I very strongly doubt that your tree has a phosphorous deficiency. I have NEVER heard of a phosphorous deficiency in container citrus, when it is regularly fertilized with a NPK fertilizer. Phosphorous does not leach from a container, as does nitrogen and potassium. Nitrogen and potassium are both VERY water soluble and readily leach with every watering, but not phosphorus. All of the leaves that are turning yellow, and dropping are the older leaves on the tree. Your tree is withdrawing the nitrogen from the leaves (thus they are yellowing) to reuse in other part of the tree before discarding them. When a leaf no longer is capable of supplying its expected quota of energy to the tree, it is quickly discarded. There is no welfare program for citrus leaves. However, your tree is also under fed. If the tree's problem was other than aged leaves, you would be seeing yellow dropped leaves from the new leaves also. Also it is not wise to add this and that chemical in hopes of coming across a cure. First ascertain what the problem actually is then correct it. I'm sure it was also not helpful to the tree to let it remain in the same container and "soil" for 5 years. Fertilize the tree using a high nitrogen fertilizer. You can add vinegar if you wish, but there are much better ways to acidify. Organic acids such as vinegar, have an extremely short life.

I may be wrong about this, but I was told the flowers have to be pollinated in order to set fruit. I had a Meyer indoors that kept dropping blooms, and when I moved it outdoors, it started setting fruit. And indoors it was in a bright room with a huge east-facing window, and it thrived other than no fruit. That was years ago- these days my Meyer (the same dwarf tree I got 8 or 9 years ago!) is in a very large pot (about 36" across and 30" tall) on a dolly in my yard. If it's going to freeze or frost I roll it into my garage until it warms back up. I got a sturdy furniture dolly from Tractor Supply for $15 and it is a lifesaver! This was in central Georgia; I recently moved to the Florida Panhandle and have no concrete on which to roll the dolly, so it's now living next to my well house, I drape a tarp over it and the well house when it's going to frost/freeze. So far the warmth from the well house, water tank, and heat lamp in the well house has kept it fine. I get literally 100+ lemons per crop off this tree now, and the tree itself is maybe 3 feet tall by around 4 feet across. It often has mature fruit (HUGE lemons often as big as a baseball!), green growing fruit, blooms, and buds at the same time. About every 3 years (it starts doing poorly when it needs it) I pull it out of its pot, shake all the soil out of its roots, repot in fresh citrus potting mix, and pray I didn't kill it! I do it when the tree has no fruit on it, because doing it while there are blooms or fruit causes it to drop the blooms and fruit. I also only prune as needed to control unwanted growth. I just nip a branch or bud occasionally, never a lot at once. I thin the lemons it sets. I wait till they are marble-sized, then remove any that are scarred or misshapen. Then I remove some if it has what I decide are too many in one spot, because leaving too many on the tree results in smaller fruit, and I like BIG lemons! I fertilize pretty haphazardly- usually I toss a handful of 10-10-10 on it when I'm fertilizing something else. I have used "citrus fertilizer" as well. Finally- I have accidentally left it out in the freezing cold, and the result was it dropped all the leaves the frost killed. It also dropped all the blooms it had and buds that were forming. However, I took better care of it the rest of the winter and in the spring everything grew back. Meyers can be tricky to get the hang of, but are totally worth it!! I have fresh lemons 9 months out of the year most of the time, thanks to its habit of setting successive crops of fruit throughout the year if it's happy.

You may want to just bookmark these guys, as they have most every popular variety between them as a graft or cutting: Florida Colors Brad's Buds and Blooms I also just use them as a quick reference for info on growth habits or fragrance. Here is a link that might be useful: TLD

Do you know which species of Garcinia it is? I gather from the name "Lemon drop" that it's perhaps one of the yellow ones. I have a yellow one, Garcinia xanthochymus, that's about 4 metres tall and fruiting right now. Never noticed it flowering, just saw the fruits developing. They're still quite green. It's under the canopy of very tall trees and gets filtered sun during the dry season. Wet season it gets direct sun through the middle of the day. I don't know how much taller they get but they're so slow growing it'll take virtually an eternity to get there. I have a smaller one that I put shade cloth over initially as they don't like too much direct sun when young. It's about 2 metres tall now and this past year I've removed the shade cloth completely (it's outgrown the poles the shade cloth was on). But it only gets midday/afternoon sun, all year long. It's not looking as good as the older one but it's in much poorer soil and gets less water. It hasn't flowered at all that I've noticed. There are a couple of other species of Garcinia that are yellow fruiters but I don't know whether they have different requirements or not.