# Soil moves through space and time *one thing about soil* ## Erosion happens in three steps: detachment, transport, and deposition The surface of the Earth is a dynamic place. Even though soils form over hundreds to thousands of years, this doesn't mean that individual soil particles always stay put. The natural movement of soil materials leads to the unique mixtures of soils and rocks that we find out in nature. Erosion is the process by which sediment and soil particles are moved over space and time. There are three components of erosion: - **detachment**: when particles are lifted from a body of rock or soil - **transport**: when particles are moved from one place to another by the aid of wind, water, gravity, and organisms - **deposition**: when particles settle in a new place at lower elevation. Let's imagine you are out picking wild blueberries to make a pie. Pulling the berries off the vine would be the process of detachment. Carrying the berries from the field and into your kitchen in whatever cute little basket you happen to have - that's transport. Dropping the berries into the pie crust, baking, and then enjoying a slice... deposition. Maybe you decided to pick some blackberries along your walk and end up making a mixed berry pie. This happens with sediments and soils, too! Once detached, loose particles are mixed together as they are transported across the surface of the Earth. Consequently, soils are often forming from mixtures of particles with different sizes, shapes, properties, and origins. ## Erosion leaves behind features we can observe We can observe the consequences of soil erosion all around us. Water is an important agent of erosion. Scientists categorize soil erosional features that are the result of moving water in three groups based on the size and intensity of the process. **Sheet** erosion occurs when water moves slowly and more or less uniformly across the surface of soil. Along the way, water gently picks up and moves particles. Have you ever seen a sheet of water flowing across a concrete sidewalk? It's kind of like that, only slower. Sheet erosion can leave behind pedestals like those in the photo below. Small rocks that were too heavy for the slow paced water to move stabilize the soil particles underneath. Meanwhile, water is gently washing away the particles around each rock. Over time, we are left with little columns of soil topped with rocks. Look closely and you'll start seeing them everywhere. ![[sheet erosion SLO.jpg]] Once a groove in a soil surface forms, water will preferentially flow into the groove and pick up more soil particles as it goes. Eventually, this process will create a channel that grows over time. **Rills** are the erosional features that form from this process. We can differentiate rills from sheet erosion because they are larger in size and have a clear channel of flowing water. Here's an example of parallel rills in a new neighborhood development. Construction crews were trying to stabilize the slopes that have been exposed during the groundworks process by seeding grasses. In this case, it wasn't enough to prevent rill erosion during a series of heavy rainstorms. ![[rill erosion SLO.jpg]] Over time, rills can grow in size from successive erosive events (heavy rainstorms). This can result in the formation of a **gully**, like the channel seen in the photo below. A gully is much larger than a rill. You can walk over a rill, but not a gully. Once formed, gullies are difficult to reverse because water continues to cut back at the surface during each rain event, leading to more soil losses into the waterway. Notice how the darker soil surface horizons have eroded more quickly than the lighter colored subsurface. This happens because there's more clay in the subsoil that holds the soil particles together and resists detachment from the profile. I took this photo during a particularly intense period of rain. The soil was saturated and you can see water ponding at the surface. Any water that doesn't infiltrate into deeper soil layers will instead flow across the surface, following the pull of gravity and (in this case) further eroding away the soil along this gully. ![[gully erosion.jpg]] ## Erosion is a natural process, but humans are accelerating it Erosion is a natural process that shapes Earth's surface over time. Mountains, hills, and valleys are carved by erosion. Floodplains, alluvial fans, and deltas form when sediments are deposited by water. Landscapes form through these combined processes. If it wasn't for erosion, we wouldn't have the marvelous variety of landscapes we experience today. Even so, accelerated erosion caused by human activities is one of the biggest threats to soils. We accelerate erosion by disturbing vegetation, tilling up hillsides, and overgrazing fields. We move soils around as we build infrastructure, leaving soil surfaces bare and exposed. It's estimated that the rate of human accelerated erosion can be anywhere from [100-1000 times higher than natural processes](https://www.fao.org/about/meetings/soil-erosion-symposium/key-messages/en/). Given that soils form over centuries, any soil lost from a site by erosion is soil we can't get back in our lifetime. Keeping soils covered with vegetation helps slow down water as it travels across the soil surface and allows more water to make its way deeper into the soil horizons. Vegetation also helps stabilize soil against strong winds that might otherwise pick up and move soil particles. While the process of erosion is natural and cannot (and should not) be completely stopped, we need to conserve and protect soils from excess erosion. ## Take home message Erosion is the natural process that describes the detachment, transport, and deposition of sediment and soil particles across the surface of the Earth. Together, these processes help form the landscapes we admire and the variety of soils that exist in nature. We can find evidence of soil erosion if we bring our attention to the edges of soils - the places where soil, water, wind, and gravity meet. When we stop to notice erosion of the soils around us, we begin to see the impact of our activities as humans. Soil conservation begins with noticing these patterns. #### Postscript > This essay is part of [[One thing about soil - an educational series]] created by Dr. Yamina Pressler. The essay was originally shared to my [newsletter](https://buttondown.email/wonderofsoil) subscribers. *created November 18, 2023*