Rooftop farming and soil performance

Since 2009, Ben and the team at Brooklyn Grange have been pioneering a possible future for supplying food to cities.

The 2.5 acres of rooftop that Brooklyn Grange is cultivating puts it on an entirely different scale than the typical rooftop garden. The Grange grows over 50,000 pounds of produce in a season. For a city like New York, using its vast amounts of empty rooftop space for growing food holds enormous promise.

But maximizing the rooftop farming model's potential depends heavily on the soil.

The development of soil for rooftop gardening is just as crucial as all the other innovations being explored by farms like Brooklyn Grange. The soil demands for rooftop farming are particularly interesting, which is why we sent the GreenTree soil team down to check out Ben's operation.

The challenges of rooftop farming

The Grange cultivates about 1,667 yards of soil on its rooftop in the Brooklyn Navy Yard. At roughly a foot deep in the raised growing beds, that means over 15 tractor trailer loads of material. Being a rooftop, all that soil had to be pumped up to the roof with a compressor and a huge hose, then spread by hand. The soil for their first farm in Long Island City was hoisted by crane.

Getting it up to the top of an 12-story building is one problem; keeping it there is another.

Rooftops are windy. Soil erosion has always been a problem for traditional (earthbound) farmers. But for Ben and his team, after the arduous process of getting 1500 cubic yards of material onto their roof, it's extra important that their valuable soil not blow into the East River.

Water retention is another issue. There is no groundwater because there is no ground -- only a waterproof membrane a foot below the soil's surface. Ben is essentially growing his crops in a giant bucket. The Grange relies on municipal water for irrigation when rainfall isn't enough. After a historically dry summer in New York State (see our previous blog post on Main Street Farms) Ben is especially focused on water management. Better water retention in the soil would mean less irrigation and lower costs.

Rooftop farming needs to be resource-efficient in order to be successful: finding the right combination of weight, density, and water retention in the soil is crucial.

(Dr. John examining the soil in Brooklyn Grange's rooftop beds.)

The best rooftop gardening soil

GreenTree's soil team is working with Brooklyn Grange to understand how soil needs to perform in order to meet the demands of rooftop farming.

Because rooftop farming is still a relatively new agricultural model, the available soil options for it are limited. The soil that's commonly used for green roofs wasn't designed for agriculture. Green-roof soil is made with rocky aggregates, which prevents erosion and suits the low-nutrient plants that are typically used. But for growing crops, green-roof soils tend to drain water quickly.

For the past three years, Yoshi Harada -- a grad student at Cornell University's School of Integrative Plant Science -- has been doing research on soil performance at Brooklyn Grange.

Currently, Yoshi and the Grange team are running a soil trial to test water retention and plant growth in four different soils. Two of the soils are made by GreenTree; one soil has coconut coir plus biochar, and the other has coir without biochar. These are being tested against the green-roof mix that Ben's crops are currently growing in, as well as a new soil mix from the same supplier.

Yoshi's trial has turned up some interesting results. The data shows that the GreenTree soils have more water-holding capacity than the standard green-roof mix used by Brooklyn Grange. This makes sense, since GreenTree soils are designed to be drought-resistant. Rather than using rocky aggregate to encourage drainage, GreenTree soils use coconut coir to absorb water.

However, one unexpected result is that the GreenTree soil with biochar holds the most water out of all three soils. And Yoshi isn't sure why. Coconut coir should be superior to biochar in terms of water-holding capacity. But something about the addition of biochar makes the soil more drought resistant. It's an exciting result that could point to a better soil for rooftop farming.

(Jim, Yoshi, Dr. John, and Ben, discussing Yoshi's soil research at Brooklyn Grange.)

Water retention still needs to be balanced against soil density. Even if GreenTree soil is better at holding water than typical green-roof mixes, it must also be resistant to wind erosion. There's some preliminary evidence that coconut coir -- in addition to holding water -- creates a birds-nest effect with its fibrous strands, holding soil together against the wind. Yoshi's ongoing research should provide more information on how the soil performs over time.

Rooftop farming is becoming more prominent as a model for both practicing and creating awareness of sustainable food production. Brooklyn Grange is leading the way in adapting traditional farming techniques for rooftop environments, and soil performance will be a key part of this model. We're proud to have GreenTree soils serving an important role in developing the future of farming.

Brooklyn Grange has two locations in Queens and Brooklyn. They offer public tours of their farming operations, as well as rooftop yoga classes and weddings. In addition to vegetables (available at their market events and through their CSA) the Grange also produces honey and hot sauce. The Brooklyn Grange team has recently been featured on Fortune and Curbed. Email [email protected] for more information on all the super-cool stuff that Brooklyn Grange offers.