GYPSUM

[Roberto A]

Hi.
I would ask you something about gypsum (calcium sulphate)...
I have never added it to my soil for C&S but I wont to try, now...
As far as I know, the "agricultural gypsum" is from just crushed natural gypsum rich rocks and is calcium-sulphate dihydrate; on the other hand, "plaster of Paris", for builders use, is the "cooked" rock, resulting calcium-sulphate monohydrate...
Well... I have been unable to find agricultural gypsum here (Sardegna Island, Italy)....: does any of you use it or the "plaster of Paris" for you C&S compost?
Which dose do you use?

Let me know your experiences, please.....

[Terry S.]

Gypsum is used agriculturally as a soil improver, particularly on clay soils. I do not see why you should want to add it to good potting compost?

[rodsmith]

I've never used any lime-based soil improver in my cacti & succulent compost. I would certainly not use plaster of paris as it reacts with water to set hard.

[Liz M]

You can buy powdered lime I believe, to add to compost. It might be of benefit to lime-loving species. I grow alpines and have grown Saxifrages on tufa, a type of honeycombed limestone, they love it and grow well. The tufa dust I have put in compost, I cannot say I have seen much difference to cacti or succulents grown in it, though. All composts should have some lime in them, maybe totally organic composts have a very low amount and extra lime would be beneficial then.

[DaveW]

I have used gypsum in the past as I was told whilst lime alters the PH of the potting compost gypsum does not. My friend, a cactophile but also a bird watcher used to pick up gypsum rocks lying on the surface for us in his travels since British Gypsum had a mine and a plaster board plant in this area that used it. I note in the following link they refer to using old plasterboard (wallboard used for drywall or studded walls) as a source of gypsum, saying it contains gypsum, quartz, portlandite and calcite. Don't know how, or if they remove the paper off it though, but this normally comes off anyway if any is left out in the rain so presumably you could soak it off beforehand.

A quote from the following link:-

"Gypsum will not correct the most common type of soil compaction
High quality gypsum will not change the soil pH, although contaminants in some by-product gypsum may have some effect (raising or lowering pH) on soils.
Gypsum will not "de-toxify" most soil problems."

http://www.spectrumanalytic.com/support ... Gypsum.htm

I can't say I noticed much improvement with it. Most S. American cacti prefer a slightly acid soil anyway so lime or limestone is not usually recommended for them. How many N. American cacti need alkaline soils too is debatable, merely growing on limestone rocks does not mean they prefer them, but are just the only place they can escape competition from more rapidly growing plants. I believe Marlon Machado once said on this Forum cacti on limestone only grew when they received acid rain and stopped growing as soon as the rocks turned the water alkaline. How alkaline the gypsum rather than limestone soils other N. American cacti grow on are I don't know?

[Terry S.]

"Lime" is normally regarded as hydrated calcium hydroxide which you might throw on your cabbage patch but not use in potting compost. What is usually present in potting composts is a form of calcium carbonate, typically powdered limestone. The John Innes formula requires the additions of such limestone to correct the pH which would otherwise be lowered by the peat which "should" be in a JI mix. I suspect that multipurpose compost, especially if based on peat, would have powdered limestone added to bring the pH back to near neutrality. As an aside, composted municipal waste which finds its way into potting compost these days is alkaline as a result of the high temperatures reached in the composting process.

Any form of calcium carbonate will raise the pH of a potting compost, whereas calcium sulphate (Gypsum) will have little or no effect on the pH. Some growers maintain that cacti which grow naturally on limestone (particularly those from NE Mexico) benefit from adding more crushed limestone to the compost, a view that I do not subscribe to. I much prefer my potting composts to be neutral or slightly acidic. Tap water is always slightly alkaline (to avoid dissolved lead) and will tend to push up the pH of the compost over time anyway.

There are a few cacti which grow on gypsum in the wild, e.g. Rapicactus zaragosae, Geohintonia mexicana, Aztekium hintonii and perhaps this is why the original question was posed on the forum. However, such species grow OK in standard potting mixes.

[DaveW]

A little girl was given a book on animals and asked if it told her what she wanted to know? She replied "yes in fact it tells me more than I wanted to know about them!" Information overload!

I think this link and its following pages will tell you more about gypsum soils than you want to know:-

http://www.fao.org/docrep/t0323e/t0323e02.htm

[fruit]

Not sure that you'd really want to use plasterboard. The gypsum used to manufacture it is often a waste product of other industrial processes such as flue gas desulphurisation in power stations

[rthr]

A few years ago I conducted a very basic experiment to see the effect of gypsum on the south African succulent Gibbaeum album, and concluded that gypsum conditions can produce more robust growth especially when combined with salt. In brief, G.album seeds were sown in April 2009 into a communal pot and seedlings transferred to 4 individual test pots in November 2009. The various treatments I was interested in were:
- salt treatment (one teaspoon table salt/litre water)
- gypsum - light sprinkling of crushed gypsum on soil surface
- salt plus gypsum
- control - no treatment

Pots were watered (250ml per pot) monthly during their main growing season (Nov-Feb) and then once at midsummer, the two salt treatment pots getting saltwater. In December 2012 I terminated the experiment, when the pots looked like this:

albumexp_end_1212.jpg (111.98 KiB) Viewed 3549 times

photo of pots, clockwise: top left - control; top right - salt only; bottom right - gypsum only; bottom left - salt and gypsum

I know which pot I think looks best after the 3-year trial! I used just a spoonful of crushed gypsum sprinkled as a soil topping in my experiment and since there are gypsum mines near where I live in south-eastern Spain, small quantities of the raw material are freely available here. I don't recommend using any processed [plaster] product as a substitute. I haven't carried out any other experiments with gypsum nor tested any other succulent species with it in such a controlled manner. And although I haven't investigated the mechanisms involved in any great depth, it does seem a topic worthy of further study.

Catherine

[Roberto A]

Thanks to all contrubutors, but I think many of you misundestood the question: gypsum is a calcium sulphate and not a calcium carbonate, like limestome for example. That is a BIG difference, being gypsum valuable in providing calcium to the soil lowering (and not raising) the soil pH, being acidic.....
As to plaster: as I can't find the agricultural gypsum here (as I wrote in the first message), I have been wondering if some one is using it (being it "just" cooked gypsum rock).....