i personalty wouldn't do nothing using that soil, just water when needed other wise ull burn the roots bad . mg soil is a hot soil so if u plan on re potting ur babies. if ur going to use mg soil cut it with some regular soil, ive seen to many beginners try using mg soil and they cant figure out why there heaving a shit load of problems. if u dont than good deal.
mix some reg potting soil with some pearl lite at least 25% pearl lite will give you good soil liteness and provide good o2 to your roots. and with the seltzer answer before thats seltzer water ( carbonated water ) not alka seltzer.
if it were me helping u id say to repot those into some regular potting soil and use ur own cocktail of nutes, instead of relieing on mg to give the plants the rite nute. that soil isnt made to grow cannabis. but good luck , time and a little tlc is the best to grow cannabis.