These issues are one reason why Shaw tells urban growers that “growing in 100 percent compost isn’t the best way to go because you can have nutrient imbalances, you can have disease problems. It doesn’t offer great physical support.”

Edwards used several different cover-crop mixes whose deep roots or abundant foliage can be used to address compaction or smother weeds. During the first year of the experiment, Edwards found that a “weed-suppression mix” of grasses and broadleaf plants made up of sorghum sudangrass, cowpeas, and buckwheat created a lot of biomass and had few weeds, although the plants struggled to reach their full size in poor soil.

A weed suppression spot at the farm plot.

At the same time, oilseed or “tillage” radish—essentially a daikon radish bred to break up compacted dirt—rooted deeply in all the test plots and produced a lot of biomass, regardless of the tillage type or lack thereof. In this way, it works on a couple of levels, creating channels in the soil for the roots of next year’s crop, but also producing crucial organic matter.

The research suggests that quick-growing crops like oilseed radish and the sudangrass mix could be used seasonally to fill in gaps in production: oilseed radish is best suited to cooler temperatures in spring or fall, whereas the sudangrass does well in the heat of summer.

Shaw had similar results with a short-term cover-cropping study. “We didn’t really expect to see a change after three years,” he says. “But after the first year, we actually saw a statistically significant decrease in bulk density in the surface because of the tillage radish, and to me, that was incredible.”

He believes that tillage radish could be a great option for breaking up compacted soils in urban areas, but adds, “it’s got some problems in that when it rots, it sort of stinks”—something that is perhaps not a minor issue when you’re growing food in a dense urban area.

Growers may have trouble working a radish crop into a crop rotation where they’re growing other brassicas, like cabbage, kale, or salad mix. Yet these complexities give support to the approach of the MSU project, which incorporates a range of treatments used in various combinations. “There’s such a wide breadth of gardeners and objectives,” Edwards says. With this in mind, he hopes to produce a flow chart that allows growers to cross-index their needs and perhaps financial resources with the data provided by MSU.

Pros and Cons of Tilling City Soils

Various forms of tillage, from tractor-scale cultivation to hand-tools, are used to incorporate cover crops, make planting easier, or kill weeds. But they can also have drawbacks, like undermining soil structure.

The MSU experiment is evaluating several tillage regimens for most of the same criteria as they’re using for cover crops, although Edwards’ own observation of the copious amounts of debris pulled out of the ground has informed his findings as well. “I definitely have come to  believe that the best thing that you can do if you want to do in-ground production is till up the first year,” he says. “Tillage is just that first early opportunity to get all that stuff out of there before you put too much equity and energy into the land.”

Although tractor tillage can be difficult to implement in urban areas, Edwards was able to hire a local farmer to cultivate a third of the test plots—which included using a subsoiler, moldboard plow, and a tiller attachment —for $1,400. He believes even cheaper options exist in the city.

On the experimental plots, Edwards pulled a large amount of debris out of the ground. Even basic roto-tilling can remove some debris, he says. The plots where this was done drained water the fastest and were the only ones to decrease in compaction over the growing season when combined with cover crops.

It’s unclear how this will play out in the long-term. Frequent tillage can cause problems, especially in Detroit’s clay soils. Tillage can break down  aggregates—clumps of soil whose crumb structure allows roots and water to move through the ground—and compromise biology  by killing fungi and earthworms. It may also cause more compaction in the long term by destroying the aggregates that keep soil fluffy and creating the “hard pan” that’s formed when dirt is repeatedly tilled at a certain depth. At its worst, this creates a vicious cycle of needing ever more tillage to break up the compaction that the tillage itself is causing.

However, it’s possible the MSU project will find a sweet spot where a minimal amount of tillage can break up the ground enough to introduce compost and cover crops that in turn create long-term fertility with less disturbance.

Beyond Farming, the Benefits of Soil Health

MSU’s experiment could have a big impact on people’s ability to produce food in their own communities, especially growing in Detroit during the COVID crisis as residents look to find ways to stretch their food dollar and limit trips to the grocery store.

But the implications of the research in Detroit and elsewhere extend beyond urban farming. Edwards believes that cover cropping practices could be used on vacant Detroit lots that are currently being mowed by the city to sequester storm-water that has caused extreme flooding in recent years, likely a result of climate change. They might also be used to reduce the amount of weed species that contribute to high rates of allergies and asthma in the city.

Urban soils can also create a carbon sink to help absorb atmospheric CO2, an important strategy for addressing the climate crisis. “There’s a whole lot more carbon in urban soils than people originally thought,” Shaw says. Additionally, he says, there’s “three times as much carbon in soils as there is any above-ground vegetation, and there’s a whole lot more in soils then there is in the atmosphere.”

A paper by Shaw and others—which studied soil in New York and Paris—found that urban soils often stored more carbon than agricultural or even forested land. It’s unclear exactly how big of a difference urban soils could make in mitigating climate change, but Shaw says that things like ash, soot, and asphalt already contribute to high amounts of carbon storage, and he’s looking into how to increase this by using compost and biochar, a charcoal-like substance used as a soil amendment.

Edwards hopes that the project “recognizes and legitimizes urban agriculture” and gets Detroit away from the “Motor City paradigm.” And he hopes it could help policymakers see urban farms as part of larger soil management and greening strategies that help cities adapt to the era of climate change.

All photos courtesy of Naim Edwards.