You can try iron chelate. I think lemon trees are more prone to winter chlorosis than kumquats, but time may tell. I'm in zone 9 and my kumquat has mostly dark green leaves. Your question reminded me to fertilize. Is your tree getting enough sun? This photo was taken January 8th (today). Here is a link that might be useful: Kumquats

Thanks for your reply. I'll take a look at the fertilizer I'm using and see what it lists. I've just been using a 20-20-20 general plant fertilizer from Miraclegrow (I called 6 different nurseries in my area (Denver) and no one carries fertilizer specific to citrus). However, I've used it very sporadically. All the leaves of my Meyers lemon were turning yellow so the first thing I did was, since it's been in the same pot for over 6 years, was to root prune and add new soil. That was over two weeks ago and the tree actually looks a ton better. There's only a couple leaves with a little yellow left and the rest of the leaves are really greening up. It also got a good soaking as we had 15 inches of rain in about three days! It's chock full of small lemons and I want to be able to "reap the harvest" and have been doing some research on how to properly care for potted citrus and came across the notion that people periodically water their citrus with epsom salts. So I thought I'd give it a go. However, you bring up good points so I'll probably hold off on the epsom salts and try to be more diligent in regular fertilizing. I have no idea the calcium, magnesium, or alkalinity concentrations of my tap water as I've never tested for them but I do know the tds is generally low, usually in the 30-40 ppm range.

If it was still outside and you lived in a citrus climate I'd have maybe gone to the production rate, but since you're taking it inside, I'd just go with maintenence. But no, citrus don't really "need" a break, they're a tropical evergreen so they don't need to go dormant or even slow down in their preferred weather areas. They do sometimes slow down in winter outdoor in citrus growing areas that are less than tropical, like here in the SF bay area, Ca, but I would bet in South Florida they probably don't slow down much in winter at all. Here it's the cooler temps and less light that causes them to not grow as much in winter. Where you are it would be a lower light level inside, I would assume. So go ahead and fertilize in winter too. imo.

Oh, I see only now that a part of my previous comment is missing. Yes the advice sounded weird to me too, I got it from a worker at the plant store. The tree is still at its old place, at the terrace which provides most sun my apartment can muster, and is regularly exposed to fresh air. I do bring it in in the evening under a light, which is best I can do for now to increase the light exposure. I was also very happy to see that the roots are not damaged, there was nothing to be removed. That is why I found it so strange that more and more leaves are falling off and the tree is almost bare now. I bought a soil mix for Mediterranean plants, like olives, citruses, palms, oleanders (or at least that is what it says on the package). I did not keep the roots out of the ground for too long, but it did take some time to remove them unhurt from the previous one as the soil was really tightly squeezed around them. I am sad though to hear that the fertilizer did not work...what would be other suggestions? Thank you all for posting!