Compost and Limestone

[Martin]

I have been working my way through my Thelocactus collection repotting them all. Most of them have been in the same pot since 2006/2007 seasons due to my time in the US.

I was considering if I should include limestone in the mix or not as the collection was approximately 50/50 with and without it before.

Looking at the plants there was very little noticeable difference in them all on the surface and being low on limestone chippings I decided to go without them. I am now about 60% the way through the 400 or so plants and have an interesting conclusion.

I have not logged this data methodically so its not a scientific study etc but there appears to be some clear differences in the two groups ie those in a limestone mix vs those not

There is generally a much better root system on those NOT in a soil mix with limestone
There is a significantly higher number of plants will NO roots at all in the limestone mix

I tend to grow all the plants a little on the harder side so there is little difference in top growth all being nice compact plants with generally good spines etc

In other Mexican plants I use limestone in the mix for eg my ariocarpus etc I have not seen this root loss like in the Thelos and I have repotted all the other Mexican plants this winter about 250 or so plants

So not sure what this means or if its an odd result of the plants being in the same pots for 5 or 6 years but I am glad I have not used it with the Thelos this time around

I guess when you have a mass repot like this in a relatively short period its easier to see things like this, potting a few plants each year may not bring this out into the open as clearly

Anyone else find plants losing the roots in limestone mixes ?

[claudinski]

Dear Martin,

Very interesting. Which si the compost you sue?

[Martin]

I use a traditional UK style mix the older plants are in a mix of

John Innes No 2 / Course Grit / Vermiculite in equal parts

To this I added limestone chippings at about 10 - 15% for those that needed it

Now I am repotting into a similar mix but its

John Innes No 2 / Course Grit / Tesco cat litter

Again equal parts of each

[Chris43]

Again, this is not a researched conclusion, rather some views from observation.
In habitat, Thelos and most Cory's favour flatter, more open environments, and not the rocky cliffs or outcrops. While I know that soils tend to reflect the underlying rock strata types, it does seem logical that where there is more humus in the soil, the acid/alkali balance is unlikely to be too extreme. So, my tendency is not to use anything more alkali in a potting mix for those two genera than is in the JI anyway.
Its different for many Turbs and Escobarias which tend to favour limestone or gypsum rocky areas.
I'm not sure about Arios, as I have seen many in flat open areas, as we'll as in pockets between limestone rocks. I suppose that A.fissuratus is an obvious limestone dweller, but not A. trigonus.

[DaveW]

You will find opinions for and against adding limestone in composts. Some claim it is advantageous, whilst others maintain it is a growth inhibiter but prefer to add it so the plants retain a more natural habitat look and don't become "show bench bloated" and out of character. Certainly no cacti actually seem to need it.

One for:-

http://www.cactiguide.com/pdf_docs/hexalog-engl.pdf

One against:-

http://ralph.cs.cf.ac.uk/cacti/Cactus%2 ... linity.pdf

I seem to remember Marlon Marchado said the plants on limestone only grow when they receive acid rain and as soon as the underlying rock turns the water alkaline they stop growing. Cacti often grow on soils they can tolerate due to lack of competition from faster growing plants that can't, not because such soils are optimum for their growth. Gypsum is not the same as limestone.

http://www.gardenguides.com/108474-gyps ... -soil.html

Many of the limestone inhabiting plants grow in fissures in the rock that probably have accumulated some degree of humus over the years. Your experiments tend to indicate limestone does produce less roots, but is that because the plants in limestone can extract all the nutrients they require with a less extensive root system than those without, or just that roots just don't like it? Presumably the above ground growth should indicate which?

[phil]

I have also noticed root loss with some of my Turbs, Arios etc since using limestone in the mix and have now gone back to using crushed Gypsum 5-10% which gives good results.

[claudinski]

I'd like to add my few cents as I started recently in this kind of passion.

Limestone and Gypsum, at my first evaluation seems to be similar. Both are Calcium salts. Limestone is Calcium Carbonate, CaCO3 while Gypsum is CaSO4, calcium sulphate.

But looking more in deep, in the Mohs scale of mineral hardness, Gypsum, has an absolute hardness of 2, while Limestone has 3. Then looking at the absolute hardness the first has 3 while limestone has 9. Practically limestone is three times harder.

Then, looking at the solubility in water, limestone is mostly insoluble even if an acid water it increase its solubility while gypsum is moderately soluble, but 100 times more soluble than limestone in neutral condition ( the condition we create if we use city water).

Pay attention to one characteristic of these salts. Both at an increase of the temperature become less soluble in water as the temperature increases, till the precipitation. As you can see in your kettler after several teas! That white powder is precipitated calcium carbonate!

Actually, personally, I use gypsum in some plants like the mammillaria!

[Keith H]

Interesting reading that first link, where can you buy small quantities of crushed gypsum, I assume that I can not just break up some plaster board off cuts.

[DaveW]

Keith see:-

http://www.ebay.co.uk/sch/i.html?_trksi ... &_from=R40

[Keith H]

Thanks Dave, the item described as 'Horticultural Gypsum Micro Granuals Garden '? When I looked up crushed Gypsum I got links to quarries and exporters of said, I should get some to spread on my London Clay lawn and keep a small quantity for the North American cacti.