Hello :)

A year ago, I bought Lemon Lisbon tree (it is in a pot 30x30 cm, now its 4 years old) which was thriving in summer, had two big branches and a lot of green leaves. It was fertilized once a week with liquid fertilizer for citruses. After that, it had a very rough winter because it had light, but it was in temperatures 1-5 Celsius for 2 months (it was covered). Almost all leaves have dropped during the winter, and I did not water and fertilize as much during the winter (maybe 1-2x per month). In February this year, I have re-pot it (same pot, but different soil), sprinkled the branches with seaweed fertilizer and watered with liquid citrus fertilizer (6-8-6). The temperatures now are mostly 15-20 Celsius in the afternoon (but it goes in between 1 and 7 Celsius in the night/morning), and I keep watering it and fertilizing it. Truth to be told, maybe the tree itself is overwatered/fertilized because I dont let the tree dry in between, because the liquid fertilizer needs to be put with water in the pot, and the soil does not dry in one week. I was thinking about watering with the pot saucer (to put the pot on the saucer which is filled with water (with fertilizer) to water it down-top, not top-down. When I re-potted it, I used a good soil, the water goes right into the pot and the tree/root is not wilting, the soil itself is not soggy, but wet. It is white, not brown, it is growing (the roots). But the tree (leaves) itself is not growing, I have a lot of new leaves emerging, but they are turning black like on the photos.

This tree should blossom in May if I could get the leaves grow by then, but I dont know what I am doing wrong. It is the overfertilizing/watering or the low temperatures? I have a mandarine tree which is also having a lot of its old leaves from last year, a lot of new leaves grow + 3 emerging flowers (and I am watering/fertilizing it the same like the lemon, just the new emerging leaves are growing very slowly, almost nothing, its standstill). I was thinking about buying fertilizer for citruses which I can add to soil 3x a year instead of the liquid fertilizer to lower the times I need to water the tree. I also have a fertilizer made by californian worms (you know, those red californian worms), so maybe I can put that instead of the fertilizer? Also, the trees have no mulch, and I have sprinkled the leaves/branches with iron fertilizer and epsom salt (on the second picture you can see that the piece of the leaf attached to the branch is yellowish, because my citrus fertilizer does not have magnesium in it.

Please help me :D