About adding limestone or not, and the use of dolomite

[mmcavall]

First of all I’d like to apologize for asking something probably already discussed many times in the Forum. But the Forum is something alive and there always will be newcomers with the same old questions. I’ll try to put in a different way and also I would like to say that I read lots of posts before formulating this question. But, again, I know this is not new…Anyway…

As I live in Brazil, most of soils are naturally slightly acid, and the pH of tap water is neutral. So, for me, the issue about substrates pH is a matter of add or do not add limestone or calcareous rocks. I currently don’t do that, and, without taking any measure, things will be good, on average, for most of my cacti.

On the other hand, I like to know about the natural habitat of my plants. Also, I’m interested on which kind of rocks and minerals they grow and which kinds I’m using. I’m aware that once the soil is inside a pot, it will not behave as it would do in nature, so it’s almost impossible to reproduce the soil of the habitat in a potted collection. Even knowing that, I’m still interested in the habitat. I use to search for images of a given species in its habitat, so I can have some clues about their natural environment, such whether the soils is sandy, humus-rich or a rocky outcrop. As much as possible, I would try to reproduce that, just for fun.

Finally, following other member’s experience, I’m more and more interested in grow my plants in soil-less, rocky substrates. When you start growing some cacti on pure mineral mixes, the kind of minerals that are being used gain new and higher importance. That’s why I’m here trying to objectively ask some questions about that (Ok, I know, I’m not being objective ).

So, how do you guys make a decision about adding limestone or calcareous rocks to the potting mix?

(a) I never add limestone and I avoid calcareous rocks

(b) I always add limestone and/or calcareous rocks

(c ) if the cactus is South-American, I don’t add limestone and avoid calcareous rocks; if the cactus is North-American, I add limestone and/or use calcareous rocks

(d) I search for information about the habitat and whenever is said that the soil is alkaline, I add limestone/calcareous rock

(e ) I don’t care

For example, I have a Mammillaria hutchisoniana, which is from Baja California. Finding information about Baja California soil can be tricky. At a first search, I found some references to soil alkalinity, but going a little deeper, I found out that the region is a mosaic of soils, and another Mammillaria species (phitauiana), also native to Baja California, likes acid soil because of the particular characteristics of its particular locality. As I don’t have any information about the exact locality for M. hutchisoniana, I can’t say for sure that it likes more alkaline soils. As a conclusion, I would get crazy if trying to find specific information for each species and try to give them these specific conditions. As result, I give up (option “a” above).

How do you guys usually deal with this?

Is there, somewhere, a list of cacti species with information about their locality and /or some information about it’s soil preferences?

This post is already too long, but I also would like to ask if calcareous rocks effectively will affect pH; in other words, should we bother about that or not?

In fact, I started this thread because I found a good and cheap source of ground dolomite, and I would like to use it in the sowing mix. Will it be a problem to Notocactus, and good for Thelocactus? Or it doesn’t matter at all?


here is a picture of the ground dolomite:


Thanks for your patience in reading all this, any comments are much welcomed.

[adetheproducer]

