Might want to add more perlite to your potting mix next time. Especially if in 5 gallon buckets. They'll retain moisture and you'll risk root rot. I drill 1/2" holes in the sides of the bucket(s) to help with moisture control.

What's your day/night temps?

Do you know what that particular soil is buffered to? Mulches are generally buffered between 5.5 and 6.0ish. Potting mixes are generally buffered to 6.8 to 7.0ish. If using a mulch, you'll be chasing the ph till harvest.

Purpling of the stems and petioles can be from a few different things. PO4 defeniency, temperatures, and phenotype of the strain are the biggest sources. Some strains do have purple genetics buried in their past, and this expression can pop-up once in a while.

Careful with any magnesium product. Overuse can and will lock-out nutrient uptake. (possible cause of the additional purpling?)

All organic is soooo overrated, but that's likely for another thread. :thumbsup: