What Do the Soil Colors Tell Us?

The stories soil colors tell us vary with the ecosystems in which the soils formed. Some soils are white because the parent material they formed in was white. These soils would have inherited the white color from the parent material. For example, the white dunes and soils of White Sands National Monument are due to the gypsum in the soil. Or the local parent material may be red, white or purple shales and soils forming from them are multi-hued, such as in Dinosaur National Monument.

Some soils may be brown or black because the parent material was decaying organic material such as some wet organic soils in Acadia National Park while other soils may be brown or black because the parent material source are sediments that are brown or black, with very little organic matter in them.

Soils of Redwood National Park are often very red because of intensive weathering in a maritime climate.

Mineralogical transformations (i.e. weathering processes) in soil results in different soil colors because different minerals have different colors.

Soil scientists can measure at what depth wetness or a water table exists is in the soil because of soil color patterns of different mineral types that form in variable wet conditions.

Landscape stability also dictates soil color by preserving soil colors formed from weathering of minerals in the soil horizons. A stable landscape allows soil weathering processes to act longer, developing different colors in the soil. Consequently by using color intensities we can judge land surface age, and make assumptions about landscape stability and past climates. Generally speaking, given a similar time frame, wet and warm climates create different soil colors than hot and dry climates.

Buried horizons in soils usually have darkened, buried A horizons (due to greater organic carbon that was in the surface horizon) which can tell us if there are risks of flooding, landslides, erosion, tsunamis, erosion or earthquakes, depending on where you are in the country.

Many soils develop distinctly colored subhorizons (soils below the surface) because of leaching. This is a prominent process in sandy wet soils such as those in Acadia National Park. Soils formed in volcanic tephra such as those at Crater Lake, Mount Rainier, Hawaii Volcanoes National Park and North Cascades National Parks may have highly contrasting colored horizons due to layering of volcanic ash and also due to leaching.

Contrasting colors in this soil from Mount Rainier National Park are due to leaching of iron and carbon. Water-soluble carbon moves down through the soil and combines with humus and iron that accumulate below in black, humic acid bands over reddish iron bands. Often, a white to light gray layer, mostly quartz, occurs below the organic matter on the surface. This process is evident in this photo at the 20 cm mark on tape. (NRCS file photo by Susan Southard)

How is the Approach to Soil Science and Soil Management Changing in National Parks?

There is growing awareness of soils in the NPS. The National Park Service has made a great effort to complete soil inventories of their lands and have trained staff to use the information. There are some NPS restoration ecologists using soils data in their work, with their efforts mostly focused on infrastructure maintenance, such as road revegetation work.

There are a few park visitor center soil displays at Pinnacles National Monument in California, Big Bend National Park in Texas and Walnut Canyon National Monument in Arizona. There may be a few others too, but soils information has never been the highlight of any visitor center display, nor interpretive program, but the interest is growing.

The effects of high-shrink swell soils on park buildings and roads, and Visitor Centers placed in floodplains and areas of unstable soils should be explored.  The effects of soil erosion after catastrophic wildfires due to historic fire suppression is evident in some parks. These are all land management issues that should involve considering the soils first.

What Is the Top Concern in Soil Management?

Overall, in my view, the top concern in soil management in the NPS system would be the effects of climate change – loss of permafrost, sea level rise and effects of catastrophic wildfires.

There is a dramatic loss of permafrost soils in National Parks and Preserves in Alaska. While we probably cannot do anything now to stop the loss, there are consequences to human health and safety, and loss of infrastructure.

In all coastal parks from Acadia National Park, Cape Cod National Seashore, Gateway National Recreation Area, Assateague Island National Seashore, Cape Hatteras National Seashore to Canaveral National Seashore and Everglades National Park, the coastal tidal marsh soils and dunes play a critical role in protecting these areas (and populations) from shoreline changes and flooding due to sea level rise.

Accelerated soil erosion after wildfires in parks like Santa Monica Mountains National Recreation Area, Sequoia-Kings Canyon National Park, Yosemite National Park, and Crater Lake National Park require progressive solutions to stabilize.

Soils offer the largest terrestrial storage of carbon which is implicated in global warming. By managing park soils for carbon sequestration by minimizing surface disturbances, NPS can contribute to the mitigation of carbon loss to the atmosphere. To me this is an important political point about the ecosystem services soils provide.

Is There Anything Else You Would Like to Share?

This gif image aggregates all the soil color observations soil scientists have made over decades of field work. The gif displays soil color in slices at uniform depths. The gif was prepared by USDA-NRCS soil scientists at the National Soil Survey Center. When all the Munsell colors are displayed in a national map of soil color we can see interesting patterns.

A glimpse of the nationwide 0 to 5cm slice has darkest colors due to near surface organic matter content. The overall map gets brighter as the image slices to deeper soil depth. Noticeable is the bright red swath associated with the Piedmont region of the southeastern United States and the Red Prairies of Texas and Oklahoma and the Arkansas valleys and ridges and Ouachita Mountains, where high iron oxide content hematite is in near surface horizons. Florida soil colors become grey with depth slices, not redder, due to near surface sandy horizons that have high quartz content that have been stripped of color through the process of podzolization. Areas south of Lake Okeechobee in Florida remain dark in the animated slices due to thick, black humified organic soils of the Everglades National Park and Big Cypress National Preserve.

The northern tip of Maine surrounding the end of the Appalachian National Scenic Trail shows the definitive slice sequence of Spodosols with surfaces of dark brown color, then grey of the bleached sand grains below the surface then brightening of the subsoil to reddish brown created by the translocation of humus-metal complexes in the subsoil.

Just west of the Appalachian Mountains, stretching from Maine to South Carolina is a swath of yellowish brown colors observed in the 25cm slice. This is goethite in the upper subsoil.

The dark black Mollisols that formed in loess southwest of Lake Michigan, surrounding the Lincoln Home, the Ulysses S Grant, and Harry S. Truman National Historic Sites, remain dark colored through sequential slices in the gif. The thick brown alluvial soils of the Mississippi River valley to the southeast are also evident. Stretching to the northwest from lower Lake Michigan is a swath of dark colored soils formed in till and loess.

The grey and white “Salt Flat” soils west of the Great Salt Lake and southwest of Golden Spike National Historic Site also remain light colored through all the depth slices. The light colored surfaces to 15 cm in the Joshua Tree National Park region are also evident in southern California, north of the Salton Sea.

Explore around the map for places you know about and think about why the soils are colored the way they are!

To learn more, check out Susan’s article in Park Science Magazine, “Celebrating Soils in the National Parks.”

Soil color is one of the most visually striking properties recorded by scientists around the world. These colors are observed by many, but there is no better expert than a soil scientist to tell the story of how and why a soil has the colors it has. Soil colors represent development processes depending upon environment and time that have influenced their appearance and geographic distribution. Image Credit: Zamir Libohova – USDA-NRCS-NSSC, Lincoln, Nebraska; Dylan Beaudette – USDA-NRCS-Davis, California; Skye Willis – USDA-NRCS-NSSC, Lincoln, Nebraska; Steve Peaslee – USDA-NRCS-NSSC, Lincoln, Nebraska

Stay tuned for more interviews in the coming weeks with soil experts as we continue to celebrate the National Parks 100th Anniversary.