Wonder why your lawn won't grow? There's a dirty little secret about western King County soils.

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Another spring, another lawn disaster: bald spots, brownouts, unrequited love.

A look below the surface with a University of Washington soil scientist explains why. Called Alderwood — a deceptively sylvan name — the type of soils beneath yards supplicated by countless weekend warriors all over the city and beyond are actually as tough as the glaciers that created them.

Only pavement sheds water with more rapidity and disdain than a lawn on this region’s grudging, implacable Alderwood soil, stripped of its top layer of decomposing vegetation and compacted by development. Consider this: When ranking runoff potential, the state Department of Ecology puts lawns one step above asphalt.

The only way to grow a good lawn on this stuff is with heavy additions of water and fertilizer. Which, among the greenerati of Seattle, makes loving deep, plush green grass about as PC as smoking or tanning.

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After all, stormwater runoff is one of the biggest threats to the ecology of the Puget lowlands. Runoff scours and erodes creeks, and carries pollutants — including lawn chemicals — to the waters of Puget Sound. And runoff squanders precious rainwater instead of storing it in the soil.

The properties of Alderwood soil were established when glaciers advanced as far as Olympia and the Puget Lobe of the Cordilleran Ice Sheet covered the Puget Sound area 13,000 feet deep. Like massive, heavy plows, the glaciers moved enormous amounts of gravel and rocks of all sizes and left the material behind as they crept north to south. Their grinding pressure also compacted everything in their path.

As the glaciers retreated, they left their mark: a hardpan about 30 inches down of impervious material, overlain with soil made since by time and nature.

But as this upper layer of soil is disturbed — the native forest and other vegetation cut, and the soil stripped off or worked over by heavy equipment — that’s when the trouble starts.

Already naturally prone to runoff because of the underlying impervious layer, Alderwood soils, once stripped of decaying vegetation or otherwise abused, can shed water as defiantly as plastic — and make a lawn thin like a midlife scalp.

But Robert Harrison, soil scientist at the UW School of Forest Resources, found in experiments at test plots at the Center for Urban Horticulture that 4 inches of organic material, dug into the top layer of the soil to a depth of about 8 inches, doubled the soil’s ability to hold water and greatly slowed runoff. Compost will work; so will leaves, pine needles, wood chips or other organic material, shredded and mixed into the soil.

At his plots, he mixed compost two-to-one into the top 8 inches of soil. In a visit to his test plots in late May, the grass growing on unimproved soil was already a sorry sight — thin, patchy and stalling out. But plots improved with compost were lush and deep and soft. Even foraging geese know the difference, selecting them over the control plots, Harrison said.

He found the improved plots needed no fertilization. He fed the control plots — and 72 percent of the fertilizer ran off the site in the first rain.

Much of western King County can be considered similar to the control plots, or worse, Harrison said. About 54 percent of the soils in western King County, and much of its developed area, is built on Alderwood, Harrison said. After all, Alderwood’s substratum provides a rock-solid building platform just right for development.

At a residential development near Redmond Ridge, Harrison knelt and peeled back a grass strip by the side of the road as easily as if unzipping it. The subdivision was built before present county codes applied, requiring developers to retain or improve soils around the building and road footprint, to restore at least some of the native landscape’s hydrologic function.

“That fluffy surface material was not stable, and it was easier to just bulldoze that,” Harrison said. “In a lot of yards, the soil was wholesale removed.” Decades have passed, but the grass never penetrated the ground, growing instead in a horizontal mat sustained only with heavy fertilization and copious watering. Even trees ran their roots along the surface of ground too hard to penetrate.

The landscaping strip looked green — but in actuality was anything but. “In terms of runoff, it’s maybe not quite as bad as concrete,” Harrison said of the strip. “But it’s close.”

Of course, it’s possible to have too much of a good thing. Bonnie Geers, vice president of community development for Quadrant Homes, remembers some of the early results at Redmond Ridge, the first housing development at which King County imposed new soil codes.

Before long, the company discovered trees drowned when compost sucked up water in landscape strips with glacial till below and asphalt on the sides.

The county has since revised its codes to require less added material.

County original requirements also had the company in the position of trucking topsoil to the site, and trucking away stockpiled native soil removed during development.

“The carbon footprint had to be worse than the environmental gain,” she figured.

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com

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