Beneath every square meter of soil run 15 to 20 miles of fine plant roots and perhaps 10,000 miles of filament-like fungal roots. Between 10 million and 10 billion microbes inhabit each cubic centimeter of soil. The right kind could decompose motor oil and gasoline, which are common in urban runoff. A cubic centimeter of humus, the fine, dark grains in soil, contains 2,000 square meters of surface area that binds to toxic heavy metals, like mercury.
Standing beside a 300-square-foot Bronx green street he helped design, Mankiewicz expounds on the value of living soil. Normally, the city has to treat runoff before releasing it into waterways. But this patch of green could absorb much of the 50,000 gallons of runoff generated here yearly. (Soil sensors will measure just how much.) Multiply that by some 2,000 green streets now in existence, and the result is some 100 million gallons of water not going down storm grates.
Over the years, Mankiewicz has emphasized the critical role of simple dirt, says Robert Alpern, an adviser in the city’s Department of Environmental Protection under former mayor David Dinkins: “His major contribution has been to sensitize everybody in the bureaucracies to the potential of soils as a filtration and infiltration medium for storm water.”
On St. Simon Stock Elementary School’s rooftop garden (finished in 2005), Mankiewicz explains the greatest hurdle to greening the city’s 35 square miles of rooftops: The average cubic foot of dirt weighs between 100 and 120 pounds, around three times the load for which most roofs are engineered.
So he designed his own soil, substituting ground polystyrene (Styrofoam) for much heavier sand and clay. The result, patented as GaiaSoil, weighs 10 pounds per cubic foot and can hold twice its weight in water. The next problem: New York gets about 40 inches of rain yearly, but it doesn’t come evenly. A recent dry spell has left some of the Little Blue Stems, one of perhaps 25 native species atop St. Simon’s roof, looking forlorn.