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Soil In The City

by Jake Farley

When people think of Manhattan, they don't think of soil, and indeed, there is very little of it on the city's surface.

According to the latest USDA Soil Survey Map, 80% of Manhattan soils are classified as "pavement and buildings," not your usual soil type. The rest is "glacial till" (sediment dropped by the mile-thick glacier that scraped the city clean about 15,000 years ago) and "fill" brought in by horse and wagon.

The man who charts this natural history of New York soils is Richard Shaw, soil scientist and project leader for the New York City Soil Survey. He works for USDA-Natural Resources Conservation Service and he's mapping the town.

The glacier scraped northern Manhattan hard, exposing bedrock such as the rocky outcrops in Central Park. The northern avenues parallel the Hudson River following the trend of the city's bedrock from southwest to northeast. Some avenues like 125th Street are in "valleys," or fractures, in the rock.

"More often than not, we don't know the past use history or if it's safe to touch with our hands."

In southern Manhattan, the glacier dumped so much till where Battery Park is now, that bedrock there is almost 90 feet below sea level. That means the foundations of the buildings are below sea level too. Retaining walls hold back the bay and river water.

Marshes once fringed the entire circumference of Manhattan and they are now all filled in and built upon. Former glacial meltwater streams, drainage channels that once rushed under the thick ice, were filled up to make the land level enough to build on.

But water still runs under the city. Freshwater steams are redirected through culverts and buried, relegated to life below earth as sewers. Who knows what life these once-fresh brooks now contain?

Central Park is one large area that contains a lot of "fill" that was brought in from New Jersey. Studies have been done on the park's soils because they are relatively "undisturbed." The results are interesting.

Wooded areas in the park show atmospheric deposition of lead, copper, zinc, mercury and nickel over a long time. "The metals are higher on the surface. They haven't been moving down through the soil profile," says Shaw. "Organic matter forms a tight bond with metal. The soils are pretty acid, so organic matter binds tight, so the metals are not going anywhere. They are not soluble." Heavy metals are immobilized thanks to soil's organic matter, itself deposited by plants and organisms over time.

Shaw says the woods in Central Park are the stablest landscape to see a record of atmospheric deposition. Once, a team of scientists studied the lake-bottom sediments in Central Park that showed heavy metal deposition from the air over a long period of time. They found the same results, and because they were able to date the sediments, they found that municipal solid waste incinerators were the source of the metals.

Questions remain. If heavy metals fell from the air over Central Park, haven't they fallen in other areas where buildings are? If the buildings and streets are impermeable, what happened to the metals?

Shaw suggests they probably washed away into the rivers and bay.

Another joy that city soil contains is "cultural artifacts," or garbage. "When a site is redeveloped, it costs a lot of money to cart junk away. Most people leave a lot. It's mixed up with the soil," says Shaw. "Because of this construction debris, road salting and concrete plaster, urban soil has a slightly higher pH than non-urban soil."

Shaw says having the soil tested is a good idea because gardeners should know what they're dealing with. But unless you know the previous land use history and what kinds of pollutants the soil could possibly contain, testing is prohibitively expensive. He suggests bringing in soil if the plan is to grow vegetables in the garden.

Shaw and his team from the Natural Resources Conservation Service spend a lot of time in NYC soils. "More often than not, we don't know the past use history or if it's safe to touch with our hands." He suggests this simple test: smell the soil, look for living plants and insects, and inspect debris. A bad odor, no life and a lot of junk mean an increased chance of contamination.

One problem characteristic of city soil is compaction - the result of squeezing the air out of the soil. Trampling by feet and wheels, especially on wet soil, causes loss of air space - great for buildings, bad for plants and microorganisms.

What Compaction Does: inhibits root growth; reduces water retention; reduces the amount of available water to a plant; reduces populations of soil organisms; reduces plant growth.

Symptoms to Watch for: poor plant growth; digging down and finding that roots grow only horizontally; difficulty in probing the soil.

How to Prevent Soil Compaction: only work in dry soil; add topsoil for seeding grass; stay on designated foot paths in gardens.

How to De-compact: add topsoil or replace with topsoil; increase organic matter; use cover crops; use mulch, compost, manure; aerate turf; use an air compressor with a metal tube around tree roots; water frequently and gingerly.
Courtesy of Richard Shaw, soil scientist, NRCS

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