A mixture of
1/3 compost
1/3 peat moss
1/3 coarse vermiculite
That's all there is to it. No dirt is needed!
It's actually no secret at all but is Mel's Recipe from his Square Foot Gardening website. It really is the best!
"A blended compost made from many ingredients provides all the nutrients the plants require (no chemical fertilizers needed). Peat moss and vermiculite help hold moisture and keep the soil loose. It's best to make your own compost from many ingredients.
When buying vermiculite, be sure to get the coarse grade, and get the more economical 4 cubic foot size bags. If placing frames over grass you can dig out the grass or cover it with cardboard or landscape cloth to discourage grass and weeds from coming up through your new garden soil."
~From Mel's Square Foot Gardening
