I would start at 2.5% and add a bit each cycle. I soak mine in fish hydrolysate at the recommended mixing rate for application for a minimum of a week before using it. I've made probably 50 yards of this recipe over the past year at 10% biochar per yard.
I wouldn't be too concerned about the pH. The soil will probably need to nutrient cycle for a week or two after adding the biochar, so I would plan for that too.
Thanks for the quick reply. I added like a cup of the fish hydrolysate to 2 gallons of char, is that going to cause any issues with using so much fish to soak? I figured it soaked up a ton of N so was the reason for such a high amount of fish used plus they were bottles Ive had around for a couple years that I rarely use so I was using them up on this.
Im not entirely sure what nutrient cycling means but I am assuming to sit and "cook" after amending? My soil mix has been amended and wet with lacto b serum and sitting for at least 6 weeks so far (with another 4 weeks until I need to use it) and when this flower cycle finishes I planned to add/mix the current runs soil together with the fresh batch of soil and char when I assemble the no till bed. I will probably just use half the char I have soaking then so its at 2.5% of total volume and reserve the rest of the biochar for later use.
(EDIT: re-read your post and I read it wrong the first time, now it makes sense, let the mix sit for 2 weeks after adding the char.)
I appreciate the prompt and knowledgeable replys guys.
Also siski thanks for the reply, unforunately I cant add this much char that I need for my soil mix to my worm bins, it would be a larger quantity of char than my current bedding/worm size haha. But I have been adding pinches of powdered char to the top of the fruits/veggies mix when I feed the bins. I figure that should only make better castings and help house microbes and store nutrients and acids, all the good stuff.