In case you've been living under a rock, sustainable agriculture and locally grown food are growing in popularity, as self-proclaimed locavores and famers markets pop up in cities throughout the United States. Growing foods in cities via urban agriculture is a seemingly obvious way of satisfying both requirements. But urban agriculture, while gaining traction, still isn't practiced on a large scale, and is hampered by a variety of obstacles.
Perhaps the most serious constraint to the growth of urban agriculture is contamination of soil, plants and food from lead, mercury and other heavy metals that are often found in heavily urbanized and former industrial sites, said Sam Wortman, a researcher at the University of Illinois in Urbana-Champaign. He co-authored a report on the challenges threatening the growth of urban agriculture, which was published this month in the Journal of Environmental Quality.
One group that Wortman is collaborating with now is trying to build a seven-acre farm in Chicago on a former industrial site, and a prime concern of theirs is contamination of the soil, which they plan to fix via an expensive remediation process.
Lead is a main concern because it was once widespread in leaded gasoline and paint, and was used even more than it is currently in various industrial processes, Wortman said.
The most common method used to "remediate" industrial sites is to get rid of the soil, or bury it. In the former case, the offending soil is removed and replaced with virgin soil and/or compost from a rural area. In the latter case, a method called "cap and fill," a barrier is place over the contaminated soil, such as crushed cement, rocks or clay. Fresh soil is then placed on top. The idea is that the cap will prevent the roots from penetrating into the contaminated soil. As you might imagine, both methods are quite expensive, as they involve "transporting truckload after truckload of fresh soil" into the city, Wortman said.
Studies have shown that plants transmit a relatively modest amount of many of these contaminants into the parts of the plants that people eat—although that doesn't mean it's safe to grow plants in contaminated soils. But an even bigger concern is poisoning via the soil itself—fresh produce is often covered or coated in soil, and a small amount can carry significant quantities of certain contaminants like lead. That's one reason why it's important to thoroughly wash your vegetables, Wortman said. Working and gardening in contaminated soil is also another obvious and legitimate concern for those who want to practice urban agriculture.
Another limitation to urban agriculture is the availability of suitable water. Ideally, drinking water wouldn't be used for crops, since it is often in limited supply—and water scarcity is a real and growing problem throughout the United States, especially in the Southwest. (And it's worse in much of the developing world.) It'd be better to use so-called "gray water" from sinks, showers and washing machines. The infrastructure required to capture this kind of water is lacking in most areas, however.
Rainwater is also a promising candidate for irrigation, but it can also be contaminated if it picks up airborne contaminants that land on flat surfaces like roofs or parking lots (and this is one reason why parking lot runoff can be surprisingly toxic.) One way to reduce contamination in rainwater is to allow the first flush of rain to runoff without collecting it. This rain cleans the roof, making the rest of the rainwater clean, and automatic systems have been developed to exclude the first portion of a rain shower's downpour and collect the rest after the roof has been cleaned by the water preceding it.
Microclimate and weather conditions also change that way plants and crops fare in cities. Some crops that thrive in heat may do better, since cities can be on average nearly four degrees Fahrenheit (two degrees Celsius) warmer than surrounding rural areas due to the urban heat island effect. But this can also hurt cool weather crops like kale, lettuce, Brussels sprouts and onions, Wortman said. The increase in carbon dioxide found in cities from exhaust fumes may also help crops. But none of this matters if the other limitations to urban agriculture aren't addressed.