an area with calcareous tap water and want a lemon tree, then I suggest:
