How to mix the perfect soil

[iann]

Probably one of the biggest differences is between a porous pot and a non-porous pot, especially for small pots. Small porous pots dry out incredibly quickly by evaporation in summer.

[Edwindwianto]

Wow...this is a nice experiment
But i'am jot sure that your initial "numbers" from this experiment will serve you well in the long run...

Let's say, you have numbers like so...
Water storage = A%
Air storage = B%

But in other hand...your medium is veried in nature
Let's see peat...
1) if you water it a little, it absorbs little....you let it sit in water, it absorbs a lot
>>> can you guarantee that everytime you water it, you water it with the same amount of water?

2) peat do degrades, the more it degrades, the more water it retains until it turns in to a mushy sludge (from my experience from growing Nepenthes in 2012)
>>> so, the same amount of water you pour this month, may not be retained the same way next month
Ex: you pour 250cc this month and is retained x%
Next month as your peat degrades more, it may not retaines x% anymore...but tends to retaine more, (x+y)%

So...this kind of experiment only works with relatively inert substrate, like pure mineral substrate
Although mineral do degrades too, but in a very slower manner and not in the same way as the organic substrate
When mineral degrades (ex: limestone is degraded by acid rain), it releases ions and then salts which could be washed up by water...
What is left, is relatively the same medium as the one you had prepared 1 year ago...

I see that this kind of experiment are often misleading and give you a false convidence with your own mix (which contain organic mix)
You tend to overlook the varied property of the organic matter that i mentioned above
You tend to water your cacti in the constant manner although your substrate acts inconsistantly
I suspect, this is what kills many people's cacti...they thought, ah there are only X amount of water left in my substrate (i did my experiment, right?), this should be OK for winter....in fact, there are (X+Y) amount of water left, since the peat degrades and absorbs more water overtime...
Shooot...my cacti died during winter...what did i do wrong? O_O

EDWIN

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Hi Keith

It is interesting to see your sand...because i always want to try it, but i'am affraid

BTW, i use this kind of mix for my mexican desert cacti

EDWIN

[bartab]

I was told this hobby did not require math.

[7george]

You tend to water your cacti in the constant manner although your substrate acts inconsistantly

it is not fully true: You cactus grows, absorbs more water because of more roots and bigger body. Also peat will not degrade in 3 or 5 years but the whole mix will be for change because of salts in it. So the fine peat is almost useless if it is not acidic bringing pH down.

[ImNotTelling]

Dont know about perfect. I was terrible at making soil
My secret ingredient? Gravel. At least 40% of the soil per volume has to be gravel.

I also have used perlite, baked clay, , lake gravel as supplementary components and crushed brick, leca and orchid bark as additions. Only peat as organic component, or re-used standard cactus soil.
Have even grown plants in pure gravel (almost, at least, still had some soil on the roots) like rebutia steinmanii ,armatocereus etc.

[keith]

"I was told this hobby did not require math.'

[Newton]

I was told this hobby did not require math.

The hobby does not need maths but the forum certainly does need some mad scientists.....

Some great observational inputs on this thread...thanks

[keith]

I made a new mix with way more pumice to sand and it holds too much water. I have to add some course sand / small gravel to the mix.

Too much water for Mexican living rock type cactus and Lithops.

The reason I know it holds too much water is the cactus and Lithops are rotting .

[bartab]

Mind if I ask what percentage of Pumice to grit were you using before you added more grit? I'm using approximately 60/40 for the desert cacti. I add a little soil mix for other cacti, like my rebutia and the like. Not much though. I tend to make a hybrid of Steve's mix and CandD Plants mix. I like the varying size of Pumice which CandD talk about, 1/16 up to 3/8. I do wash away the dust though

[keith]

Hi Bartab, I guess like 80 20 ? Its Small pumice called dry stall and it seems to hold water for days then go bone dry by the time I find the rotted cactus and upend the pot. Some cactus are doing much better so its not all bad. its a mystery but I guess to wet for too long for some cactus.

" I add a little soil mix for other cacti, like my rebutia and the like." I do that also for Sonoran desert cactus and south american types like rebutia.

