Looked around the net about biochar and its advantages and found quite useful information.

I will just add charcoal in my potting mix. will need to decide how small to bust up the charcoal.

I done a 10% calculation.

5 gallon potting mix, 10% charcoal=0.0024 cu yds = 0.48473766 gallons= rounding off @ 0.50 gallons = 8 cups

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Found this calculator to come in handy.

https://pacificbiochar.com/biochar-calculator/