Kumquat, did you say the soil was too heavy? If so, how long were your citrus growing in this soil?
Have you checked leaves/stems/soil for insects? Are leaves/stems sticky? Do you see or feel any oddities? bumps, white spots, webbing?
Actually, we're not supposed to fertilize a sick or infested plant (of any type) Though many think by adding fertilizer it'll change the condition of a plant.
Sometimes after repotting, leaves drop..and of course, too much fertilizer will burn roots if a non-organic type is applied.
But you stated leaves were yellowing before fertilizing so most likly that isn't the problem.
The first thing I'd do is inspect plants thoroughly for insects..see if you come up w/anything..Toni

the kumquat was fine before i repotted it, so i'm thinking it must have been something i did (nothing happened to the mandarin tree though, and they were potted exactly the same way). i repotted it again yesterday, but left out the fertilizer. today i ended up cutting off the rest of the dead/dying twigs, and i'm left with 2 branches that haven't turned brown *knock on wood* though a couple of leaves have fallen off. i never noticed bugs or anything on it. will the twigs grow back eventually, or will i be left with practically a trunk?

i did find a few egg cases (i think) on the lemon tree and i cut off the twig (they were all on the same one). there was a little sticky substance near where the twig meets the trunk. there was one more on a leaf further up, so i just plucked it off. there are only a couple more yellowing leaves right now, on the inside. the leaves towards the ends of the branches are dark green.

I believe the problem has anything to do with fertilizer, nor do I suspect the problem has to do with an insect infestation. The problem sounds like the tree's root system was damaged during transplanting. When this type of damage occurs just after transplanting into a new container, the cause is almost always root damage. The fact that not only is the tree loosing leaves, buy that the branches are also dieing back points to root damage. Over fertilization damage shows as a scorching on the margins of the leaves, and not as a general broad yellowing. Because the leaves that are turning completely yellow, are only the older leaves at the bottom of the tree, this could be nothing more than the tree discarding old leaves that no longer are capable of producing more energy for the tree than they consume. When this happens a citrus tree will discard them.

is there anything i can do to salvage it? if the rest of the branches die it's just garbage isn't it?

Here is a link that might be useful: here are some pictures

Kumquat, what type of egg cases are we talking about? What did they look like?
Considering the brown, crispy leaves, Silica could be right, but when you mention stickiness and eggs, that's got me thinking insects.
Continue checking for stickiness/eggs.
The dead, brown leaves almost looks like insect damage.
In the meantime, I'd keep the two trees separated. Toni

i found out that what i saw on the lemon and mandarin trees were scales. i just scraped them off. the kumquat didn't have them, and that's the one that's dying. i haven't seen bugs on it at all, aside from the millipedes that were living in the soil. and the other trees had those as well. now even part of the trunk - well above the graft - looks brownish.

the leaves were brown, but still soft, not crispy. and i'm noticing that they're turning brown on the underside first.