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Rooftop Farm Report: Fertility Problems for Soil

Over the last 2 years, Uncommon Ground has delivered food from roof to kitchen. But a worrying trend has begun to develop here: many of the plants are slow to grow while others are stunted or discolored.
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A cool, rainy weekend followed by a couple muggy days has been benefiting plants all over Chicago. We harvested garlic scapes, and snow peas this week. Early next week, we'll be plucking up a whole bunch of arugula microgreens.

For regular readers of this blog, you'll know that the rooftop farm at Uncommon Ground is on its third growing season, but its first with yours truly as Farm Director. This blog represents a report on how we're doing this year but is also a journal of my experience as I figure out how to grow food 30 feet up in the air.

Over the last 2 years, Uncommon Ground has delivered food from roof to kitchen, supplementing other local, organic, sustainable or humanely raised ingredients. Talking to the owners and to the previous Farm Director, however, the second season seemed to be less productive than the first and a worrying trend has begun to develop here: many of the plants are slow to grow while others are stunted or discolored.

Any hopes this slow start being related to anything else were quickly dispersed after a few pieces of data. The first, a soil test conducted by our interns, showed nitrogen depletion. Second, a trip to my community garden showed me direct comparisons of certain plants I knew were planted within days of each other. Kale at Uncommon Ground was perhaps a third the height of kale at my community garden. Third, our earthboxes are going gangbusters (more on these in a moment). Finally, our beans and peas are doing just fine as well.

In times like this it pays off to mobilize the finest minds around you, and in my case, that means my interns. On Thursday we field tripped down to the Chicago Center for Green Technology. We met with the resource librarians who introduced us to a whole range of innovative materials design, primarily for green roofs.

(A side note: I want to take a moment to reiterate the difference between a green roof and a rooftop garden. Perhaps this seems obvious, but the two often seem to be used interchangeably. Green roofs are engineered systems that create a living, growing surface to a roof which helps insulate, manage water, and reduce heat reflection. In green roofs, the soil profile is usually only a few inches thick. Rooftop gardens, however, are spaces designed to actually grow crop plants and as such require deeper soil depths and more irrigation.)

Our understanding of what's happening is roughly this. Our planter boxes are made from cedar planking and lined with weed cloth, a semipermeable plastic fabric, to allow drainage. Both rain and our irrigation system are watering from the top. Nutrients that enter solution (which would be easy for the plant to pick up) will be flushed out the bottom as the net solution movement is always down in our system. The earthboxes are bottom watered and therefore retain nutrients in their water reservoir and, hence, are doing great. Furthermore, the peas and beans are legumes that self-fertilize nitrogen with a symbiotic relationship with fungi that grow on their roots.

What we saw down at CCGT was fascinating and exciting, getting our creative engines running. The interns loosely designed 3 systems for improving our growing system. In essence the three systems are this: a) replace our soil volume with a whole bunch of earthboxes, b) convert our large planter boxes into super-sized sub-irrigation planters, c) figure out a water catchment and recycling system, so water (and by extension, nutrients) is retained.

As I type the interns are researching materials and costs. We'll finish these designs over the next couple weeks and install them in a couple of test beds upstairs. Of course, the proof is in the produce, so we won't know how well these work for many months. In the meantime, we're working on figuring out amendments to help improve the plants already in the ground.

Make sure to come on by during our Farmer's Markets to see how we're doing and how we're trying to address the age-old farming dilemma: how to keep soil fertile. The roof is always open on Fridays from 4 - 8. Hope to see you then!