Green City Diaries: Dig this

People have been growing food in Detroit, off and on, for centuries. Today, it's clear that urban agriculture is an essential part of our post-industrial identity. Even city government has gotten behind the movement, finally passing an urban ag ordinance just a few months ago. As more and more people choose to live lives here that are at once urban and rooted to the earth, important questions emerge: how much do we really know about the soil in Detroit, for instance, and how has it changed over time?

Urban soils, it turns out, are a vastly understudied phenomenon. In Southeast Michigan, as in many parts of the country, regional soil surveys conducted by the USDA have historically stopped at the city limits. It's a problem of both scale and complexity: urban activity has rendered the soil conditions so variable that the undertaking would be enormous. Two samples collected just a few feet from one another are likely to have wildly different compositions. Multiply that by 139 square miles and you start to see the problem.  

There are Detroiters, however, who spend much of their lives working in and thinking about the soil, and their insights can provide a fascinating glimpse of the complex and surprising world beneath our feet.

Dr. Jeffrey Howard is a Wayne State geologist who studies the phenomenon of urban soil formation in Detroit's vacant lots, a process, he explains, that starts with demolition.

In 2011, Dr. Howard excavated a site in Roosevelt Park, in front of Michigan Central Station. There, a number of houses from the 1860s and 1870s were demolished in the early 20th century to make room for the train station and its elaborate formal gardens.

As he dug beneath the rich layer of topsoil, formed over about 15 to 30 years by bacterial decomposition of plant matter, Dr. Howard was surprised to find a deep red layer of subsoil, comparable to soils elsewhere in the country that have been enriched by naturally-occurring iron oxides. He didn't understand what he was seeing until he found decaying wrought iron nails, left over from the demolished 19th century houses. As the nails break down, he realized, they slowly deposit iron into the earth, forming an enriched subsoil in the process.

After a building is demolished, Dr. Howard explains, the land is typically cleared of debris and prepared for future development by the addition of fill dirt. Often, though, some debris is left on the site, and when workers dump the dirt on the land and spread it, grade it, and compact it, that debris gets mixed in.

After about 35 to 40 years, the debris begin to weather and, like the nails, to deposit minerals into the layer beneath the topsoil.

Dr. Howard has also observed the generative effects of mortar and concrete, both of which deposit calcium carbonate into the soil as they decompose. In tandem with the breakdown of plant life into humus, the decomposition of these man-made artifacts actually creates soil. "In 50 to 100 years," he says, "the soil seems capable of generating as if under completely natural conditions."

While Dr. Howard's concern with urban soil is largely mineralogical, Patrick Crouch, the manager of Earthworks Urban Farm, is more interested in the complex networks of organisms that live within the soil.

"When people point to a parcel of land in Detroit and call it 'vacant,'" Patrick says, "it makes me cringe, because it discounts the millions and millions of organisms inhabiting that space. We tend to think of soil as this inert material," he continues, "when really, it's not inert and it's not one material. It's thriving, both full of life and life-giving."

Patrick illustrates his point with a vivid account of what he sees when he looks at "vacant" lots, many of which have tightly compacted soil as a result of the process by which they were prepared for future development.

"Right now we have a lot of sweet clover, chicory, and alfalfa blooming in these lots, all of which have extremely long taproots that break up the compacted soil as they scavenge for nutrients. When they drill through the soil, they open it up for other lifeforms, worms and other creatures, who can then follow the root tunnels. At the same time, they're taking up nutrients from deeper down and pulling them up into their leaf matter. When they die, those nutrients get deposited on the surface for other organisms to access more easily."

At Earthworks, Patrick and his colleagues enrich their soil through careful cultivation of microbial ecosystems, rather than relying on the artificial stimulation of fertilizer. With the knowledge that annual crops, for instance, tend to thrive in an environment rich in bacterial activity, they add more organic material to the soil to foster bacterial growth. With perennials, which require more fungal activity, they create conditions for fungi to thrive. The results are healthy, productive, naturally balanced soil ecosystems.

Some of Earthworks' crops are grown in raised beds, but others are planted right in the earth, which brings up an issue of some importance: toxicity. What exactly are the effects of Detroit's industrial history on the soil?

The Earthworks community takes no chances about this question. Potential plots (and neighboring parcels) are extensively researched, analyzed, and tested for contaminants. Sites of onetime residential use, for instance, run the risk of lead contamination from lead paint. Former industrial sites offer a host of nasty possible contaminants including cadmium, lithium, and arsenic.

Research suggests that different kinds of plants absorb different quantities of heavy metals, and make them bioavailable to humans at different levels. Patrick stresses that there is minimal risk of locally grown vegetables absorbing dangerous quantities of heavy metals; most plants tend to avoid the metals because they can't metabolize them.

The greater risk is in contact: eating vegetables with contaminated soil still on them, for instance, or children playing in a garden and putting soil in their mouths. To minimize this risk, Earthworks designs its plots accordingly. On sites that tested higher than they'd like for lead, they've built raised beds or planted orchards, in which the soil is covered with sod and the fruit, high on the trees, runs little risk of contact with the soil.

I ask Jeffrey Howard if urban soils, formed in part from human activity, are healthy. "Well," he answers, "that's what we just don't know."

At Roosevelt Park, Dr. Howard was heartened by the abundant earthworm activity he found. "That's really encouraging. Sites of earthworm activity tend to be sites of microbial activity. If the earthworm population is doing well, it'll make the soils healthier and promote the microbes which also makes the soils healthier."

But, he cautions, "there's some weird stuff in these soils, and we're not completely sure what the consequences of it all might be." There's soot and coal, for instance, which can bind with toxic organic compounds, such as PAHs from fossil fuels, and hold them in the soil.

On the question of lead, which can also still be present due to the use of leaded gasoline, Dr. Howard's research suggests that the man-made artifacts can actually have a beneficial effect, since iron oxides immobilize the stuff.

What's clear is that we have adversely affected the soil over the years, and that anyone growing food in it should make careful decisions based on research and testing.

That research, I'm happy to report, should be a little easier in the coming years. Remember the USDA soil surveys I mentioned, the ones that typically don't touch urban areas? Dr. Howard tells me that about a year ago, a new Wayne County survey was mounted, and this one, ambitiously, will include Detroit. He expects it'll take another five or so years to complete, but when it's done, we'll know more about the great, teeming world underfoot than we ever have before.

Green City Diaries
 is a co-production of Model D and the Green Garage Urban Sustainability Library. For more information about growing food in the city, see our Urban Agriculture resource page. Many thanks to Kirsten Lyons and Helen Bradley of the Green Garage and John Lamb of McDowell & Associates for sharing their abundant knowledge about local soils with me.

Photos by Marvin Shaouni

Read more articles by Matthew Piper.

Matthew Piper is a writer and photographer covering art, architecture, and sustainable development in Detroit. Follow him on Twitter @matthewsaurus and on Instagram @matthewjpiper. Find more of his work at