Silt vs. Soil

“A nation that destroys its soil destroys itself.”

Franklin Roosevelt wrote those words in a letter to all State Governors in support of the act that created Soil & Water Conservation Districts. That was in 1937 and the nation had just passed a series of laws in response to the devastation caused by the Dust Bowl.

Eighty years later, our understanding of the strength and fragility of native soils is greater than ever. Yet erosion will always be a concern -- even here in The Intertwine.

We know that soil erosion occurs when soil particles detach and move around, usually caused by water, wind, gravity and even ice. And we know that erosion can be devastating, both to the natural landscape and to our homes -- compromising foundations, clogging drains, and dislodging whole gardens.

Lastly, we know that our native soil is precious. It takes about 200 years to form one inch of soil!

But you might be wondering, why is erosion a concern here in the fertile Willamette Valley -- sheltered from Gorge winds, hundreds of miles from from drought-stricken California and half a continent from the locus of the Dust Bowl?

One word: silt.

In Portland, the most erosive areas are in the West Hills. Most of the soil there was formed during the Ice Age following events known as the Missoula Floods, which would have covered Portland is as much as 400 feet of water carrying soil and rocks from distant parts of Washington, Idaho and Montana. After the flood water receded, it left mostly sand and silt, which was then carried by wind up into the West Hills.

Silt causes two main problems.

First, silt is the most erosive type of soil, its fine particles easily carried by wind and unlikely to bond chemically, as do small clay particles. This windblown matter, called loess, dominates the soil of the West Hills.

Second, silt can bury preexisting soil, forming a layer that engineers call a slip plane. Rain and irrigation water seeps through the newer soil, but stops at the slip plane, blocked by less permeable older soil.

With enough water and not enough support (from cover, roots, or other erosion control measures) a slope can fail. These types of landslides have occurred for thousands of years. More recently and close to home, we've seen the media pictures of homes sliding into each other in the West Hills.

So, how do we combat loess and the slip plane factor?

One of the best ways to stabilize both slope and soil is to plant grass, shrubs and trees. Here are just a few reasons why vegetation is a soil savior's best friend:

• Root systems, and the fibrous mycorrhiza fungus that attach to them, literally hold the soil in place.
• Roots can also create holes, known as pores, which allow water to seep into the ground, rather than pond on the surface and wash soil away.
• Plants pull the water they need from the ground, they help prevent soil in steep areas from getting too saturated and heavy.
• Plant roots also pump organic matter, formed from the breakdown and composting of living material, deep into the soil -- forming a "glue" that holds soil together. This organic matter contributes to a virtuous cycle, holding water deep within the soil  (more efficiently than mulch, compost and other amendments, I might add) and providing nutrients for crops, trees and ornamentals in your garden.

So now that you know erosion could be just a rainstorm away, take a look around your home. Does your native soil need more cover? We're here to help.

Contact West Multnomah Soil and Water Conservation District for more information on soil, erosion, or other conservation practices. Or check out WMSWCD's Soil School -- an April 5th course intended for gardeners and beginning small farmers. Held at Lewis & Clark College, you’ll learn what’s in your soil, how to test and analyze your own soil sample, and determine the best way to amend your soil for your growing needs.