Sludge Watch ==> Don't underestimate value of native topsoil

[Maureen Reilly]

Sludgewatch Admin:
Beware of these replaement 'topsoils' which may be very much more 
contaminated than the original  native soil.  We are seeing farms slated for 
development sludged up ... then the contaminated soils are put back around 
the new homes...to dreadful effect.

..........................................


Don't underestimate value of native topsoil

Published Saturday, April 28th, 2007

By Marianne C. Ophardt, Special to the Herald

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.

http://www.tri-cityherald.com/hg/ophardt/story/8835636p-8736247c.html