I found a reasonable source of pumice to use as a soil amendment. But I have a question: does anyone here use fine grade pumice as a germination medium for cactus seeds? Because of my very humid climate I want to avoid rot and fungus on tiny cactus seedlings. Will pumice reduce that threat?

Yes and yes.

Run some Physan through it first to disinfect, then rinse well.

J

Emphasis on rinse well. Pumice is sturdy but there is always "dust" that will obstruct drainage. I personally find pure pumice not all that great for anything. After rotting out an Aloe I realized how much water is retained in pure pumice alone. For reasons I don't understand, I find better drainage with equal parts pumice, sand (silica #12), and peat or other similar material. This is what the CSSA sites recommend and I have had good results. This is how I germinate aloe seeds and I imagine cactus seeds would do well too. Cactus roots are generally shallow so smaller pots will do. You can always dust with sulfur or rinse with captan it fungus is a major concern. If conditions are humid then good ventilation and allowing the mix to dry will help prevent fungus.

Not it sure if this has been mentioned elsewhere, but there is "dry-stall" sold at horse feed stores that is a nice, uniform; smaller-sized pumice that is good.

 

Good luck!

Great stuff Meg but do run it through a fine mesh screen first then rinse/wash it. It's my #2 preferred ingredient in my bromeliad mix also.