There are many topics in soil science, and soil and water conservation that are worth a thorough discussion. One is soil erosion. Erosion is a vast topic so it will not all be addressed in one post, or even one blog for that matter. However, this blog post is the first of many in a series of blog posts that will address different aspects of soil erosion. First, I’ll define erosion and discuss why it’s almost always a bad thing.
The Soil Science Society of America defines erosion as follows:
erosion (ii) the detachment and movement of soil or rock by water, wind, ice, or gravity
I’ll stick with soil erosion by water and wind for this post.
The following descriptions of water and wind erosion was paraphrased from Chapter 14 of Brady and Weil (2010):
Water erosion is a three-step process including steps 1) detachment, 2) transport, and 3) deposition. In a rain storm, falling rain drops impact the soil surface with tremendous force. When that occurs, soil particles are sent flying (detachment). When rain falls at a faster rate than what can infiltrate (enter) the soil, rainwater begins to pond on the surface and eventually flows downhill. The soil particles that were sent flying can eventually land on this flowing water and move with it downhill (transport).
If the flowing water has enough force it can also cause further erosion. If the water is flowing as a sheet of water (think water flowing smoothly over the windshield of your car) it is termed “sheet erosion”. If the water gathers together into tiny channels it is termed “rill erosion”. If the water collects even more, and erodes deep into the soil (on the scale of feet or meters) it is termed gully erosion. Most water erosion occurs by the impact of the rain drop.
Eventually the water will slow, and some soil particles will fall out of suspension and collect (deposition). The transported soil may travel just a couple of feet, or travel thousands of miles to be deposited in the ocean. Just think of why the Mississippi River is called “The Big Muddy”. It is because of all of the eroded soil that is in suspension in the river, and is being transported toward the Mississippi delta, and the Gulf of Mexico where it will eventually be deposited.
Wind erosion, like water erosion, involves soil detachment, transportation, and deposition. Just the movement of wind is enough to detach some soil particles. Once the air is full of soil particles the wind becomes much more abrasive and can break down soil aggregates (detachment) and add more soil particles to the moving air. Once those soil particles are detached, they move by suspension, saltation, or creep. Small particles move by suspension, which is where the soil particle is suspended by the moving air. Saltation transports largger particles like sand. The particle is too heavy to be suspended for long distances in the air, but instead, the particle bounces along the surface of the soil in the direction the wind is blowing. The largest particles move by creep when they roll along the surface as they’re pushed by the wind, or as they are bombarded by particles being transported by saltation. Eventually the particles are deposited somewhere when the moving air slows down, or in an area that is protected from the wind.
Wind erosion has been a topic of interest recently in the news. Here is a video of a sandstorm that engulfed the City of Phoenix, Arizona in soil particles eroded by wind (a dust storm)
Also, a huge event in American history involved wind erosion – the Dust Bowl. The Dust Bowl will be a topic of several future blog posts as well so I won’t go into too much detail on that chapter of American history today.
The Good and Bad of Soil Erosion
Erosion has good and bad things associated with it. It is bad when a farmer loses the best, most fertile soil on his land (near the surface) to erosion because this eventually makes his or her land less productive. Erosion by water is bad because it can bury benthic habitat in streams (where insect larvae like to live – among the small pebbles in the stream bed) which is a crucial part of the food web. Erosion by water is also bad because transported sediment can carry nutrients such as phosphorus, which can contribute to algal blooms, fish kills, and the dead zone at the mouth of the river systems like the Mississippi. Wind erosion has many negative health effects on humans, can damage our machinery, and can be harmful to plant life as well (think of what a sand blaster would do to a blade of grass).
Erosion also has a good side. Wind erosion and deposition contributed to the development of some of the most productive soils in places like Iowa. Loess (wind blown sediment) was eroded from the Missouri River valley during glaciations and was deposited all over the region. This loess is mostly silt, which stores a large amount of plant available water, among other desirable soil properties. It has also been found that soil particles eroded by wind in the Sahara blow all the way across the ocean to fertilize the nutrient-poor Amazon rain forest.
My lunch break is over, so that is all for now. I hope you found this quick summary of erosion interesting. Look for future posts on this topic that dive into specific aspects of erosion.