4 x 4 x 6 \ 12 = 8cu feet.
That is what all my boxes are roughly.
They probably say to use use 8cu feet in a 4x4x7 so your soil will just sit down in the box a little bit instead of being filled to the brim.
i use 1/3 vermiculite, 1/3 peat moss and 1/3 compost.
remember that peatmoss expands... a 2.2cuft bag is more than 2.2 when expanded. the exact measurments i don't know and it varies...
i use a little garden tractor trailer that is roughly 10cuft inside. i measured into 1/3's on the inside and used paint marker to make lines. i start by adding the peat moss up to the first line, breaking it all up so it is expanded, then i add vermiculite and finally the compost or topsoil. then i just use a shovel and mix it. it's light enough with all that vermiculite and peat moss in there.
Edit: Don't use fertilizer if you are using nutrient rich soil\compost. And you add more compost when you pull the old plants and add new ones, just mix in a garden shovel\spade scoop of it.
enjoy your garden! :D
I use Blue Wave, but don't expect one of THEIR silly taglines.
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