Hello, dahlialady! Welcome to the forum!

Soil questions are heavy around here, for many reasons. Our differing climates, range of available materials and varying results over time make it almost impossible to say, "This is the mix that will work for you." I can just FEEL my friends hovering over this one like a day-old donut... I'll bite, since your bait is so delicious to me.

Any succulent mix needs to be well draining. Every "Cactus Mix" I've tried has been too rich, not enough drainage, too much bark. Here's something it took me too long to figure out... bark is bad! It's still decomposing, too hot, roots don't like it.

I don't even bother with cactus mix anymore. I'm going to give you my formula for mesemb mix which is a greenhouse mix intended for controlled use of water. I'll then explain the difference between that and my outdoor mix. You must keep in mind that I live in a very moderate part of southern CA. Nothing freezes here.

I start with Miracle Grow moisture control mix... What???... I know! It blows my mind too! It turns out it just has a nice mixture of peat and forest products, light fert and other components too tricky to find and measure yourself. Plus, it's very nicely mixed/fluffed up and it has very little bark. I sift it at 1/4" to get the bigger chunks of bark out. This also makes sure all the peat is broken up.

To this, I add:
washed sand - about 1/3 the amount of potting soil
vermiculite - about 1/4 the amount of sand
The sand and vermiculite have their own qualities but what they do in a simple sense is cut down the ratio of organics/inorganics in your fines.

All my fines are together now. I like to mix them very well at this stage, thoroughly amalgamated.

I then add Perlite and Volcanic Pumice to equal about 1/2 of the entire mix. I use more Perlite than Pumice - 3 parts Perlite to 1 part Pumice or 75/25, maybe 70/30.

Mix all that up and you have it! Plants that are grown in greenhouse conditions where they get plenty of light to transpire their water and watering is controlled, do very well with this mix. Outdoor plants, Indoor plants, or plants that have been accidentally overwatered do not
do so well with this mix. It's still well draining but stays wet too long for shady or rainy conditions.

Don't worry! I didn't make you read that for nothing! All it needs to become outdoor mix is more drainage which just means a bit more Perlite and Pumice. I would call it about 65% grit, 35% fines.

There are other things people use for grit like turface "chicken grit", coarse vermiculite, scoria... you use what you can find. If anybody has any other thoughts on soil, feel free to jump in... I'm about done. I'm not going to jump on the defense if anyone disagrees so have at it :)

I'm glad you found a Pleios 'Royal Flush', beautiful plant. If you haven't already, remove all of the old soil with the hose. Remove the fine hairlike roots. Don't be afraid, just pull them off with your fingernails, they will be damaged anyway. Let it dry out for a day and then pot it in the mesemb mix or something like it. Mesembs generally do not grow well in too gritty of a mix and they love sand. Put it outside but in the shade, maybe under a table, for about a week before you increase light. I usually use a couple stages of increased light over the next week until the plant is in it's final position. Mist on the 2nd or 3rd day after transplanting, give it a good watering on a hot day once you get it in the sun.

Pleiospilos can take lots of light, growing nice and compact with good coloring. Not full sun but maybe 75%? Water when outer leaves are absorbed. Water when the inner leaves feel soft. Decrease watering when plant is splitting as it is consuming the outer leaves. They still need mist and a periodic drink, though... don't want to kill the roots. When the old leaves are obviously on their way out (you can tell they would not plump up again if you were to water) then you can give the roots a good drink, you will be itching to by then.

Oh, man... I have stuff to do. Good luck!

Some nice advice given already. Mesembs have very fine roots and like already mentioned, a gritty mix that would suit many other plants will weaken mesembs. A dense mix works best. Even though mesembs in general come from some harsh locales, they will only survive where they can get some sustenance - cracks in between rocks are cool and can funnel dew and heavy fogs, for example. So even though it may rarely officially rain, they are still getting water consistently for part of the year in habitat. The fine root systems survive better and withstand drought better in dense soil.

I first started growing Lithops and other mesembs before the internet became what it is today. Other than a few useless books it was hard to find good guidance. I went through various combinations of ingredients until I found one that worked for me. I mixed about equal parts clayey garden soil with perlite. This satisfied the density and aeration requirements, but still the pots need to be in a controlled environment because a few days of rain would be bad news. You can always add more perlite to reduce water retention.

As you gain experience and want to try new things, you can seek out other substrates like pumice, turface, etc. Vermiculite is usually not a good choice for long term because it breaks down into a clay aggregate, but for mesembs clay can be a good thing in the right amounts.

Pleiospilos have deeper roots than most other mesembs, so select a pot accordingly.

Hello and thank you very much for your detailed responses. I have read some of your posts and I salute the two of you for all your wonderful work. BRILLIANT! Your information is well appreciated, highly informative, and will be put to use well. I'm afraid that when I delve into things, I delve into them head first. I placed my first order of lithops with mesa garden. The list is as such, if you are curious. All lithops species except for a few cacti.

1596.9-hallii Klippunt, Upington, wonderful bluish reticulate form $1.10

1625.6-karasmontana ssp bella C143A $1.10

1571-dorotheae C124 /D/ bright red lines on pale top $1.40

1550.1-aucampiae `Kuruman' C12 $1.10

1549.3-aucampiae v koelemanii C16 red network, matte $1.10

1616.18-hookeri C142B dark brown, red jagged lines $1.10

1616.93-hookeri 'vermiculate' C336 furrowed channels $1.10

1628-karasmontana v lericheana C267 dark jagged pattern $1.10

1633.5-karasmontana SB2162 Signalberg Mt, milky to dark lines $1.10

1636.5-lesliei C28 brown background, jagged green veins $1.10

1671.9-olivacea v nebrownii Aggeneys, rose-brown form $1.10

1731.71-terricolor 'peersii' C131 blue-grey, spots $1.10

1720-schwantesii 'grey' C250 few red lines, dark grey body $1.10

1755.5-verruculosa C120 fine red pimples $ 1.10

1772.5-villetii ssp kennedyi C229A maroon top $1.40

1608.5-hallii v ochracea C303 grey orange, red lines $1.10k

Cacti

250.706-reichenbachii v perbellus Major Co, OK, black tipped spines $1.10

1196.84-kieslingii R694 Valle Grande, Salta, bright orange fl $1.00

Will be moving into a new house soon. I hope I have a huge southern facing window!
Here is a pic of my new baby waiting to be repotted.

Do not start the Lithops seed now, you will be disappointed. Wait until September when the nights start to cool down.