Hey all, and thanks. So im heading into my first big solo grow, and ive got some great soil coming in for my 30 gallon pots. But i have a bunch of bulk soil left over from someone elses grow and have decided to use that for some big boxes that were already on the property. Im going to be amending that soil with earthjuice rainbow mix pro, and just wanted some advice on determining how to calculate the amendment amount. Some of the labels and info arent super clear to me on if its for amending soil, or just feeding. I will share all my numbers, and hope someone can chime in. thanks!!

Box size- approx 4'W x 5'L x 2' TALL
Rough volume of soil calculation = 40 cubic feet

Information from earth juice website-
Use as a Pre-Mix, top/side broadcast fertilizer, soil booster & plant tea. General use rates: 1-4 Tbsp. per gallon or 1/4- 1 cup per Cubic Foot of Soil. General frequencies of use: Every 12 weeks throughout the growing season. Planting mixes, coco coir, native soils. Formulated to be used on indoor or outdoor plants.

Ive also included the label from the rainbow mix bag.

From the info on the site, i would assume that 1/4-1 cup per foot is their suggestion ( assuming this if for mixing into soil as amendment) so roughly 40 cups per box?
The label from the bag is what kind of confuses me, as it gives suggestions for feeding plants by top dressing, but for "gardens" states 1-2 cups per 25 square feet. So assumign my box is ALMOST 25 square feet, how do i quantify the depth of the box to get that number right.
Im hoping ive provided enough info to get me started :)

I also have several 300 gallon pots im going to plant multiple plants into, so once i figure out the answer to this, i think i can extrapolate and figure out the same for the 300 gallon pots. thanks!!!