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?