Hello. I've been using a product I like. It's Wollammo at wollammo.com. It's a calcium-silicate mined in Quebec Canada. It's a powder that can be mixed with or put on any medium.
It has in it SiO2 55% - CaO 29% - MgO 3.1% - K2O 1.4% - Fe2O3 0.65% - Mn2O3 0.60%
This provides a source of plant-available silicate, carbon neutral calcium, magnesium, and other beneficial trace elements and micro-nutrients.
Check it out.
Chuck.