On the question of limestone growers...

[MikeInOz]

There is a lot of conjecture about the need for limestone in potting mixes made up for cacti from limestone areas. I find people in both camps claim success one way or the other and I thought I might add a bit to this subject for anyone interested. Besides cacti I also grow orchids and there is just as much - if not even more - argument in the orchid circles regarding the need for lime in the mixes particularly for the Paphiopedilum species which grow exclusively on limestone (usually in a mixture of decayed vegetation and limestone ''mud'' embedded in cracks in the rock. The pH of these soils is usually measured at about neutral. (7) There was a trial to study the reactions of ferns which are said to be alkaline growers coming from limestone cliffs. The results of the trial was very interesting. Basically it found that the important factor was actually not the pH of the substrate but the ratio of calcium to magnesium ions.....

P1130862.JPG (56.4 KiB) Viewed 3213 times

To explain.... These plants grew equally well in pH of 5.5 and 7.5 provided the calcium to magnesium ratio was high enough. For example the ratio of 4 - that is 4 times the Calcium to magnesium in the mix lead to excellent quality plants of scolopendrium at pH 5.5 (acid) and 7.5 (alkaline) and for anything over 4 for the other species. Note that even at pH 7.5 plants were DEAD at Ca/Mg ratio of 0 (that is - equal parts Mg and Ca)

What does this mean for cacti growers? It means that the reason people have success growing with or without limestone in their mix is most probably that the ratio of calcium to magnesium is high enough. Including soil for example (loam) in your mix will most likely give you the correct ratio of Ca/mg even if it is acidic because most medium loams contain high calcium levels regardless of their pH.

So what would we need to make sure we get a good supply of calcium and be sure it is higher than magnesium? We probably don't need to do much at all but adding gypsum or limestone or both would ensure it. Most gypsum samples I have checked are contaminated with calcium carbonate anyway so they will raise the pH of the soil slightly. Gypsum is also about 10 times more soluble than limestone so it should be added in granular form if possible. Limestone could be much finer. I use both together for the limestone species and only gypsum for the acid growers and rain water whenever possible and use an acidifying form of nitrogen in your fertilizer (ammonium sulphate). A very experienced Paphiopedilum grower when talking about growing a difficult limestone grower once said ''use acidic pH 5.5 or so) water and/or water + nutrients, but always have some calcium carbonate available to the plant...it may seem strange but that's the way to grow it''.

[esp_imaging]

Interesting too see that the pH in itself doesn't seem to be the key factor:

Basically it found that the important factor was actually not the pH of the substrate but the ratio of calcium to magnesium

FWIW - surely this is wrong, a ratio of Ca/Mg of 0 implies zero Calcium, not equal Ca and Mg:

Note that even at pH 7.5 plants were DEAD at Ca/Mg ratio of 0 (that is - equal parts Mg and Ca)

Another take on the results is that the plants that "need alkaline conditions" were very intolerant of Ca deficiency, but in comparison fairly tolerant of Mg deficiency.

[MikeInOz]

=esp_imaging post_id=376089 time=1595243591 user_id=8785]


FWIW - surely this is wrong, a ratio of Ca/Mg of 0 implies zero Calcium, not equal Ca and Mg:

Yes my mistake. Equal parts would be a ratio of 1. But 0.33 implies 3.3 times as Ca as Mg not zero Mg. So you would still the same ratio if you had 100ppm Mg and 300ppm Ca. = 0.3 Ca/mg.

Another take on the results is that the plants that "need alkaline conditions" were very intolerant of Ca deficiency, but in comparison fairly tolerant of Mg deficiency.

Correct.

[keith]

Bonemeal for calcium ?

[MikeInOz]

Bonemeal for calcium ?

Sure but because it needs bacteria to break down and bacteria need constant moisture it will be pretty slow. I still think the best form is Gypsum. (if you don't use ground limestone) You can buy bags of mined gyp here at the local hardware place. Called ''claybreaker''

[DaveW]

Interesting what we can learn from work done on other plant families. We got the idea of using cat litter as a soil ingredient from the Bonsai growers. So no doubt some scientific research is applicable to plants growing on similar substrates. I was told by a botanist cacti that inhabit limestone only grow when they receive acid rain and stop as soon as the bedrock changes the water to alkaline? Obviously it depends on how soluble the bedrock is before the waters PH stops growth until the next rainstorm.

I thought limestone made the PH alkaline but Gypsum did not affect it? But as you see I am no chemist therefore I was confused when I read this link and it said limestone was acidic?

https://www.differencebetween.com/diffe ... limestone/

[greenknight]

Limestone is not acidic, that's an error. It has a pH of 8 to 9, anything above 7 is a base.

[DaveW]

I thought it must be Spence. I occasionally get a similar brainstorm and write the opposite term for what I mean, left for right etc, the problems senility causes! Therefore in that link change acidic for alkaline and it makes sense since both begin with "A" so probably it was a bad day for the author?

[Aloinopsis]

https://www.ebay.com/itm/Espoma-Garden- ... SwhhJfEJMX

Is this the kind of gypsum product you all mean?

