I couldn't tell you much about nutrient usage in soil. One thing I will tell you is get rid of the MG dirt. Drop some money on a better quality soil or mix your own because MG, regardless of their "organic" branding, has a bad rep in the canna community. Once I switched to
Fox Farm I saw a world of difference when I grew in soil.
Since I'm in coco, I start feeding almost immediately. Wait till they have a couple nodes then I start feeding 1/4 of what my max feeding will be and slowly work it up. In pre-amended soil, I wold probably wait at least a month, maybe till a week before flip, before I feed. Should allow the plant to develop a solid root system and be able to take a potential over feeding easier. Problem with using liquid nutes in soil is the possibility of overdosing your plant and then having to take a week to realize it then another three to correct it. This is why I switched to coco. See nearly immediate results (not like in hydro, where you'll lose your entire crop for one fuck up) yet having the leeway to make mistakes and not cost you everything.
Best of luck on the next run bud! That OG Shark sure looks dank! Nearly as dark as some Grape Ape I picked up last week.
One Love,
tobh