You have just joined the exclusive club of growing container citrus trees inside during winter. This exclusive club has a membership of only 99.9% of the people like me who bring potted citrus trees inside for the winter. If your pot is on a cold window sill you might want to put a Styrofoam block under the tree. If your widow is double glass low-E window you will not get any solar heat to the pot and a block of insulation might help between the pot and the glass. Add a tablespoon of vinegar to your feed water when watering. If you can find something like a feed with a 1-1-3 ratio with micros you will be able to feed your tree without over feeding it with nitrogen. This ratio is for winter feeding only. Otherwise I would say your tree look nice.

6b Steve

Thanks a lot! I've got a 7+4+5 fertilizer from when I was having a Calamondin(if everyone wonders: I've killed it two years ago by over watering it), I'm gonna search for a winter fertilizer. The decorative ceramic pot isn't cold, so I don't think that I need to isolate it(I have my Phalaenopsis Orchids in the same window and they don't mind). Just out of curiosity: What does the vinegar do? It makes the soil more acidic? So the problem in an insufficiency? Or do I have to wait for the ''Citrus Master Gardners'' answer?=)

Vinegar make the water more acid and thus helps with the up take of Iron and Zinc. The 7 -5 -4 will do fine this winter.

6b Steve