Urban agriculture includes everything from a farm in the city to a raised bed next to your apartment. No matter what the scale of your garden, start by learning about the soil you will grow in. What used to be there? What may have been left behind? Take steps before you put your hands in the dirt to make sure you and your community will have a safe place to grow.
In the City of Portland, there are several places to go to find out about a site's history. Talk to long-time neighbors: they may remember when an empty lot was once a home or business. Check the property's address on portlandmaps.com for historic permits, and also check the Oregon Department of Enviornmental Quality's Environmental Cleanup Site Information database to see if your site or a close neighbor have known contaminant releases. Another useful stop on your historical tour is the downtown public library, for a look at the Polk Directories. These reference books are like reverse phonebooks - they can tell you if your vacant lot was once a gas station.
If you discover past uses that may have left behind contaminants, a more thorough investigation may be the appropriate next step. Contact the Portland Brownfield Program for information on available resources and funding.
The Environmental Protection Agency has an up to date website on brownfields and urban agriculture, filled with resources to help you garden successfully in urban soil. The EPA fact sheet Growing Gardens in Urban Soils is a great place to start.