Background: Many factors influence nutrient density. A key factor is the soil in which a crop is grown. Or as the new axiom goes: You are what you eat eats (e.g., if a soil is very poor in Calcium, how can the leafy green grown in it be high in Calcium?). What does this mean for rooftops where light-weight, engineered “soils” are used to accommodate strict load limits?

Rooftop growing media are great at doing what they do, but agriculture falls way outside of thier intended use. That said, we sent a sample of Rooflite® off to an agricultural lab and were surprised to discover that it is incredibly well balanced from a food-production standpoint (at lease mineralogically).

Looking for information about the nutritional quality of foods grown in rooftop media turns up very little. The makers of Rooflite report that to their knowledge no one has investigated the issue. It’s possible that highly nutritious food is already being grown on existing rooftop farms. We just don’t know yet.

Another concern is the tremendous amount of energy required to produce most rooftop growing media. Generally, their light weight results from heating the mineral portion to 2,000-3,000 degrees at which point they expand, becoming lighter per volume.

It occurred to us that a natural, carbon negative material called biochar might a good substitute for at least a portion the “expanded” materials.

  • Biochar can be made from just about any carbon-based material. This means it can be made locally, just about anywhere.
  • It drains well but also retains a good amount of moisture.
  • It is extrememly light weight.
  • Producing biochar generates usable energy rather than consuming it.
  • It has a high capacity to hold nutrients.
  • It can be made to manifest a wide range of properties.
  • It can sequester carbon over hundreds, if not thousands of years.
  • It provides great habitat for soil microbiology.

Caveat: For anyone else thinking about doing this, it’s worth noting that the standard rooftop media are complete products. They have structural longevity and comply with strict FLL guidelines for green roofs. Biochar is fragile and may require a raised bed system to protect against foot-traffic. Furthermore, the term biochar is used to describe a material that can have an incredible range of characteristics depending on the feedstock used to make it and the way in which it is processed. Johannes Lehmann, a leading biochar researcher, points out that with the same feedstock one can create a char with a pH as low as 3 and as high as 12. If you plan to incorporate char into your growing, make sure you know what you are getting or making and do some research about how to apply it without damaging crops.

But again if done right incorporating char has tremendous potential to increase yields, reduce irrigation, improve soil texture,  retain nutrients, sequester carbon… It’s definitely worth investigation.

Let’s get growing shall we?



Objectives: To determine the nutrient density of foods grown in rooftop media that maximize the use of biochar.


Mixes: The mixes will contain three major constituents:

  1. Rooflite
  2. Biochar “B100” from New England Biochar
  3. “Garden Primer” compost from the Stone Barns Center for Food and Agriculture

Soil testing: All three constituents have been tested at Logan Labs using their standard soil test – a Mehlich III (M3).

Designing an amendment program: Using results from the 3 Logan tests, excel was used to estimate results of a hypothetical M3 test of Rooflite/Biochar/Compost mixed at a ratio of 10:7:3. Using formulas from Michael Astera’s book, The Ideal Soil, deficiencies were identified in major and minor elements.

Biology: Because Rooflite and biochar are both sterile, the mixes were inoculated with biology through the addition of compost and also compost tea. Actively aerated compost was made using a 10 gal. brewer at Brooklyn Botanic Garden using Stone Barns Garden Primer and microbial foods.

Amending: The biochar and compost will be mixed at a ratio of 7:3 by volume. This mimics the ratio of Rooflite’s mineral and organic components which are mixed at a ratio of 7:3 by volume. These two mixes will subsequently be referred to as Rooflite (RL) and biochar/compost (CC).

RL and CC will were amended with different minerals depending on their deficiencies.

Both RL and CC were amended with the same proportion of Agricola’s Best for biological innoculants, humates and trace elements.

Minerals were mixed with each other and then incorporated dry into the RL and CC.

Both RL and CC were inoculated with compost tea.

Both materials were allowed to sit for two at least two weeks to allow the biology to develop and incorporate the soil amendments.


  1. RL unamended
  2. RL/CC 9:1 amended
  3. RL/CC 4:1 amended
  4. RL/CC 7:3 amended
  5. RL/CC 3:2 amended
  6. RL/CC 1:1 amended

Crop testing: Crops representing four plant families will be grown in each mix. (All seeds from Fedco Seeds).

  • Solanacea (Tobacco family): Green Zebra Tomato
  • Compositae (Sunflower family): Sweet Vaentine Lettuce
  • Chenopodiacaea (Beet family): Detroit Dark Red Beet
  • Brassicaceae (Cola family): Lacinato Kale

Tomato, kale a lettuce will be started in flats in a potting mix from Stone Barns and then transplanted. A fall succession of lettuce may be direct seeded.

At least 5 replicates of each crop will  be grown in each medium.

The following observations will be made in the field:

  • Germination rates for those things that are directly sown. (Separate germination tests may be conducted).
  • Growth rate
  • Yield
  • brix
  • Pest, disease and drought tolerance if those condition present themselves.

As crops mature sample will be harvested and taken directly to the USDA food laboratory at Cornell university. Though specific tests remain to be determined, they will likely include major and minor elements along with a limited phyto-nutrients.

Publication: Results will be publicly available on this website and possibly others, such as IBI.

We hope that the information gained from the trials will help establish of a new wave of more eco-friendly rooftop farms capable of growing food that exceeds USDA averages for crop nutrition.

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