I went to the garden center, found a ph-meter and a chelated iron additive for chlorosis. The man at the center suggested to try this, since the past year the DLs were fine, maybe having to use a lot of tap water with a high ph made things worst this year. He explained that clay someway traps iron and these kind of addictives break the "traps" and are availaible for the plants. So the ph of the soil looks around 7.5 - 8.

I will look for something to help with this soil, since the spaces I use for plants are not so big I can even think of replacing it entirely. In part it's something I already did with a little square near the entrance when I was planting previous potted DLs: I took out a lot of soil and replaced with common garden soil. But I guess the problem lies underneath the soil, there is limestone everywhere. I will check the DLs to see how they'll do.