I am in zone 6a. I have a Poncirus trifoilata that I planted in ground this spring. It is about 3 feet tall. Do I need to provide winter protection? If so, would I need to include a heater?

I am looking to purchase Italian lemon cuttings for me to graft. I am in Massachusetts and there is no quarantine for citrus.

https://www.washingtonpost.com/science/2020/01/14/dogs-are-helping-save-floridas-citrus-groves-devastating-disease/

I have read on many websites that the best fertilizer to use for citrus is one with a ratio as close as possible to 5:1:3.
Where did this idea come from?
Are there any peer reviewed scientific papers to back this up?
Is it based on the NPK ratios in citrus tissues?
Has anyone actually shown that citrus take up these nutrients in a ratio of 5:1:3?

I am about to move to a new home and cannot take my greenhouse with me. I will have only a few areas with plenty of sun and so am considering overwintering them in the basement. However, the basement has few windows and so is rather dark.
Will I be able to successfully over winter them in semi-dark?
At what temperature?

Just saw this video on rootstocks: https://www.youtube.com/watch?v=unTjLRQyY5Y

I read this thread on a fig forum. https://www.ourfigs.com/forum/figs-home/291461-rooting-cuttings-can-we-reduce-the-process-to-its-essence
It caused me to ask the same question about rooting citrus cuttings. From this posting: Question: What are the essential components of an environment that will cause wood to sprout? Under proper conditions, we should need no medium at all, other than air -- a stick of fig wood suspended magically in the air could sprout roots, if other conditions were suitable. If so, then everything non-essential might simply complicate the process and reduce the probability of success. The non-essentials could be eliminated, boosting the probability of success.
The poster in this thread found that fig cuttings will root without being stuck into a growth/rooting medium. Can this be true of citrus? In other words, what are the essential conditions required for rooting citrus cuttings?

Does it make a difference if you use a wide, squat pot or a deep pot of the same diameter?

Anybody know of a good online vendor of jaboticaba plants?

Can someone recommend a soil mix recipe for containers of 15 gal and up for growing citrus? I have tried commercial cactus potting soil but lost a nice calamondin as a result (root rot). I also tried Talpa's gritty mix on a few small trees but I need to re-pot 5 large trees and the gritty mix takes way too much work to prepare. Any suggestions?

I have a 40 year calamondin that has been in the same pot for about 5 years. Leaves look good but it has not put out any new growth in a while. But the root ball is pretty dense. Should I leave it in the same pot and prune the roots? Should I also prune the branches? Or should I just put it into a larger container without pruning it?