I use limestone only on cacti I have found information on showing they live on limestone or in limestone deposit areas. The region the cacti come from is not a good indicator as there are plenty of cacti from mexico an area usually quoted plants needing limestone but are strictly found in volcanic soils/rocks and others which cannot flower with out humus or live in forest or grass lands.
The purpose of limestone is either to achieve a specific look of growth or like you said to provide a more natural environment. In practice limestone is in no way required for cacti. It's pretty much right what has been written in things like the stone eaters addition of xerophilia limestone is only added as a growth retardant. It limits growth so takes the bloated look out of cultivated plants but slowing growth and avoiding the bloated look are easily done without limestone. In my opinion I think if they grow in it in the wild then it's pretty much worth using it in my pots. I am not trying to mimic the local soil but offer at least the right raw ingredients the plants are exposed to in the natural environments.
Mineral content is where pure mineral mixes get tricky. You need to find the right macro and micro nutrient minerals. The more varied rocks you can get the better as you are more likely to cover your bases. A bit of carefull research into geology and lithology will go far when pinning down good mineral ingredients. For knowing what plants live on limestone naturally you just need to search the Internet, try the usual cacti encyclopedia sites they will usually have something noted llife.com are pretty good. Then hit mining information, local geological surveys and even colledge/university papers there's lots of papers freely available done so people can get their grades. Use http://ralph.cs.cf.ac.uk/Cacti/fieldno.html for location data sometimes you can even find GPS data of locations online and just cross reference your research. The main thing to remember is if you don't know if it lives in limestone naturally don't bother putting it in a mix with limestone, as long as your soil is free draining and you can keep salts from building up if you have hard water you are pretty much good. Add ingredients which provide the mineral requirements and the and them for structural reasons. I have access to local dolomites, limestones, shales, hard and soft sandstones then getting gardening supplies for granites and volcanic minerals that covers most mineral needs. The sandstones also add structure for roots to spread across and water retention, many of the minerals will not be particulaly absorbant so sandstones and bake/fire clay product are great (clays can also offer good mineral inclusions for nutrients too) you add the absorbant materials relative to your local weather. I use little soft sandstones and baked clay cat litter as my local climate is pretty damp and cold for most of the year so water retention is low on my agenda. Don't be afraid of experimenting to get the right balance, you want the soil holding mosture just long enough to supply the plant and not realy to much on evaporation to dry the pot you want the plant to suck up the water. Test a pit of just the soil mix by weighing it when dry, soaking it them letting it dry until it gets back down to it original weight then you know it's dry. You want that time to be no greater than a couple maybe 3 days at most you know then the plant will only sit in water for 3 days defore it's dry which should be enough for the plant to drink and not rot if it's not thirsty.
Having said all this I do think it's a personal choice with the limestone, if you do your research and feel confident go for it, I did and my plants look good in my opinion. I still think of my self as a newbie/rookie, which ever tern you prefer, to cacti cultivation. I still make mistakes and loose plants but from what I have seen on this forum even the most experience growers end up with rotten plants now and again. Practice makes perfect and you learn from your mistakes so just get stuck in as deep as you want and enjoy its just a hobby to fill you mind with.

[mmcavall]

Practice makes perfect and you learn from your mistakes so just get stuck in as deep as you want and enjoy its just a hobby to fill you mind with.

=> true, good to read that!

Hi Ade, thanks for your reply (I was indeed expecting it). I have something about minerals to discuss, but I have not much time now, so this just to thanks for your reply. The link for field numbers is very very helpful, thanks a lot.
For all you said, I would'nt add dolomite grit in the sowing substrate, at least not in high percentage. It would be a choice for one or other species only. The same for limestone in the pots.
I'll come back latter. Thanks.

[jfabiao]

I believe it is a matter of personal taste rather than need of the plant. It has been said by many that the plants that live in the limestone outcrops and associated habitats do so because it is only there that they can outcompete the rest of the flora. I can understand the argument but it lacks hard evidence to me, and to say it in such broad sense as to cover plants of widely different genera seems unwise. But that is just my several-thousand-mile-away opinion.

One other thing you should bear in mind is the influence of mycorrhizae on the exact conditions in which a plant lives, especially for species that live in cracks, crevices, moss accumulations, etc. Looking at geological maps and soil charts will give you an idea that can be quite unreal when compared to the actual needs or even acceptable limits of a given species.

I use limestone chips (around 3-5mm) on my mixes, usually about 5% or less. It puts my mind to rest as to the possibility that plants such as Lophophora, Ariocarpus, Mammillaria lasiacantha and its cousins, Strombocactus, Turbinicarpus, etc., need some more calcium. I would use dolomitic limestone if I had it, but since I do not I supplement magnesium with fertilization. I would love to find some gypsum rich soils to grow some Aztekium or Turbinicarpus lophophoroides in it and see the difference.

