Limestone is available in many garden centers as: agricultural lime (ground limestone - predominantly CaCO3); dolomite lime (ancient seabed deposits - more magnesium than other limestones); marble chips for landscaping (almost pure CaCO3); found in pet stores as coral sand (mostly CaCO3), for use as substrate in saltwater aquaria. Lime is limestone CaCO3 (Calcium Carbonate) heated to high temperatures until the extra CO2 is driven off, leaving CaO (Calcium Oxide). Calcium oxide is called lime, or quicklime, when water is added to calcium oxide it forms Ca(OH)2, also called calcium hydroxide or slaked lime. Calcium hydroxide is moderately high in pH and when in solution reacts with CO2 to precipitate calcium carbonate. The natural tufa,
Pinguicula moctezumae is said to inhabit is usually formed when calcium is leached by cold water from underground deposits, then reacts with carbon dioxide in the atmosphere to precipitate as soft crystiline deposits of CaCO3 (tufa).
A description of tufa can be found
here.
Many other Mexican
Pinguicula species grow in and around formations of gypsum, gypsum is also a calcium bearing natural mineral, calcium sulfate dihydrate(CaSO<sub>
4</sub>·2H<sub>
2</sub>O). Gypsum, most often has a neutral pH reaction, neither being alkaline or acid in reaction. Though, if these plants can benefit from supplemental calcium, then even crushed egg shells could quite possibly supply this need.
As flytraplady5 has said, species, such as
Pinguicula esseriana have a very adverse reaction to higher pH levels, so be careful what calcium source, if any, you use in your
Pinguicula media.