Building a raised bed is a great option for city back yards where there may be no soil or where the soil's health is questionable*.  Raised beds allow you to control the amount of moisture and nutrients in your garden and can help keep grass and other weeds from invading your veggies.  Raised beds are also a great way to garden with children without everything getting trampled!

Important things to consider:

1. Get a soil test!  Many areas of the city have dirt contaminated with lead and other harmful substances that can be picked up by the roots of your veggies or your hands.  If it is contaminated you will need build a deeper raised bed with a barrier between the bed's soil and your backyard soil.  Fruiting crops are generally safer for questionable soil than leafy or root crops.
2. Pick the sunniest spot in your back yard.  A good vegetable garden needs at least 6 hours of direct sun per day.  This is especially true if you want to grow fruiting vegetables, like tomatoes or strawberries.
3. Having a hose that reaches the bed will make watering much easier.
4. A gentle slope is fine (in fact it can aid in drainage), but the area must be leveled out before building the raised bed frame so the soil doesn't wash out under gaps in the frame.
5. Think about an appropriate width so you don't need to step on the soil to plant or weed.  Most people can reach across a 4ft wide bed if there is access from all sides.  If you garden is along a fence or other barrier, make your bed 2ft wide or less to ensure easy access.
6. Plants like peppers, carrots, and lettuce use less space in your raised bed than tall or spreading crops like corn, strawberries, or pumpkins.  Try growing cucumbers, tomatoes, and herbs in smaller pots or containers to save space.

Building the bed:

Make sure you pull up all plants, turn over the sod, and level the area before building your bed. 

Your bed should be 8 to 12 inches deep and the frame should be sturdy enough to hold the soil that will be placed in it.  Cedar lumber is ideal because water won't cause it to rot for many years.  Another, cheaper option, is to use untreated planks of regular building lumber at least 2" thick.  These will last for 2-3 years before they need to be replaced.  Screw the planks of wood to 2"x2" or 4"x4" corner posts using 4 screws per board (2 at each end to attach the boards).  For better stability sink the corner posts 6inches into the ground before attaching the side boards. 

Get creative in building your beds, but use caution in selecting materials.  Used railroad ties, landscape timbers, or treated 4X4s can be laid on the ground and stacked in log cabin fashion to the desired height.  Another fine option is to use old tires, large rocks, or even whole trees to create your borders.  However, wood and other materials that have preservatives or industrial products on them can leach chemicals into the soil, and most people choose to avoid these materials. 

Soil:

Figure the cubic measurement of the bed to determine how much you need: width x length x depth = total volume.  For example, a bed that is 4 ft wide x 8 ft long and 1 ft deep =  4x8x1 = 24 cubic ft (almost 1 cubic yard).  The mix you fill your raised bed with should be one half soil and one half compost.  If the soil is dense it might help to add some sand to aid in drainage, especially if you are placing the raised bed directly on concrete.

You can buy compost by the ½ ton from Fairmount Park (assuming you live in Philadelphia of course) (but you will need a pick up to go get it), or top soil and compost from a landscape supply company.  Some places will deliver the soil or compost for you.  If your raised bed is small, or access is an issue, you can purchase top soil and compost from a nursery or hardware store by the bag.  

Links:

• Get your soil tested by the Massachusetts State Cooperative Extension office : www.umass.edu/plsoils/soiltest/
• Fairmount Park Recycling and Compost Center: www.fairmountpark.org/RecyclingCenter.asp
• Article on many different types of raised beds: www.planetnatural.com/site/raised-bed-gardening.html
• An article on how to plan the layout of your garden bed: www.humeseeds.com/vegplan.htm
• Laurel Valley Soils: laurelvalleysoils.com/garden.html
• See our related articles on brownfields and testing for lead