I can report that among the cacti that I have which are claimed to prefer
limestone, they seem to grow better when the PH is lowered to the 6-7
range. Unlike the CP species, they depend on root absorption for all
minerals and most minerals have their bioavailability peak in the 5-7
range. I do start out with rather hard well water which raises a
different question.
One thing that I cannot measure in cacti is calcium deficiency. The
symptoms are much less obvious than those seen in leafy plants. The
symptoms ascribed to low PH may not be PH at all but rather calcium
deficiency.
An obvious test is to supplement the mix with granulated gypsum rather
than ground limestone. You don't need much because it dissolves faster
than limestone.
In the past, I have only used gypsum outdoors to flocculate clay soils.
Large amounts (10-20 lb/100 square feet) can be applied to correct salt
pollution without apparent phyto-toxicity.
Bob Cruder - bcruder@miaco.com