If pre-buffered, what is adding cal-mag expected to do? I presume it better to 'buffer' (and also charge with nutrients) new pre-buffered coco pots with base nutrients which are made for this (buffering feed solutions) vs. adding unpredictable cal-mag and potential overfeeding at youngest stages with Ca/Mg.
I presume(?) adding Ca/Mg in solution to pre-buffered coco will just result in that solution retaining much that same concentration/ppm, not any significant altering of the levels of coco-bound ions (Ca/Mg), which is what 'buffering' coco involves. Seems overkill, risky, to add cal-mag to try to 'buffer' pre-buffered coco. If concerned about 'buffering' your feed water (such as to stabilize pH in a reservoir) then add cal-mag to the feeds. Cal-mag solutions and supplementation are not going to 'buffer' the coco (which should be inert, not readily release, not be the plant's source of Ca/Mg vs. getting Ca/Mg from feed solution).
I use bagged Canna coco, do no pre-treatment, just saturate it with ≤1 g/gal. 2-part MegaCrop and transplant seedlings from the ~quart/liter yogurt plastic containers they were each sprouted in. After about 2 weeks, I start feeding full-strength nutes. I essentially never do drain-to-waste, never add cal-mag (have no need); also never even measure pH or ppm much less adjust.
