A gritty mix is generally considered to be one where all particles are at least 1/8" in diameter. Smaller ingredients like sand and potting soil (peat), will create a perched water table, where water is held in the lower portion of the soil like a sponge. If conditions do not allow this water to be quickly removed through transpiration or evaporation, the roots begin to die and rot. The solution is to switch to a true gritty mix with no fine particles.

The ingredients can vary, but the important thing is the particle size. As an example of particle size, here's what I use:

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Many people also add pine or fir bark as an organic ingredient. Other potential ingredients are Turface, Floor Dry, pumice, perlite, granite, lava rock, etc. Locating the ingredients can be a bit difficult for some, but the result is well worth it. There are many discussions here about gritty mixes. The standard gritty mix that many people use is one part pine/fir bark, one part Turface, and one part granite grit, but there are many variations that will work if you have different needs, or have trouble locating certain ingredients.

-Chris

So I shouldn't be using standard potting mix at all? Like I said, lots of conflicting opinions on internet. Mostly I've read 1 part potting mix, 1 part builder's sand and 1 part perlite (or a variation). I assume they don't live on the coast or keep cactus outside. Thanks so much for answering.

I guess I need to ask a couple more questions. If I use the mix with no potting mix, there would be no need for rocks on the top or bottom of the pot, right? And if I repot the cactus that seem to be rotting, will they be alright? Also, the most silly question, do I need to repot ALL my cactus and succulents? As I said, I have lots and most are fine, for now anyway.

I'm curious as to why do people put rocks on the top of the soil?

"So I shouldn't be using standard potting mix at all? Like I said, lots of conflicting opinions on internet. Mostly I've read 1 part potting mix, 1 part builder's sand and 1 part perlite (or a variation)."

Adding perlite to potting soil is often suggested because it's an easy solution, and it's better than using straight potting soil. But it's not a gritty mix, and it still suffers from a perched water table. You could try doing a 50/50 mix of perlite and potting soil, leaving out the sand, but if you're going to go to the effort of repotting your plants, you should at least consider using a more ideal mix. "If I use the mix with no potting mix, there would be no need for rocks on the top or bottom of the pot, right?"

There's never a need for rocks on the top or bottom of a pot. Rocks on the bottom are used by some people to improve drainage, but this is a myth. A rock layer has no affect on drainage. Rocks on top are often used as a top dressing for aesthetic appeal, but are not necessary. "And if I repot the cactus that seem to be rotting, will they be alright?"

That depends on how far gone they are. If a cactus begins to rot from the inside out, there's not much you can do. If it's just a bit of surface damage, it may pull through. But leaving it in a water logged soil is the worst thing you can do, so repotting is a good idea. "Also, the most silly question, do I need to repot ALL my cactus and succulents? As I said, I have lots and most are fine, for now anyway."

Of course it's up to you how to proceed. Repotting a large collection can be a lot of work. And you may find that your new mix needs a little tweaking if it dries out too quickly or too slowly. I'd start by repotting the plants that are looking bad. See how you like the mix, and then consider repotting the rest.

-Chris

Those are good answers by penfold and I could also add that builders sand is the worst sand of all because it clogs too easily. If you do use any sand make sure it's Sharp sand or Silver sand.

Sand of any kind is really not a good thing to use in a potting mix. All it does is fill pore spaces. Unless it's extremely coarse sand, which would be more accurately described as gravel.

-Chris

Thanks for all the help. I really thought I was doing good with my mix after all my reading. Now I just have to do the work and figure out what to do with all the sand and rocks I bought.

Yet another very informative thread about soil, definitely have some better idea for my next repotting job now. I just have to finish using that bag of cactus soil, before I change it to gritty mix, also I need to get my hands on all the ingredients needed anyway. Is repotting every year too often? Also, is fir bark the same or interchangeable as orchid bark?

This post was edited by DavidL.ca on Sat, Aug 9, 14 at 0:54

"Is repotting every year too often?"

It all depends on the plant. Some will benefit from yearly repotting, while some do very well when left in the same pot for several years. One of the advantages of a gritty mix, though, is it's ability to last several years without losing its porosity. "Also, is fir bark the same or interchangeable as orchid bark?"

Most orchid bark is fir bark, but there is some pine sold as orchid bark as well. Pine and fir bark have a slightly different shape, but can be used interchangeably.

-Chris

If you would like to gain even more understanding, read about Crassulacean Acid Metabolism, which is one factor in how some plants (not just Crassulacea) in arid environments survive. It's not just the root systems that are affected by high humidity.

Here is a link that might be useful: CAM explained at wikipedia

If one uses 50 - 75 % perlite with your potting soil, One will want to use rocks on top to weight the perlite down so it does not flow out of the pot when you water it. it acts to HOLD the soil. In hot climates , Some believe that it acts to insulate the soil from the sun. IT helps slow down the evaporation rate that is unbelievably ferocious in the sun in Texas.

I don't know if it has been mentioned that using a gritty mix will require more frequent watering. I agree that a gritty mix works well with succulents, but with other plants there are other options that can work successfully as well.

Christopher