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The title of the post kinda says it all. For my North American calciphile plants I don't think I'm at a point in my cultivation skill where I should be adding limestone to the substrate itself, but would a few large-ish pieces of limestone (1/2" to 1") on top just to give a nod to the plant's habitat be detrimental?

Can't recommend it. Here's a very small example of the types of gravel top dressings I use:
Looks like it's mostly or all river gravel, which is what I've preferred for many years. Just a matter of personal preference, though -- use whatever appeals to you. The point is to apply a nice thick layer of gravel top dressing all around the plant's base. And in case you were wondering, that's my Turbinicarpus pseudopectinatus under the flower.

zpeckler wrote: ↑Sun Mar 24, 2024 1:07 am The title of the post kinda says it all. For my North American calciphile plants I don't think I'm at a point in my cultivation skill where I should be adding limestone to the substrate itself, but would a few large-ish pieces of limestone (1/2" to 1") on top just to give a nod to the plant's habitat be detrimental?

It won't make any difference since the amount dissolved from such large pieces will be infinitesimal. In fact I am of the opinion that placing rocks on the soil mix is detrimental because it holds moisture around the base of the plant. If you have a good instinct about when to water then it will be ok I suppose.

MikeInOz wrote: ↑Sun Mar 24, 2024 8:20 amIn fact I am of the opinion that placing rocks on the soil mix is detrimental because it holds moisture around the base of the plant. If you have a good instinct about when to water then it will be ok I suppose.

I've been using gravel top dressings on all of my cacti ever since I started the current collection back in 2011 -- zero problems with it.

on top just to give a nod to the plant's habitat be detrimental? "

It shouldn't do anything it's more decorative than anything else, maybe hold water in soil a little longer as Mike points out. Many growers add Gypsum to the soil for limestone native cactus. Gypsum is calcium sulfate wont change the PH at least that's what Ive been told. Many cactus grow in almost pure limestone rocks and pockets of Limestone powder soil making a super reflective landscape. Less competition from other plants like grass etc so these slow growing cactus have the desert to themselves with a few very other very hardy plants.

keith wrote: ↑Tue Mar 26, 2024 12:25 amIt shouldn't do anything it's more decorative than anything else, maybe hold water in soil a little longer as Mike points out.

Nothing wrong with the decorative aspect, although for growers who find that stuff floats out and gets all over their plants when they water, top dressing is functional as it keeps the floaty stuff down. However, I can see where top dressings might be a problem if soil-based mixes are being used in a very humid climate. With soilless mixes and/or dry desert-type climates, IMO no problem.

keith wrote: ↑Tue Mar 26, 2024 12:25 amMany growers add dolomite to the soil for limestone native cactus. Dolomite is calcium sulfate wont change the PH at least that's what Ive been told.

Hi Keith.

Dolomite is Calcium-Magnesium Carbonate. It has almost the same liming power as pure Limestone-Calcium Carbonate. It will raise pH in potting soils.

Gypsum is Calcium Sulfate. It has a neutral pH reaction and is very slightly soluble so it is a good source of Calcium without changing soil pH.

Opps I meant gypsum I add Gypsum granules to potting soil for certain desert cactus. Dolomite where did I get that from ?

Thanks Jerry

keith wrote: ↑Wed Mar 27, 2024 1:12 am Opps I meant gypsum I add Gypsum granules to potting soil for certain desert cactus. Dolomite where did I get that from ?

Thanks Jerry

It comes with old age Keith! I'm finding some similar bloopers.

I have used limestone chips, about quarter inch in size as a top dressing in the past, when I did not want to include them in the potting soil. These were obtained cheaply from a local builders merchant in the UK for spreading on bitumen felted roofs to keep the sun off and render them more fireproof.

The plants did seem to do a little better with them on, but as cacti grow slowly it is always hard to tell if slight alterations to growing conditions makes much difference. No use at all if you bottom water, but I presume if you water from the top there will be some slight degree of alkalinity washed into the pot each watering. Obviously the finer the chips the more alkalinity will be dissolved each watering. Powdered lime obviously will react with water quicker than large limestone rocks!

Marlon Machado (a Brazilian Botanist) used to maintain that plants that grew on limestone only grew when they received acid rainwater and stopped growing again when the rocks turned the water alkaline.

For most N. American cacti limestone in the mix is not necessary and S. American cacti seem to prefer slightly acid potting mixes. As usual if they grow OK for you stick to what you are doing.

From the "Cal-Mag of my dreams?" thread I started in July 2022, here's a brief quote from MikeInOz:

MikeInOz wrote: ↑Thu Dec 29, 2022 6:52 amI don't think giving gypsum (calcium sulphate ) would hurt any cactus even epiphytes...We have to standardize the culture because if we try to replicate the habitat for every species insanity would soon follow.

His comment came from an inquiry I made regarding the list of cacti in my collection -- which species can absorb calcium from limestone vs. the species which can't. All cacti can absorb Ca from gypsum regardless of whether they're North American or South American species, desert cacti or jungle cacti (including epiphytes). Mike's point about standardizing the culture is worth noting. If growers need to give their cacti an additional calcium source beyond what the plants get from fertilizers and/or acidified tap water, go with gypsum for everything -- simplifying without being simplistic. Re. top dressings that are used for the decorative and functional reasons I mentioned, add a dash of gypsum granules to the top dressing. If growers are watering their cacti from the bottom, dig the granules into the soil. By the way, Mike recommends 2-3 mm granules, although I think we can go a little bigger. Since they're a slow-release form of Ca, they'll last for a growing season or two.

Steve Johnson wrote: ↑Sat Mar 30, 2024 1:47 am which species can absorb calcium from limestone vs. the species which can't.

Just to clarify Steve, all pants can absorb Ca from any source as long as it is in solution. The problem arises with Calcium carbonate raising the pH and interfering with uptake of the metals like Fe, Mn, Cu etc, by plants which have [eevolvvved - this word is disallowed apparently ] on acidic soils.

DaveW wrote: ↑Thu Mar 28, 2024 10:44 am


Pelecyphora.jpg

Pretty much my all time favourite cactus. How old is this one Dave?

Must have been about 30 to 40 years old as my plants don't grow very quickly. Alas it decided it had lived long enough and with no discernible change in growing conditions died two years ago. A species that does not show much visible signs if rotting since it does not go soft but you find out it is a hollow shell when repotting. Pity as with many plants if the main head shows signs of rot you can quickly detach the offsets and root them so saving it. One of those for "Plants we used to have"!

DaveW wrote: ↑Sat Mar 30, 2024 9:42 am Must have been about 30 to 40 years old as my plants don't grow very quickly. Alas it decided it had lived long enough and with no discernible change in growing conditions died two years ago. A species that does not show much visible signs if rotting since it does not go soft but you find out it is a hollow shell when repotting. Pity as with many plants if the main head shows signs of rot you can quickly detach the offsets and root them so saving it. One of those for "Plants we used to have"!

A tragedy!