I do wash away the dust though" I wash away the dust from the gravel but add it back as diatomaceous earth powder. So my mix is dusty needs to be slightly wet or wear a mask when using it. Gives it a gray white color. Insect control the little gnats don't like it.

60 -40 I think That's better I'm going to add more gravel see how it goes.

[Steve Johnson]

I made a new mix with way more pumice to sand and it holds too much water. I have to add some course sand / small gravel to the mix.

If you're talking about fine sand like beach or playground sand, that's no bueno for cactus and succulent mixes. Don't know if you've already seen it, but just in case -- check out part 1 of my 2017 review detailing the "how" and "why" of the mineral mix I use:

http://www.cactiguide.com/forum/viewtop ... 92#p339492

The reason I mention this is that "play sand" generated by decomposed granite is the type of sand we need to avoid. Keith, I know that Thousand Oaks is hotter and drier than where I live, so a straight pumice and granite gravel mix may dry out too quickly for you. If such is the case, add a little soil -- not sand -- to the mix for better moisture retention as the roots take up water. By the way, I have zero experience with succulents, so what I'm recommending applies only to desert cacti.

[bartab]

Hmmm. I have some Sonoran cactus but I don't put any soil in the mix at all. Should I? I have several Echinomastus, E. vivipara, coryphantha, and a couple of M. grahamii. I've been wanting to have the Echinomastus and others dry out as soon as possible and I have them in 60/40 grit and pumice. I think the Echinomastus are in 50/50.

[keith]

I like adding sand for South African Mesembs like Lithops.

[The_Nikon_Guy]

I made a new mix with way more pumice to sand and it holds too much water. I have to add some course sand / small gravel to the mix.

If you're talking about fine sand like beach or playground sand, that's no bueno for cactus and succulent mixes. Don't know if you've already seen it, but just in case -- check out part 1 of my 2017 review detailing the "how" and "why" of the mineral mix I use:

http://www.cactiguide.com/forum/viewtop ... 92#p339492

The reason I mention this is that "play sand" generated by decomposed granite is the type of sand we need to avoid. Keith, I know that Thousand Oaks is hotter and drier than where I live, so a straight pumice and granite gravel mix may dry out too quickly for you. If such is the case, add a little soil -- not sand -- to the mix for better moisture retention as the roots take up water. By the way, I have zero experience with succulents, so what I'm recommending applies only to desert cacti.

Hi Steve,
I have extensive experience growing cacti of different varities, in pure sand. You read it right - pure beach , builders or playground sand.
I have grown the following cacti in sand:
(1) Pure builders sand (this sand is fine and has a small amount of clay mixed in): various Mammilaria spp., Astrophytum myriostigma var. nudum, and Gymnocalycium mihanovicii
(2) Pure washed find river sand: Discocactus horstii, Melocactus matanzanus, Fenestraria aurantica (babies toes) and Frailea castanea

I can categorically state that the cacti have grown WAY better in pure sand than in any other medium. I am absolutely convinced beyond any doubt that pure sand is the way to grow cacti.

Having said that - I thing I know why a lot of people are petrified of pure sand - they grow their cacti in plastic pots - yes, its the nemesis of all cacti.
Plastic pots are one of the worst ideas when it comes to growing cacti - I grow all mine in terrracota and its porosity helps a lot in wicking any excess moisture.
To prove my statement above, I have posted a few photos of cacti growing in absolutely pure sand - enjoy
Note, for the Astros, I did not clear away the top dressing to prove the pure sand substrate, but believe me, it is pure sand.

Astros.jpg (153.86 KiB) Viewed 14343 times

Babies toes.jpg (125.19 KiB) Viewed 14343 times

discocactus.jpg (123.46 KiB) Viewed 14343 times

Fraelia castanea.jpg (126.29 KiB) Viewed 14343 times

Mammilaria.jpg (130.97 KiB) Viewed 14343 times

Mysriostigma.jpg (85.3 KiB) Viewed 14343 times

Melocactus.jpg (117.9 KiB) Viewed 14343 times

[The_Nikon_Guy]

and another photo of 3 baby Gymnos I planted in fine washed builders sand...

IMG_20200516_125100.jpg (135.88 KiB) Viewed 14337 times