Topsoil: The World’s Urban Sponge

All those urbanites growing organic food in the city has a certain appeal for the media, but to the average person, it may feel like a temporary marginal fad at best.  So why are city governments around the world taking it so seriously?  As it turns out, this trend has the potential to solve some of the worst problems that cities face – namely, climate change and water shortages – with a simple element: Topsoil.

Over the last several decades, many of the rainforests that act as our “carbon sinks” have been slashed and burned to make way for agricultural production.  Likewise, grasslands and savannas in Africa and America are routinely burned to make space for agriculture.  The farms that consequently inhabit those places feed the world’s cities – from Buenos Aires to Anchorage, Tokyo to Sydney, and everywhere in between.  Moreover, as cities expand to make room for sprawling communities, former farmlands are converted to suburbs because land-holders typically sell to the highest bidder – developers.  Consequently, more farmland must be created and more wild places (habitat) destroyed to make room for more farms.

The global market for agricultural products has obvious implications for climate change, as carbon-sequestering forests are cleared and products are shipped long distances using vast amounts of fossil fuels.  However, what may be less obvious is the solution to feeding the world’s cities without encroaching on our wild lands and carbon sinks.

Most people know by now that forests pull carbon dioxide out of the atmosphere, helping to fight climate change.  What might be less apparent is that soil sequesters carbon with far less risk than forests.  As temperatures rise due to climate change, bark beetles have begun to infest many of North America’s forests, killing off thousands of acres of forest and priming these vast swaths of land for massive forest fires.  Once the trees are dead, one lighting strike or one match will be all it takes to send all that sequestered carbon back up into the atmosphere.  If sequestering carbon in forests is our plan, this is quite a gamble.

Healthy topsoil, on the other hand, can soak up carbon with a remarkable rate of absorption and no risk of loss to the atmosphere during forest fires.  Collectively, tillage management and cropping systems in the U.S. are estimated to have the potential to sequester 30–105 million metric tons of carbon per year, says R. F. Follett in an abstract on ScienceDirect.  Unfortunately, we are losing topsoil around the world at an alarming rate. According to Allan Savory and Christopher Peck of Natural Investment Services, LLC, it is estimated today that our crop and range lands lose 4 tons of soil every year for every person alive. That’s 21 gigatons of soil lost to the sea, lost to productive use on land and releasing vast amounts of carbon (New Scientist, December 2006).  Thus, the problem with our current practices lies not only in deforestation, but also in our astronomical loss of topsoil to the world’s ocean because of overgrazing, poor farming practices, resulting erosion, and urban runoff.

Topsoil is not the only thing we are giving away to the world’s oceans.  Fresh water is systematically being diverted from our aquifers in an attempt to avoid flooding.  The unintended consequence of our diversion strategy is that we are depleting our aquifers and causing severe water shortages for ourselves and for species that rely on fresh water.  The water wars that happen every year in communities around the U.S. have as much to do with our ecological illiteracy as with a drought in any given year.  Our cities’ lack of permeable surfaces and topsoil to store the water mean that it’s not sinking into the ground and reaching our aquifers, nor is it being caught and stored for use in the dry season.  Instead, this fresh, drinkable rainwater is often contaminated by chemical lawn fertilizers, motor oil, and other products before hitting the asphalt and concrete gutters that will carry it to storm drains and ultimately, to the ocean.

Although the system may seem too set in asphalt and concrete to change, cities are catching on and, along with community-based organizations, pioneering a new pathway to solve many of their woes at once.   They are addressing climate change and water shortages (and epidemic obesity) simultaneously by building sustainable local agricultural systems that feed their residents on-site while acting as a giant sponge for both water (to recharge the aquifers) and carbon.

One example of such a city is Petaluma, CA.  On October 24th of this year, the City of Petaluma, along with nonprofits Daily Acts, Rebuilding Together Petaluma, and Petaluma Bounty, came together with over 200 citizens to sheet mulch 25,000 square feet of unused lawn at City Hall and install edible landscaping, community gardens, and a rooftop water catchment system.  Leaders at the event spoke about carbon sequestration in the soil, replenishing the aquifer, and providing a source of local organic food for city residents.  Large-scale private-public partnerships include the City of Detroit and Hantz Farm, which together may soon create the world’s largest urban farm, although it’s unclear what their plans are as far as sustainable farming practices go.

According to a U.N. climate change paper on agriculture last year, by 2030 an estimated 5.5 gigatonnes of CO2 equivalent a year could be avoided by agriculture with about 89% achieved by soil carbon sequestration.  Cities have an opportunity to build carbon sequestering capacity, thus potentially qualifying for carbon credits while also reaping the benefits of tax revenues from the sale of agricultural products within their borders.  By creating permeable surfaces and building topsoil, cities will also begin to recharge their aquifers, avoiding the water wars with farmers that are so common in today’s system.  Perhaps those urban farmers are really onto something.

To learn more about urban agriculture around the world, consider attending the Eighth Annual International EcoCity World Summit.   A highly influential community of architects, planners, designers, policy makers, green businesses, political and nonprofit leaders, with the added participation of international experts and delegates will be convening for the conference to present papers and ideas on the EcoCity and its role in the escape from dangerous climate change.  Participants from Australia, the U.S., the U.K., Israel, France, Senegal, Egypt, Singapore, India, Nepal and more will join together in the discussion in Istanbul this December.  In addition, more than 100 papers will be presented in concurrent sessions from more than 40 countries representing young emerging and pioneering talent from around the world.

For more information, go to:

Stacey Meinzen


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