Avocado tree growing unhealthy - Brown spots, frizzle leaves

Question

The issue: I have a 1.5 year old Avocado tree which has been growing for the worse since the summer.

The medical history: (Lots of pictures ahead)

It started before I did its first pot change. It started looking dried and gloomy (after a quite vigorous and healthy growth from a seed) and since it was getting quite tall, and watering wasn't helping the dryness, I decided to move it to a larger pot, because I assumed it was getting tight in there.

I...wasn't gentle (even though I read that the roots are fragile) - I had suspicion that the previous soil had been infected (small white looking gnats shooting around the dirt when I was watering it) so I did my best to wrangle as much of the soil out of the root ball, as possible, which caused damage.
This is how it looked like right after the transplant (already looking a bit sick):

The follow-up result had did not really improve things and the dried looking leaves continued to darken and dry, so I took them off, in hopes to spare the tree from feeding them resources when they were goners:

Still, it did not improve by any noticeable amount, but I could see it was alive so I took it outside, to see if the extra sun and additional water(rain) would help it.

In an unprecedented hot summer however (northern Europe), the leaves got a sunburn, but it still looked more invigorated then when staying inside, so I kept it in the garden for the summer/autumn, and new leaves started to sprout. Currently, with the cold rapidly stepping in, I brought it back in, but first I changed the soil again as the previous one was meant for citrus trees, and I suspected the pH factor was a bit too high for an Avocado (I did my best to be gentle to the roots this time). Worryingly enough, the root ball did not seem as strong and blooming as it was the first time, even though the plant was smaller (which furthers my belief that I may have damaged the root system badly)

Here's how it currently looks. The leaves keep on browning a while after they grow, but they are still sprouting, at least! However, some of them are looking a bit stunted in their growth. The current soil is with pH 6 factor, and is bought off the shelf. The only "garden centre" available near me is Bauhaus and nobody in it knows or can recommend me a soil for an Avocado tree (asked different people, multiple times), so I'm trying to make do with what I find there paired with what I read online.

Full-size view. You can see the newcomers on the stem and some of the older leaves on the crown browning:

Already stunted new-ones at the top:

Sick old leaves:

The pot - H:35cm. W:40cm.:

The current soil:

The soil was supposed to be one that drains quickly, although I'm not too confident that this is actually the case (how can I check the lower layers?).

Any recommendations to help me save this fella?

• What am I doing wrong?
• What should I change?
• What can I do better?
• What should I avoid in the future?

It's the first time I grow anything from a seed and as you can see, the tree had to go through a lot because of my inexperience in gardening. Nevertheless it's precious to me and I really want it to become strong and healthy - I'm not really after the fruit itself.

Answer 1

I think the way to look at avocado pear is as a soft tropical small tree. It comes from Southern Mexico where it enjoys warm temperatures and moderate to high humidity. Consequently it produces well in other locations such as Florida or the Bahamas and can even be cultivated in drier landscapes such as California but the water management is critical in these desert places. Unlike a pothos or a Ficus the leaves are not thick water storage organs - when the top starts to dry out the roots must deliver or there is trouble.

Where humidity varies a lot it is hard for Persea to keep the foliage and root in balance. One such situation is movement between indoor heated dry air and outside warm and more humid conditions. In front of that radiator in your picture it would get all the heat it needs and more but would probably be very dry unless you have special humidification arrangements.

The potting soil can probably contain enough moisture for the plant, particularly considering the size of the pot. However if the plant continues to favour the basal growth over the top canopy then might be well to cut off the top just above a healthy green side shoot and use the top leaves for avocado tea. Then let it readjust to the new drier air, keeping the root ball moist but not wet.