Fall Update, 2011


The trials weathered all sorts of mayhem this season – extreme heat and a hurricane among other things. As hurricane Irene rolled in, Lise, Uli and I battened down the hatches by moving the smaller pots – carrots, beets and soy – indoors.

Tomatoes stayed out during the hurricane and came through with flying colors. They produced fruit into November.

Having weathered the storms and produced everything we needed them to, the trials came down last week. It had taken months of planning, 23 people a full day for installation and a season of active management. It took four of us two hours to dismantle everything and give it the heave-ho.

In addition to growing mind-blowing tomatoes we did learn a thing or two from our trials. We’ve gleaned some things about the use of biochar, some things about irrigation, and some hints about what might be expected in terms of nutrition and toxicity in this new world of rooftop farming.

As we began our research, a few people inquired about the possibility that food grown in the urban environment, and even the soil itself, could become contaminated from airborne pollutants. What if, after the installation of a giant rooftop farm, the soil became unusable?

Two critical concerns began to emerge: nutrition on the one hand, and toxicity on the other. Many of the buildings in NYC that are best suited for rooftop farming are near a pollution point source. Most are in industrial areas and many are straddled by highways. Our trial site is spitting distance from a superfund site and almost directly across the street from a cement factory.

Dr. Joshua Cheng, Director of the soil laboratory at Brooklyn College, has made a career of studying soil toxicity. He explained that with all of the changes brought about by the Clean Air Act, soils that are clean today are unlikely to become contaminated with heavy metals unless they are very close to a heavy point source. With respect to heavy metals, the real culprit these days is soil that is already contaminated, mainly by lead.

Dr. Cheng was kind enough to test the lettuce that came out of our trials in the spring. Specifically, he looked at:

As Arsenic
Cd Cadmium
Co Cobalt
Cr Chromium
Cu Copper
Mn Manganese
Hg Mercury
Ni Nickel
Pb Lead
Tl Thallium
Zn Zinc

Some of those metals, in the right quantities, we think of as nutrients, e.g., Copper and Zinc. At higher levels, however, they may become toxic. Others, such as Mercury and Lead, are generally avoided altogether, though at low levels they do not present significant risk.

The report that came back on our lettuce from Joshua’s lab showed it to be squeaky clean, or as Joshua put it, “All of the toxins are within regular background levels”.

This is good news considering the massive number of Green Zebra tomatoes that I ate this season from the trial site. More to the point, this is great news for urban farms in general. Many thanks, Dr. Cheng.

We will continue to get more information on both nutrition and toxicity as we comb through data from Ray Glahn at Cornell Agricultural Extension/USDA. When I dropped off lettuce at Ray’s lab, he had the bright idea of testing the samples both washed and unwashed. The differences in heavy metals between the washed and unwashed should indicate nastiness that settled out of the air and onto the plants. From the looks of the small dataset from Joshua, I think we can expect good news.

That data from Ray is already in. It’ll take us a while to analyze it. Right now it is a blinding series of numbers:

While the initial peek into toxicity has been reassuring, the meat and potatoes of the trials was to compare the nutritional value of foods grown in engineered, rooftop soils to foods grown in the highest quality fields. I consider one of the best fields to be the one managed by Zach Wolf at the Stone Barns Center. So while we grew crops in rooftop soil mixes on a roof in Brooklyn we also grew the same crops in the field at Stone Barns.

Ray pointed out that if the food coming from the rooftop was less nutritious, it would be hard to know if the variation was due to differences in the soil, or due to the extreme differences in the growing conditions, e.g., very hot roof –vs- nice, cool field.

To address these environmental factors, we took rooftop soil to the Stone Barns Center to grow samples in both the engineered and natural soils, side by side.

After seeing how stunted things can be on a rooftop I was impressed to see that at Stone Barns the yield was similar between the crops grown in the rooftop mix (carrots on right) and the veggies grown in the natural soil (carrots at left). This would seem to confirm Ray’s assertion that location can play a significant role.

Five of us tasted the carrots side by side and all had the same impression: Both sets were good – snappy and fresh. Yet the carrots from the natural soil were somehow a little more ‘carroty’. Were we tasting a significant difference in nutrition?

We sent a very small sample of those carrots to Logan Labs in Ohio. The data came back indicating that indeed, the carrots from the field were generally a little more nutrient dense. On the other hand, the carrots that came from our specially tweaked rooftop mix tended to beat USDA averages pretty handily. Aha!

It’s worth noting that the nutritional difference between the field-grown carrots and those from the rooftop mix isn’t necessarily indicative of a deficiency in the rooftop mix; We used a lot of biochar in our rooftop mix and very fresh biochar has the potential to compete with crops for nutrients. (feel free to email me if you want more details on that). It’s likely that some of this nutrient competition was happening and quite possible that if the same crops were grown in the same mix next season – after the biochar were more fully charged – the resulting carrots would be nearly identical to what came from the field. As it was, the carrots from our rooftop mix were very similar to the field-grown carrots, despite our competition being one of the best fields around. It’s also worth noting that the food grown in our mixes isn’t necessarily representative of the food grown on rooftops elsewhere. Just as there can be dramatic differences in the quality of foods from different feilds (see the gulf between Stone Barns carrots and USDA averages) there can be big differences between rooftop farms. And lastly, this was a small sample from a single crop and may not be representative of a larger sample set or different crops. We’ll have more data on lettuce and beets soon.

In the table below, you’ll find the numbers from the carrots grown in the natural field soil (left-most column) next to the numbers from the carrots grown in the rooftop mix and lastly the USDA averages for fresh carrots.

A few interesting things about all of this:

  1. This indicates that rooftops, if very thoughtfully managed, can produce food that may be superior in quality to some of what is found in local bodegas, grocery stores, schools, etc. If that’s the case, then NYC’s 38,000 acres of rooftop may be able to make a meaningful contribution of high-quality, fresh produce in some areas.
  2. It tells us that we have some more work to do to close the gap between the best field-grown foods and the best roof-grown foods.
  3. How cool is it that the data confirms the sensitivity and accuracy of our taste buds in perceiving nutrition?

Regarding the location effect, Ray was on the ball with that one. Check out the beets below. The big fellas were grown in the rooftop mix in pots at Stone Barns. The wee lads were grown in the identical soil mix in pots on the roof of the trial site in Brooklyn.

Our hunch is that the diminutive size of the beets grown on the roof has to do with our struggle to properly irrigate up there. A full-scale rooftop farm would have a continuous layer of “soil” rather than a series of pots. In that arrangement conditions would be similar to a natural field in terms of moisture and temperature and not as grueling as they were on our hot, silver roof.

The carrots and beets will be analyzed by Dr. Michael Rutzke, Director of CNAL – the Cornell Nutrient Analysis Laboratory . He’s been experimenting with some very cool, new protocols for nutrient testing. We’re excited that our samples will be part of his effort to get his new systems up and running. More on that to come.

Meanwhile, the next round of research is already in the works and considerably more grand than the first. We are going to get to the bottom of the nutrition gap and grow some amazing stuff. We’re also going to look into the specifics of the environmental benefits of rooftop farms.

Stay tuned for more on that…and tales of from our journey to create a truly massive rooftop farm in NYC.

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