how much soil

the area im planning on planting my plants next year has horrible soil. Im planning on buying a ton of potting soil and digging a hole for each plant, then filling it with the soil. But my problem is i dont know how much to use for each plant. The average bag is aobut $4 and 1cu foot of soil. Is 1cu foot or soil enough for the roots to grow?

Try asking this question in the "outdoors" forum. Probably more qualified answers there.

Vi