Rock Doc: Soil erosion has posed threats throughout history

This past fall I had to have a hole dug in my front lawn, part of a project to fix the sewer pipe that runs from my house to the city’s sewer line in the street. The work is long since done and the hole filled in. But every time it rains loose dirt is moving downhill into my lawn. It’s surprising just how much soil moves now that it has a chance, an example of what erosion can mean even on land that isn’t particularly steep.

Whenever soil is on a slope it moves downhill. If there is a network of roots from vegetation helping to hold the soil in place, the total movement may be small, but it always occurs.

When we remove all vegetation from a patch of earth, the rate of erosion jumps. The most common way we have of removing vegetation on a grand scale is through agriculture. In other words, plows disturb soil and make it particularly subject to erosion.

I got to thinking about the broader context of soil erosion and what it means for civilization recently when I heard a talk by University of Washington professor David R. Montgomery, the author of “Dirt: The Erosion of Civilizations.” Like me, Montgomery started life as a student of geology and was taught the interesting things in the world were the rocks under the soil. But because he has a naturally inquiring mind, Montgomery came to understand the significance of soil for human civilization. That got his attention and it’s worth our cogitating on for a moment.

If you look at all of world history, you can see that societies rise and fall for a number of reasons. Climate change, invasion by barbarians, and other factors can create “regime change.” But if you look at places like Greece, you can argue that civilizations that exist outside major river floodplains seem to run their course in roughly similar amounts of time. Why should that be so?

One reason may be soil loss. In earliest Greece, only the valley bottoms were farmed. But a while after that, as population in the region expanded, people started to till fields that were on slopes, up the sides of valleys. Tilling the soil like that leads to erosion rates of about 1 millimeter per year.

That rate may not sound fast, but it’s significant for a geological process. Here’s the important point: soils on hillslopes are often half a meter to a meter thick (that’s about 1.5 feet to just over 3 feet). It takes 1,000 years to erode away a meter of soil at the rate of a millimeter per year, and 500 years to erode a meter of soil at that same rate.

In other words, going all the way back into antiquity, societies may have been limited over time because farming exhausted the soil on which the people depended.

The take away lesson? Soil conservation matters. And that’s as true now as in the ancient past.