Let’s talk soil. If you ask a farmer and a green roof expert how to mix the ideal batch of soil, they will give you completely different answers. Farmers are interested in high levels of organic matter, which help to retain soil moisture and provide nutrients for the plants. Green roof experts, by contrast, prefer soil that drains water efficiently and is low in organic matter (generally between 8% and 10%). This low organic matter helps green roof soil (or “media,” as it’s called in the industry) to remain lightweight, which is crucial for most buildings.
So is there a happy median? How do you build lightweight soil that contains enough nutrients for agricultural crops? The answer is not so simple, and in fact, there are quite a few differing opinions on the subject matter.
Tactic 1 | Some farms, such as Eagle Street Rooftop Farm in Brooklyn (rooftopfarms.org), build soil every year. Their soil is high in organic matter and therefore heavy, but luckily the old manufacturing building below can sustain the heavy load. This type of row farming mimics that on the ground plane, where weight is not as much of an issue.
Tactic 2 | Some rooftop farms and gardens utilize lightweight planting media in their raised beds or containers. This type of soil is engineered to contain more mineral content, and may contain elements such as expanded clay or expanded shale. This type of production relies on heavy nutrient inputs, usually in the form of fertilizer or compost tea.
Tactic 3 | The lightest soil is no soil at all. Certain rooftop hydroponic operations grow crops in nutrient solution, which can circumvent the soil discussion altogether. In the U.S., rooftop hydroponic farms are situated in greenhouses, where the environment can be precisely calibrated. The weight in these systems is a function of the greenhouse and the equipment inside.