I read in a few threads on this forum that when transplanting lemon trees it's essential to make sure to get rid of as much of existing soil as possible so that there aren't two different layers of soil which would restrict the root growing. Maybe they were only referring to repotting after a few years, not fresh out of the nursery? But also like Josh said, even before I was about to seriously meddle with the root, the soil kinda just fell off and a part of the root ball fell with it. After that I didn't dare to touch it much. I noticed that this plant doesn't really have a distinct trunk above the rootball. The tree starts branching off immediately on top of the rootball, giving me almost no space to pile soil on top of the rootball. I did my best to sprinkle soil on the root ball but they rolled off after a few days. As a result the top of the rootball is pretty much exposed. My soil is also mostly barks with perlite, but the rootball is too shallow to support the tree, especially when there's no soil on top of the rootball. I don't know if staking helps much because the tree is pretty straight after I put the brick on the rootball. So if I were to replace this tree, I should just repot (I need to repot it to a bigger pot or else it's really ugly) the new tree without losing any nursery soil into my new mix?

I was not recommending slow release fertilizers. My point was that the instructions on that fertilizer amounts to binge feeding. Feast or famine. Second, you can always bump up a slow release fertilizer if you want to feed more. Or you could apply say monthly depending on their growth cycle. So I do not buy that quote from the Citrus guy. May be the quote can make more sense in context. No matter which fertilizer you prefer, if you want to increase say nitrogen availability you just have to bump up the quantity. But you would also bump up everything else it contains by the same fraction. If there is a lot of say Phosphorus in the fertilizer then it will increase that too whether it can be utilized or not. The excess will either remain in soil (if it happens to be less mobile like P and to some extent K) or leach away (like N, Mg, etc). In a container situation, that is mostly devoid of any nutrition source, the ratio of the nutrients in fertilizer is the key. In soil one has to first know the nutrient profile first through a soil test and adjust the fertilizer accordingly. In either case one has to know the ideal ratio that must be provided. I am not going into the ideal ratio because it opens a can of worm. Nevertheless it is widely agreed that N > K > P. There are two types of slow release fertilizers: one which dissolve slowly (like Shake n Feed) and the other like Osmocote are temperature dependent. The release profiles of these two are very different. There are other problems with temperature dependent release types in container culture. Regarding CEC you seem to quote some text that basically corroborates what I said. That volume is the important factor and by volume it does not make any difference between 100% peat or 100% bark according to your table. Larger bark pieces will have smaller surface area so it effectively reduce the surface area (for cation exchange) available to the roots. But at the same time increases air porosity. CEC affects Ca, Mg, K, and Na uptake primarily and secondary it includes Iron and few other cations. High CEC soil can hold on to ammonium ion (only cation that has nitrogen) for a short time (less than 4 days) before it converts to nitrate which is an anion. So it has very insignificant impact on nitrogen holding capacity in the long term. So how does high CEC help for citrus that happens to love nitrogen the most?

That looks like a clay pot. I think a single drain hole is OK, if you do understand how water behaves in the particular mix you are using. Clay pots for overwintering indoors were your house temps are low risks root rot issues. I have grown citrus in clay pots, without issue. The full blazing sun sure could be an issue: keep an eye out for wilting, of course, plus sunburn on leaves and stems. I measured the temp of the surface of my wood deck one day at noon. It was 150F. Leaves touching the deck rail get burned, and reflected light is an issue, too. Last summer I did move some plants to a more shaded area under the eaves, and I think they benefited. Hopefully you will be able to tell by looking at yours, if it is stressed.

What you need to do is get the pot itself out of the sun. You could put somethng in front of it , around it, or, as you have done, bury it. I have all my plants in pots: trees, veggies, flowers, you name it. The one thing I've learned is that the pots can't sit in the sun all day and get roasting/streaming hot; the plants will die. Mostly my pots are bunched together so they provide shade for each other, as well as a cool-mass; others have afternoon shade... hottest part of the day. I find that terra cotta gets the hottest, but considered the best because it breathes. My trees are in huge pots; all my plants are in pots much larger than what would normally be considered acceptable. I suppose that helps too.