I have a bag of that exact product in my shed which has been there for years. I wonder if I should open it and sprinkle it my cactus pots.

[keith]

"Diatomaceous earth is a rich source of calcium, magnesium, and other minerals, but most of its beneficial properties can be attributed to its abundant silica content."

Got this off the internet . I use it to keep away bugs like sciara fly and root mealy bug . Don't know the ratio of CA to Mg is accurate ? And I don't know if its really a rich source of either mineral . Been using it for 30 years and switched to food grade type about 10 years ago. As much as 30% and as little as 5%. approximately. Turns the soil a grey color.

"Diatomaceous earth is made from fossilized water plants and is a naturally occurring siliceous sedimentary mineral compound from the remains of algae-like plants called diatoms. The plants have been part of Earth’s ecology system dating back to prehistoric times. The chalky deposits the diatoms left are called diatomite. The diatoms are mined and ground up to make a powder that has a look and feel much like talcum powder. Diatomaceous earth is a mineral-based pesticide and its composition is approximately 3 percent magnesium, 5 percent sodium, 2 percent iron, 19 percent calcium and 33 percent silicon, along with several other trace minerals. When using diatomaceous earth for the garden, it is extremely important to purchase only the “Food Grade” diatomaceous earth and NOT the diatomaceous earth that is and has been used for swimming pool filters for years. The diatomaceous earth used in swimming pool filters goes through a different process that changes its makeup to include a higher content of free silica. Even when applying the food grade diatomaceous earth, it is of the utmost importance to wear a dust mask so as not to inhale too much of the diatomaceous earth dust, as the dust can irritate the mucous membranes in your nose and mouth. Once the dust settles, though, it will not pose a problem to you or your pets.

Read more at Gardening Know How: Uses For Diatomaceous Earth – Diatomaceous Earth For Insect Control https://www.gardeningknowhow.com/plant- ... ontrol.htm

[MikeInOz]

"Diatomaceous earth is a rich source of calcium, magnesium, and other minerals, but most of its beneficial properties can be attributed to its abundant silica content."

Got this off the internet . I use it to keep away bugs like sciara fly and root mealy bug . Don't know the ratio of CA to Mg is accurate ? And I don't know if its really a rich source of either mineral . Been using it for 30 years and switched to food grade type about 10 years ago. As much as 30% and as little as 5%. approximately. Turns the soil a grey color.

"Diatomaceous earth is made from fossilized water plants and is a naturally occurring siliceous sedimentary mineral compound from the remains of algae-like plants called diatoms. The plants have been part of Earth’s ecology system dating back to prehistoric times. The chalky deposits the diatoms left are called diatomite. The diatoms are mined and ground up to make a powder that has a look and feel much like talcum powder. Diatomaceous earth is a mineral-based pesticide and its composition is approximately 3 percent magnesium, 5 percent sodium, 2 percent iron, 19 percent calcium and 33 percent silicon, along with several other trace minerals. When using diatomaceous earth for the garden, it is extremely important to purchase only the “Food Grade” diatomaceous earth and NOT the diatomaceous earth that is and has been used for swimming pool filters for years. The diatomaceous earth used in swimming pool filters goes through a different process that changes its makeup to include a higher content of free silica. Even when applying the food grade diatomaceous earth, it is of the utmost importance to wear a dust mask so as not to inhale too much of the diatomaceous earth dust, as the dust can irritate the mucous membranes in your nose and mouth. Once the dust settles, though, it will not pose a problem to you or your pets.

Read more at Gardening Know How: Uses For Diatomaceous Earth – Diatomaceous Earth For Insect Control https://www.gardeningknowhow.com/plant- ... ontrol.htm

I've often wondered about silica. I know that the spines on nettle are basically a kind of glass which is silica but I haven't been able to find out what cactus spines are made of. I've always been sceptical that the limestone in mixes leads to stronger spine development as often claimed. Anyone know?

[MikeInOz]

https://www.ebay.com/itm/Espoma-Garden- ... SwhhJfEJMX

Is this the kind of gypsum product you all mean?

I have a bag of that exact product in my shed which has been there for years. I wonder if I should open it and sprinkle it my cactus pots.

Yep that's the stuff.

[Aloinopsis]

https://www.ebay.com/itm/Espoma-Garden- ... SwhhJfEJMX

Is this the kind of gypsum product you all mean?

I have a bag of that exact product in my shed which has been there for years. I wonder if I should open it and sprinkle it my cactus pots.

Yep that's the stuff.

Thank you. I will probably use it as a top dressing for some of my cactus and see how they do. I don't remember why I even have it, it has been in my shed for years.

[DaveW]

Found this on the Web, but can't find any information on proportions of gypsum to add to cactus soils.

http://www.cactusnursery.co.uk/ph.htm

[bartab]

I'm curious about which cacti would benefit from Gypsum? I've read before where Turbinicarpus would possibly be helped. At this time my Turbs seem pretty locked in and doing well. Lophophora or Mammillaria? I'm not going to repot anything just for this, but maybe add a little when repotting in the future if it is recommended. At least experiment with a few.