Combining soil
Raised beds are essentially container gardening on a larger scale. The soil recipe you use to fill them can be matched to the specific needs of the plants you want to grow. For a raised bed designed for rhododendrons, azaleas, and other ericaceous plants, an acid mix of oak leaf compost, bark, topsoil, and just a little sand is ideal. For general gardening, you can select a commercial mix or a combination of topsoil, mushroom soil or peat humus with some sand, compost, and well-rotted manure. The mix you select, and especially the organic materials you choose, will depend on what is available in your part of the country. After your bed is prepared, have the soil tested to see if you need to add any nutrients or lime to your mixture.
Many gardeners have turned to low raised beds as the optimum way to raise vegetables. This method saves space, is economical in its use of water and fertilizer, and is extremely productive. Best of all, the soil is never compacted by foot traffic because the beds are designed to be worked from either side. By building beds narrow enough so you can reach the center without stepping into the bed from 3 to 5 feet wide, depending on how far you can reach you can greatly reduce soil compaction and increase yields.
Once you've prepared the soil, you can space the plants evenly throughout the bed. You won't need to allow for conventional rows because you'll be tending the plants from the side. (Be sure to use proper spacing between plants, so they won't be overly crowded.)
In addition to increased yields, spacing plants in this way provides another benefit. By growing the vegetables closer together than suggested for typical row plantings, the leaves weave together and shade the ground. This keeps most weeds from germinating and also helps conserve soil moisture.
You can always ask an adviser at your local store for more information about mixing soil as they will be motivated to sell you the best option for your your garden.