For the most fertile and friable garden soil, create raised garden beds. Walls are constructed using various materials and the resulting area is filled in with rich garden loam, most often purchased from garden supply centers. This fertile soil will grow the healthiest flowers and vegetables. Raised garden beds are generally not walked upon, so the soil does not become compacted, which creates an ideal environment for the plants' roots to grow and absorb nutrients. The soil in raised beds also warms up sooner in spring, because the sides are exposed to sunlight.
Perhaps the most common way to build a raised garden bed is to use 6-by-6 wood timbers. They are readily available at lumber yards, where they can be cut to length by qualified personnel. Lay the timbers on their sides around the perimeter of the bed. Stack them on top of each other to create a higher wall, thereby increasing the depth of the raised bed. Wood timber used as an edging for a raised garden bed will last 5 to 10 years before significantly rotting enough to need replacing. Do not use chemically treated lumber to construct garden beds; the chemicals will leach into the soil and may be cancer-causing.
A less expensive alternative to wood timbers is half-inch plywood cut to the desired height of your raised bed. Stock plywood comes in 8-foot lengths, so this will somewhat limit the size of your beds, unless you plan to patch two pieces together to form a longer wall. Although they will eventually need to be replaced, expect the plywood side walls of your raised bed to last three to five years before they begin to rot. Do not use chemically or pressure-treated plywood as the chemicals will leach into the soil.
For a raised garden bed that will last a long time, create the walls out of concrete blocks. A single course laid around the perimeter will create an 8 to 10 inch deep growing bed. If your soil is extremely poor or your property has a high water table, a double course will create a bed nearly a foot and a half deep. For safety, the second row of concrete blocks should be secured to the first row using traditional concrete block mortar.