To be truly green, you have to get down and dirty, it seems. As Baltimore officials begin to tackle the polluted runoff fouling the harbor and the Chesapeake Bay, they are turning to a technique long used by farmers.

It’s not enough simply to strip off some of the city’s ubiquitous pavement and plant grass. The ground beneath that asphalt and concrete often remains as hard and impervious as the man-made surface it’s replacing. And the rainfall will just keep running off – washing fertilizer, pet waste, oil and other contaminants into storm drains and nearby streams.

So to make that urban hardpan act more like a natural sponge and cut down on storm-water pollution, city officials are trying out the agricultural process known as “sub-soiling.”

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At Yorkwood Elementary School in Northeast Baltimore this week, a tractor plowed deep into a half-acre patch of playground that until recently had been covered in asphalt. The farm vehicle towed a claw-like contraption with long curved blades that slice far into the packed earth.

“That’s what Freddy Krueger would have done if he was trained as an agricultural engineer,” quips Stuart S. Schwartz, a senior scientist with the Center for Urban Environmental Research and Education at the University of Maryland Baltimore County. Schwartz, who’s been studying how to reduce runoff for years, has a contract to monitor the Yorkwood project.

The ground behind the 51-year-old school probably hasn’t seen sunlight in decades. It’s so tamped down, though, that when Schwartz and his assistant, Brennan Smith, test the dirt’s capacity to absorb moisture, it takes almost an hour and a half for an inch of standing water to soak in. Any but the gentlest of rains would just run off, though Schwartz says it’s not the most water-resistant soil he’s seen.