Citrus (as with other plants) have very tiny feeder roots. If these feeder roots are dead (or not constantly regenerating) then the plant cannot take up the necessary nutrients. (Such as iron) If a plant is waterlogged (not your plant) or dries out too much, you loose those feeder roots. You probably need to water it daily while it is actively growing in your small pot. When you water you flush out all nutrients. A citrus uses the general macro-nutrients that all plants need, but it also needs micro-nutrients. If you pot-it-up, only go a couple to three inches bigger. You will need to be rigorous with your watering/fertilizing either way. I would fertilize with every watering. Those nutrients will leach readily. I would use 2 TBL Fish Emulsion and 2 TBL of Seaweed per gallon of water. Use this every time you water. You could use other fertilizer options. (Chemical/salt based Citrus & Avocado liquid fertilizer or a granular Citrus & Avo type fertilizer, both have drawbacks) Liquid salt-based fertilizers have .... well....salt. Not a big problem when you disperse said salt into a large soil area, not so good with a small potted citrus. (Can damage feeder roots) Granular feed, if organic should be scratched in or mulched on top. Again not practical with your window sill baby. If you pot-it-up be forewarned that citrus roots are sensitive and if you break up those pot bound roots, it Will fuss. (You must break up spiraling roots)You'll have to be patient during it's rebound. I would only do that when you are going into the season that it would be getting the most light. (growing the fastest) Good luck to you
