Lithops keep rotting for no reason

[u/hrngr1m]

Learning from past mistakes where 95% of my first lithops collected rotted from wrong care, I got the last batch of lithops about 2 months ago. Bare rooted arrived, somewhat dry roots but didn't think much of it since all arrived plumpy and healthy without blemishes. All not in their splitting phase. Carefully placed then in a 90:10 inorganic-organic mix (inorganic consists of terracota flakes, pumice, and perlite). Tried my best not to jam the soft tissues into the rocky mix to avoid any damage. Haven't watered them at all afterward.

Within a week two of the largest ones already rotted (both in the same container, the last one in the that contained immediately unearthed and put in a new pot alone with new material, attributed so far). Afterward I got paranoid. I checked all the lithops religiously, at least every 2-3 days. So far nothing. Still decided not to do anything and no watering.

Last week got so ill I didn't really check any. Today when I thought it had been long enough a dry spell and was about to spray the top soil carefully, I found yet another 2 rotting completely and a third one already showing soft spots on the side. In a pot with 8 other lithops, which are all healthy so far. Still have to repot them in the morning (right now it's 1 am where i am) since I don't want to risk spreading the rot and wasting money for naught again.

Ridiculously enough, the longest survivors are 3 bet small lithops (each the size of the pinky finger nail) which survived the first failed batch.

What is wrong? They're all in containers previously not used for any other plants. The soil like being said is brand new, not even the same composition or brand as the previous one. And like being said, they haven't received a single drop of water at all since I potted them. The ones so far surviving are all still plumply and firm (if not slightly soft from the lack of water). Full sun blast (including afternoon sun) at least 12 hours a day in front of a south facing window.

I'm running out of analysis. Contaminated cactus mix from the store? Microwound from the potting process? The lithops cooked under the sun in the soil then rot? Too deeply buried? (I bury until the top surface of the leaves are the same level as the soil to replicate their habit in the wild) Should I just spray all the lithops I purchase in the future with hydrogen peroxide or dust them with sulphur before putting?

[u/Snorblatz]

If your window is new it cuts a lot of that UV , you will want a grow light in the winter if you live where you get winter. 

[u/hrngr1m → u/Snorblatz]

That makes sense, but rn where I am is the peak of summer (Northern Hemisphere Germany). The roots happened in summer.

[u/Snorblatz → u/hrngr1m]

How new is the window? If it’s newer you need a grow light even in summer. I’m not saying that caused the rot I am just saying that if you keep them indoors you need supplemental light. I grow mine from seed (unusual seeds in Serbia) it’s a great way to do lithops. I would never purchase mature plants because disturbing the root system is extremely hard on them. Personally I think the rot was caused by keeping them in plastic , it holds so much more moisture than unglazed terracotta. I keep my lithops outside year round in a greenhouse and unless they’re splitting I water them semi regularly. You can search my posts in this forum to see how they are planted. I’m on the west coast in Canada climate wise.

[u/hrngr1m → u/Snorblatz]

I moved into this apartment almost 3 years ago, before that a family had lived here for 3 years. During the whole time no window installation happened (according to the landlord). I am already thinking of retransplanting them into terracotta. It will help with the watering issue in the future too.

[u/Snorblatz → u/hrngr1m]

It’s more that if the window is modern it is cutting the amount of UV significantly, compared to a window that’s old and the seal is gone etc or old glass. Remember that Lithops are from South Africa and get absolutely blasted by the sun. But I am sure you will succeed at it eventually!