With more people moving to cities than ever before, many are looking to urban farming as the future of sustainable agriculture. But growing crops in the city isn’t as simple as dumping soil on a rooftop and planting some seeds. If farmers were to use real dirt in their rooftop gardens, they’d likely end up with dead plants, leaks, soil compaction, and in extreme cases, ceiling collapse.

That’s why instead of dirt, rooftop farmers cultivate their crops in something called “growing media.” This futuristic-sounding soil substitute is carefully engineered to mimic the properties of natural dirt and improve them as well. It’s made from a blend of minerals and organic matter, which could include rice hulls, ground coconut husks, pumice, or sand.

Annie Novak, co-founder and farmer of Brooklyn’s Eagle Street Rooftop Farm, tell Atlas Obscura that compost blends can vary greatly. The growing media she uses at her rooftop farm is made from mushrooms, peat moss, and additional components like perlite (a mineral made by air-puffing volcanic glass) and vermiculite (a mineral particulate).

In order for growing media to be suitable for rooftop gardening, it must first pass a strict set of regulations. In addition to not containing any organic soil, all growing media must be sterile, stable, and capable of properly retaining and draining water while providing enough air for the plants to breath. Nutrients, salt content, and pH levels must all fit specific criteria, and most importantly, the growing media needs to be heavy enough to resist wind and water while not so heavy that it compromises the structure of the roof.

Today there are only six rooftop farms in New York City, but there's plenty of potential for the city's future. In just New York City alone, there are approximately 1 million buildings with a total of 38,256 acres of roof space. That's a lot of rooftops waiting to be filled with fake dirt.

[h/t: Atlas Obscura]