Don't want to bore you with gory detail, but "yes", I have done a lot of experimentation with growing both Malesian and Neotropical aroids at media having different pH (native and amended) and, "yes" some otherwise tricky aroid spp. respond well to this fine tuning of the root-soil interface in captivity. "No", not all putative calciphiles require alkaline substrates to thrive and many prove to be quite adaptable to a wide range of pH and physical media structures.
Note: many (but by no means all) plants originating from karst formations or regions with underlying limestone substrates are actually near- or functional calciphobes. It very much depends whether or not the plant is actually rooted into the underlying rock or grows in the (acid) decomposing leaf litter layer that covers it. You really need to know a bit about the plants' ecology prior to just assuming that since something is associated with karst formations it necessarily needs a substrate of >ph 7.5 to thrive. While not as evident in aroids, this phenomenon is well known to growers of tropical butterworts (Pinguicula spp. Lentibulariaceae) and understory palms.
I would also take up your point about chemistry: that there are some not insignificant nutritional/toxicity considerations to take into account when one couples very acid or very alkaline substrates and prima donna plants.
> Date: Tue, 19 Oct 2010 11:19:00 -0400
> From: firstname.lastname@example.org
> To: Aroid-L@www.gizmoworks.com
> Subject: [Aroid-l] Limestone Substrates
> I thought I might add a word here inspired by my reading of Eduardo
> Goncalves’s article in the recent Aroideana. For those who might not
> have gotten into the issue that far, the article is entitled “The
> Araceae from the Limestone Outcrops in Central-Western Brazil”. As the
> title suggests there are aroids that are adapted to limey substrates.
> I was surprised that they are as common as he indicates.
> Of course, a substrate dominated by limestone (calcium carbonate) will
> have an effective pH in the range of around 7.5 to about 8.5. Compared
> to the typical horticultural blend, containing generous amounts of
> organic material and, perhaps, pH-neutral amendments like vermiculite
> and silica sand, limestone-dominated substrates are very distinct
> growing media. Organic-dominant soils will have pHs in the range from
> about 6.5 to as low as perhaps 5.
> Chemically, the pH difference between 6 and 8 is drastic all by
> itself. In addition to pH, it’s pretty certain that the nutrient
> content and elemental availability of whatever might be present is
> also very different between soils at the extremes. Anyone familiar
> with soil science will know what I am talking about. I am thinking
> that plants adapted to grow in calcareous substrates (those containing
> limestone and its variants) will do poorly in the typical planting
> media used by domestic growers.
> From time to time we’ve had discussions in this forum about substrate
> likes and dislikes. For example, I remember a rather lengthy
> discussion a while back of the merits and demerits of using coir. My
> query to our readers is whether anyone has attempted to pay attention
> to soil pH and/or limestone content in the growing of aroids. It might
> be that certain “difficult” aroids are seen as difficult only because
> they have been grown in inappropriate soils.
> Ted Held.
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