Only the lower leaves are getting yellow....it could be a normal process, the bark doesn't have leaves :) !! If also the upper leaves are becoming yellow (I can't see from this photo) you should add microelements and nutrients :)

I have fertilized. is there something else I should use since it's in a gritty mix? Thanks

Usually when the bottom leaves are turning yellow first it indicates that the plant is undergoing a hormonal change and is deciding to drop all its leaves.

Are you overwatering? Or maybe the outdoor temperature is getting too cold?

Lower leaves yellowing and falling off is a classic sign of too much fertilizer. Mango trees do not like too much fertilizer.

A lot of people kill their tropical plants with too much care. Just make sure it gets consistent watering in the hot summer.

I'm not overwatering and we have had our share of rain. It's in a gritty mix that is draining well. I will keep a close eye on it.

Its not too much fertilizer- Mango leaves are fast to show burn. Black spotting and a dull dark finish.

I think it needs more fertilizer. Try one that has iron in it. Organic fertilizers are very good too. Guano,Seaweeds,etc.

Maybe it could use a slightly bigger pot (but I would wait on that until February or March).

Fertile soil with some organic compost is a much better choice than fertilizer here.

I live in South Florida. Did you suggest Feb becuase the temperature is colder? I have another potted Mango tree right next to this one, but is in a soil,perlite,moss mixture and looks healthier.

If you're in South Florida you should still have a bit of growing season left; and yes the suggestion would be because usually for most growers it's getting cold around now. Perhaps take it out of its pot and look at the rootball? See if the gritty mix is staying too dry or wet, how the roots look. How often are you watering it? It does look like it needs some nitrogen/iron since the leaves are so pale and small.

As a side note I'm doing something similar as you; growing one that my mother sprouted from a seed. She just planted it and wasn't taking care of it so I'm rehabilitating it. First picture is early this year and second is a couple weeks ago after some fertilizer, mulch and lots and lots of rain :)

I check the gritty mix to see if it is dry before watering. it has been raining every other day so I haven't watered it. I'll look into the iron/nitrogen nutrition. I figured I needed to add something additional because of the gritty mix. Your tree looks good!!!