Oakland is rapidly becoming the city of A Thousand Farms, with spades turning up soil in backyards and empty lots all over town. Most city gardens are not planted in former toxic dumps, but the issue of legacy pollution and urban farming is an important consideration before sitting down to dine on those plump heirloom tomatoes.

According to Nathan McClintock, a PhD candidate at Berkeley, and a local food justice and community food security activist, it is a good idea to check out your soil before planting. (McClintock has written several interesting papers on urban farming in Oakland.) There are two ways to investigate your soil. The first is to do a mail-away soil test. The Ecology Center in Berkeley has a list of soil labs, firms, and universities that will perform tests for a fee. City Slicker Farms, who build backyard gardens in West Oakland, use the University of Massachusetts soil test. For nine dollars, the lab will do a basic soil analysis. The two-page report contains all the information a weekend farmer needs to know. Another mail away test, A & L Labs will also perform tests for specific metals and contaminants. These labs will also analyze pieces of dehydrated plants, so if you have a lone citrus tree and are curious about the health of the fruit, you can send a chunk away to the lab.
          
McClintock explains that the two pervasive concerns for backyard gardens in Oakland are the atmospheric deposition of lead and zinc. Since freeways and major thoroughfares criss-cross the city, it is likely that most backyards are affected by depositions. The lead comes from pre-1980 leaded gasoline, while the zinc comes from wearing tires. Another issue is what McClintock calls “old hazard stock”, a reference to things like lead paint chips and gasoline dumps behind the garage.

McClintock has tested over a 100 sites in Oakland for lead, and has found that levels are lower than expected. But he says that lead levels are very site specific, and can vary widely even in a single location. Below is a map of lead levels in West Oakland. McClintock said a map of the entire city will be finished later in 2010.

It's considered safe to eat produce from soil with a lead content of 300 parts per million or less.

Another way of investigating soil is through land use maps. The land use history of a neighborhood, street corner, or section of town, can better tell you if your garden is at risk. Studying this history may also tell you what kinds of toxins or metals to be looking for. The City of Oakland has land use and zoning maps on their website, or you can check the EPA’s enviro-mapper by zip code for potential hazards.
          
All this should not scare the urban-back-to-the-landers. While it is important to know what lurks beneath your soil and while it is more than likely that most backyard gardens have some sort of legacy deposition, not everything found in your soil is dangerous. Lead, for example, is found in all soil. It's just a matter of the concentration that is important to know. According to City Slicker Farms, who have done more than 90 of these mail away tests, only about 10 to 20 percent come back with medium lead levels and less than a handful have high lead levels.
          
There are plenty of solutions if you are concerned about your soil. The most basic is to simply build a raised bed above the ground. City Slicker Farms recommends building a planter box about a foot off the ground, and if you are extremely concerned, you can just keep building higher. The bottom of the box can be lined with plastic and filled with organic compost. Another tactic recommended by McClintock is to use plants that are bio-remediators. Sunflowers and Indian mustard, for example, are considered hyper accumulators and will actually take-up metals like lead from the soil. Amending your garden with mature, stable compost will also help dilute any contaminates and make them less bio-available.

The upside of getting a soil sample done, is that usually the reports come with recommendations, so even if you have a finding of low levels of metals or contaminants, the lab will tell you what your ph levels are, and what kinds of amendments - like fertilizers and nutrients - you need to make your tomatoes extra tasty and your roses bloom bigger.