Technically, topsoil is the natural top layer of soil.

It has been created over thousands of years by weather, climate and decaying organic matter. Topsoil is usually about 6 to 8 inches deep and contains more organic matter and microorganisms than the soil beneath it.

Our native topsoil isn't the dark, crumbly, rich soil found on forest floors. We have a very different climate and, as a result, very different soil.

It's recommended to scrape topsoil off of a building site and stockpile it with the intention of returning that topsoil. This procedure is often not followed exactly as prescribed.

Too often the topsoil is not removed, leaving it vulnerable to severe compaction. Once a structure is complete, builders may bring in something that's called topsoil, but it's usually not native -- it's often a designed topsoil or a topsoil mix. This may contain a variety of three or more different materials such as sawdust, compost, manure, biosolids, sand and soil. There are no legal standards regarding what can be sold as "topsoil."

WSU Extension Specialist Linda Chalker-Scott says these topsoil mixes often are composed of 15 percent organic matter by weight. An "ideal" soil contains about 5 percent. Local garden soils have typically tested at less than 1 percent.

Chalker-Scott says that in a permanent landscape, the organic matter eventually (in 10 years or less here) breaks down, causing the soil to settle and compact. This leads to the trees, shrubs and grass sinking to below grade. She says it's better to use the soil that is in place and mulch trees and shrubs with wood chips.

The other problem with these topsoil mixes is water penetration. When the topsoil mix is simply laid over the ground, the soil is usually not broken up or loosened. It's hard for water or roots to penetrate highly compacted soil. The soil should first be loosened by tilling or ripping to help improve water movement into the soil.

Mixes also can present water movement and drainage problems, even when the lower soil is disturbed and loosened. This is because water easily moves through the typically coarser texture of the mix but doesn't enter the finer soil beneath it as quickly. This leads to what is called a "perched" water table. This creates soggy conditions and can lead to root rot.

There are no easy solutions. If you must bring in new topsoil, the compacted soil should first be loosened. The topsoil should then be tilled into the top layer of the existing soil to avoid the interface problem of two different textures. Better yet, the compacted soil should be loosened and native topsoil returned to the site. No significant amounts of organic matter should be added.

If you're buying topsoil, check out what you will be getting before it's delivered. Ask the seller what the topsoil contains and ask for the producer's test data regarding pH, salt level, nutrient levels, organic matter content and texture. Also, find out if the soil has been screened to remove rocks.

* Marianne C. Ophardt is a horticulturist for the Washington State University Extension Office in Benton County.