Due to lack of time to source proper amendments, I have been using it sporadically in week 2 to 5-6 of flower depending on strain, and or what the plants look like. Having the proper ratio of available calcium to magnesium in the soil helps stabilize the ph and make all other nutrients available.
I prefer not to use it, but for the time being, it makes sense for me. I use at half dosage or less, this little amount of synthetic should not affect microbes population. As far I know, there isnt really an cal-mag organic equivalent that can be use for ''quick fix''
ultimately you want to make sure to have the proper amendments in your soil to break down like gypsum, lime ... and then you should never have to use synthetic cal-mag.
I have been debating ''to use'' or ''not to use'' cal-mag and only broke down again recently after over a year of not using it. When I get around to mix soil for a big ROLS pot I will address the situation, but Id appreciate more input on the topic and I welcome your question nvhak49!