Otherwise, I do what plant growers have been doing for ever: if a plant seems unhappy, repot. If nothing seems wrong, tweak the mix. Trial and error, all the time.

[7george]

I rather do point (d) but: - gravel mixes I use often have different type of stone in one or another %%;
- Don't have observations that support the theory that limestone dwellers don't feel happy in pots without these rocks. The tap water I use contains enough diluted carbonates so maybe cacti don't lack absence of these rocks in the soil mixes or in pots.

Dolomite is crystalline rock that will not affect the pH of the root environment so to use it or not would be matter of choice either for top dressing, bottom drainage layer or a soil mix component.

[mmcavall]

Thanks all for replying.

I see that you guys agree that 1) adding limestone is not obligatory even to species that dwells in limestone rocks in nature; 2) information about the location from which the species is originated will be a good indicator, but one may take care, because there are several different niches in the same location, plus the fact that in nature there are symbiotic associations, etc. and 3) one may try and see what happens.

I'm doing some tests. Usually I use two plants of the same species and same age/size, and give different potting mixes to each one. I'm impressed with the results of pure mineral mixes! At the moment, I can't say anything about the effects of the dolomite, time will tell. Another rock I have here is laterite. I'm testing it too. My concern with laterite is the high Aluminium content (does anyone knows anything about that?).

7george: I dont know nothing about minerals. But in the internet I've found some information about the effects of dolomite on the ph. See wikipedia for example. In the "Stone Eaters" the author says:

"Dolomite  is a sedimentary rock, improperly called sometimes limestone, being a calcium and magnesium carbonate, sometimes also called dolomite limestone. Dolomite is very interesting in soil for plants, in particular for calcicole plants, whereas they contain magnesium.
ATTENTION!! As it has an increased solubility! (9) – this is why it will be added in smaller amounts than limestone, the effects being similar to those obtained with a larger amount of the latter. If you add dolomites to the mixture, do NOT add limestone at the same time!"

I'm not very convinced of the effects of one or another rock type on the pH of a very draining and aerated mix. Usually I dont think about rocks as "soluble" things, so I would tend to agree with you... however, I've found different information in the www. Do you have more information about it?

Anyway, I'm not even sure that my crushed rock is indeed dolomite . All I can do is testing. It is a pretty good material (physical aspect) and very cheap, so I hope I can use it without problems.

[greenknight]

Put some in vinegar, it will fizz if it contains lime.

Dolomite used as a soil amendment is finely ground so it dissolves quickly. As gravel it will dissolve very slowly, so it will have much less of an effect on pH.

[mmcavall]

Put some in vinegar, it will fizz if it contains lime.

thanks Spence I'll try that

[mmcavall]

Put some in vinegar, it will fizz if it contains lime.

Yes, Spence, it fizz. I did the test. I also tested with pure water, just to be sure that the fizz was due to the vinegar. So there were fizz only in the vinegar, not in the water.

I tested some of my other materials:

White marble fizzed a lot. It is still fizzing since yesterday. Looks like Sprite.

Followed by this dolomite and by the coarse grounded dolomite.

The other did not fizzed: basalt, laterite and "mineira" (kind of a Brazilian rock for aquariums).

[jnewmark]

I have a 4x6, 24" deep outdoor Winter hardy succulents bed, that is mostly various grades of lime stone gravel, and crushed limestone sand ( builders sand ). Why ? Because it is readily available locally. I grow the typical Winter hardy cactus, Echinocereus, small Opuntia, and Agaves. So far, the plants don't seem to mind this mix at all, and I will give a liquid fertilizer in Summer months when I see growth. I also will use the crushed limestone sand mixed with lava rock for potted plants that like a mineral mix.

[mmcavall]

thanks jnewmark for your comment