Soil profiles of all 12 soil orders including Mollisols, with dark, fertile topsoil in the midwest, and Ultisols, with fairly weathered subsoil in the Mid Atlantic.

Just as a botanist classifies plants or a zoologist classifies animals, soil scientists use soil taxonomy to categorize the diverse soils found around the world. This classification system was developed by the USDA and is hierarchical, moving from broad to specific categories. This allows scientists across the globe to speak the same language when discussing land management or conservation.

At the highest level, we have the 12 soil orders:

  • Alfisols
  • Andisols
  • Aridisols
  • Entisols
  • Gelisols
  • Histosols
  • Inceptisols
  • Mollisols
  • Oxisols
  • Spodosols
  • Ultisols
  • and Vertisols.

Mollisols are the most common soil order across the United States, accounting for about 21% of the land area. Mollisols are considered the most fertile soil order, and most common in the Midwest and Great Plains, contributing to the high fertility in these regions. Ultisols are most common in the Mid Atlantic, accounting for about 9% of U.S. land area. Ultisols are highly-weathered acidic soils that can be very fertile when treated with fertilizers and lime.

As we move down the hierarchy, we eventually reach soil series. This is the most specific level of classification and is usually named after a local town or landmark where the soil was first identified. This is where ‘State Soils’ come from, each state and territory in the U.S. has chosen a specific soil series that represents its unique heritage and landscape, a process similar to that of a state bird or flower.

 

Soil Formation

Two different soil profiles showing how extreme differences can be between soils, based on the forming factors. The soil on the left is a young, fertile, grassland soil and on the right is a mature, highly leached, forested soil.

There are five major factors that influence soil formation: climate, organisms, relief, parent material, and time. There are currently more than 20,000 soil series recognized in the United States, with that number growing as more soils are classified. Each soil series is unique because of these five factors. If you change even one of these factors, you end up with a completely different soil series.

The climate, primarily moisture and temperature, determine how fast rocks and organic matter break down. Soil is a rich ecosystem of plants, bacteria, fungi, and small animals. These organisms mix the soil and add essential nutrients. Relief, height differences across a landscape, influences the speed of erosion, movement of water, and slope aspect. Parent material is the base of what soil is made of. This can be bedrock, such as limestone, or materials moved by glaciers, water, or wind. The final key to soil formation is time. Soil formation does not happen overnight, it can take hundreds of years to form just one inch of topsoil, making it a non-renewable resource on a human timescale.

 

Soil Monoliths & Soil Pits

All-American Soils

The Smithsonian Institution has a ‘Dig It! The Secrets of Soil” exhibit, which houses 54 monoliths representing each U.S. state, territory, and the District of Columbia.

To truly understand a soil, you cannot simply look at the surface. You must view the soil profile, a vertical cross-section of the earth that reveals the story of its formation through distinct layers called horizons.

The primary tool for a soil scientist is a soil pit. These are large excavations, often several feet deep, dug to allow them to stand inside the earth. By examining a pit, we can identify the major soil horizons:

  • The O Horizon: The organic surface layer.
  • The A Horizon: The “topsoil”, rich in minerals and organic matter.
  • The B Horizon: The “subsoil”, where minerals and clays leached from above accumulate.
  • The C Horizon: The parent material, which is partially weathered rock.

Because it is impossible to bring a soil pit into a classroom or a museum, scientists create soil monoliths. These are preserved vertical sections of a soil profile. To create one, a scientist carefully peels a slice of soil from the pit face using a frame and specialized resins or glues. Monoliths allow us to observe soils from around the world without ever needing to travel!

a shovel sitting next to exposed soil

You don’t need to be a soil scientist, farmer, or engineer to learn more about soils! Learning about the soil formation and taxonomy in your backyard can deepen your appreciation for this amazing resource, and take your gardening game to the next level.

Understanding soil is the first step to understanding our environment. It dictates what we can farm, where we can build, and how we manage our natural resources. As we move through this series, we will see how these principles manifest in the Chesapeake Bay watershed. Stay tuned for the next installation on Hazelton, the official state soil of Pennsylvania.

Want to learn more about the soils in your backyard? Check out Web Soil Survey, a free online tool operated by USDA NRCS. Simply enter your address and draw a box around your area of interest. You can learn what the dominant soil series is in your backyard along with the suitabilities and limitations for use, and soil properties and qualities. This information can help you improve your backyard garden productivity!

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