Feed it with a high Nitrogen content fertilizer, which promotes growth of new leaves. The down side of that is that high Nitrogen discourages fruit production.

I am no expert, but the soil looks heavy to me. Does it drain well? How often do you water the tree?

I hope it recovers.

I usually agree with John since he is one my favorite experts here, but I have to digress this time..

If your tree is suffocating in that mix which is most likely, or if the roots are being killed off by fertilizer and tap water salt deposits, then you will only kill your tree even further with fertilizer..

It looks like you have dead branches appearing which is a sign of very poor root function due to a lack of oxygen within the root zone..

If it were my tree, I would find a better way to allow you to water more frequently without fear of root rot until you re pot in summer into a much better porous mix, stick that tree in as much light as possible since it is inside and provide a good warm room to encourage growth at this point..Watch for mites for sure since you will probably get new soft growth if you do what your tree will respond to. I would not feed until I saw new bud or growth coming on..

To do this, I would or could use the pot in pot method to draw the water out from the bottom of that pot, or you could transplant, not bare root, meaning I would shake off as much of that crappy mix as possible without disturbing the roots into a fresher mix that matches that one into a smaller pot if the root ball is smaller than that pot...

Depending on the light set up and the environment, one that is conducive to extremely good growth, you could bare root into a 5.1.1 mix, but since I am not sure of your conditions, I won't encourage that at this time of the year with your plant in that condition.,.

I would repot. That mix looks very dense. Once it's happier, then give it fertilizer like John said. Listen to these guys...they know their stuff!

Yes as Laura says. a repot is in order at this time too..I call it a 911 ..If you can do without manipulating or moving the roots, then all the better!

Then again, if you have great light and great conditions, I would use the 5.1.1 mix which requires cleaning all the soil from the roots..

Thank you all so much!! I'll try my best with trying not to disturb the roots and new lighter soil!

I think the problem may be it's too cold. But turning on the heater isn't a good idea either since that can cause the tree to dry out. (Cold air that has been warmed through a heater can be desiccating because warm air can hold more water vapor)

And no, just putting more water in the soil cannot compensate for desiccating air, not when a tree is that young.

Is it possible to maybe put that plant in a bathroom?

I'm with Mike/Laura here, the soil doesn't afford good root drainage. Heavier soils like that aren't bad in the middle of summer when the evapotranspiration of the tree is high and the roots are warm, but in the winter, they are nearly a death sentence because of low evaporation, cool temperatures, and low light. As a stopgap measure, you could add a bunch of perlite to the mixture to try to aerate it. Better yet, replace the mixture with "5-1-1 mix", which you'll easily find with a little searching.

-Tom

Katie, don't let the ingredients of the 5-1-1 intimidate you. I buy reptibark off Amazon and the other ingredients are easy to find at any Home Depot or garden center. I just use a plastic pot to roughly measure.