Soils tell us a story; a story of the history of the landscape. What are the origins of South Carolina’s landscape? What sort of environments helped shape today’s landscape?

Soil science covers a broad range of topics, ranging from the use of soil as a medium for plant growth to the use of soil for the foundations of homes, roads and cities. In this lesson we will discuss two readily observable soil characteristics, texture and color, and relate them to the environment in which the soil formed. This sort of study falls under a branch of soil science called soil morphology and genesis.

We have over 300 different types of soil in South Carolina. There are about 15,000 different types in the United States as a whole. There are perhaps millions of different soils throughout the world. How can this be?

A soil can be said to be the product of five environmental factors:

1. parent material
2. climate
3. organisms (microbes, plants, burrowing animals, humans, etc.)
4. landscape position
5. time

The five factors of soil formation are not necessarily independent of each other. For example, climate affects organisms and landscape position can affect climate. However, it is still useful for soil scientists to think of soil as a product of these factors.

Altering any one of the five factors can result in a different soil.

A Very Brief Explanation of the Geologic History of South Carolina

About 250 million years ago, what is now known as the Piedmont of South Carolina (the upstate) was a large island in the Atlantic Ocean. Due to continental drift, this island crashed into North America. Eventually, North Africa and Europe crashed into North America behind this former island. 200 million years ago, the Atlantic Ocean started opening up once again. The former island remained as a part of the North American continent. The collision of the continents folded, heated and compressed the rocks of the Piedmont. This rock lay bare at the surface for many millions of years.

The parent material of the soils of the Piedmont is the partially decomposed rock that was transformed by the collision of the continents. This decomposed rock is called saprolite.

About 20 million years ago, the coast of what is now South Carolina was where the Carolina Sandhills are today. Roughly, this is a line that extends diagonally across the state from Aiken to Bennetsville. The parent materials of this area were deposited by the ocean millions of years ago. The sand of the Sandhills were once dune and beach deposits.

A series of geologic uplifts brought the ocean floor above sea level in a series of steps. This part of the state is called the coastal plain The youngest part of South Carolina’s landscape is at the coast. The age of the landscape becomes progressively older as one travels inland (northwest) from the coast.

The parent materials of the coastal plain and Sandhills are mostly made up of unconsolidated marine sediments.

The Basics: Soil Constituents

Roughly half the volume of topsoil consists of mineral and organic solids. The other half is pore space, which contains air and water. While the contents and character of the pore space is extremely important, this discussion will focus on the solid portion of soil

The mineral portion of soil is often described in terms of texture. A soil’s texture is the relative percentages of sand, silt and clay found in it. Texture is determined by separating these particle sizes in the lab, but a close estimate can be made just by feeling the soil with your fingers.

Sand: Open the bag marked sand and rub a pinch between your thumb and fingers. Sand will have a familiar gritty feel. There is little cohesion between sand particles, even when damp. With damp sand you may temporarily be able to form a ball or other shape, but it will fall apart with slight disturbance, such as gently bouncing it in your hand. Soil Scientists estimate sand content in the field by spreading a small, damp sample out in the palm of the hand and estimating the percentage by eye. You can see individual grains of sand with the naked eye.

Clay: Open the bag marked clay and work and mold it in your hand. Add water if necessary. As you know, clay can be molded in a great variety of shapes and retain those shapes because of the great cohesion between the clay particles. One method for field determinations of the clay content of a soil is to form a ribbon out of the sample between your thumb and index finger. Continue to push this ribbon out from between your fingers. Clay content is estimated by the length of the ribbon just before it breaks under its own weight. This length will vary from person to person. Generally, with the types of clay minerals we have in South Carolina, every inch of ribbon corresponds to 20–25% clay content. That is, if you can push up a ribbon two inches long, you probably have 40–50% clay in the sample. Some soil scientists use the wire method, where a thin wire is rolled between the hands, and then it is picked up at one end to see if it can support its weight.

Individual clay particles cannot be seen with the naked eye, nor an ordinary light microscope. An electron microscope is needed. The individual particles look like tiny flat plates, very similar in appearance and structure to mica.

Silt: Open the bag marked silt and rub a pinch between your thumb and fingers. Silt will have a smooth feel, similar to that of flour. Damp silt will have more cohesion than sand, and you can mold a variety of shapes out of it. Unlike those made of clay, shapes molded from damp silt cannot support much weight. Using the ribbon method, described in the clay section, you will find that the sample can only support a small ribbon. Pure silt would support no ribbon at all, but it is extremely difficult to find pure silt in South Carolina. The sample you have is approximately 90–95% silt. When determining the texture of a sample in the field, the percentage of silt is estimated by subtracting the percentages of sand and clay, determined above, from 100.

Individual silt particles cannot be seen with the naked eye, but can be observed through ordinary light microscopes. They look like tiny sand particles.

Organic Matter: Soil organic matter includes plant litter and the decomposing products of these plant parts. Litter is easily recognizable. In the next stage of decomposition, duff, the plant parts are no longer recognizable. Finally, the organic matter decomposes to humus. Humus is the black substance that darkens topsoil. In some wetlands, soils with high percentages of humus can be found. Humus will feel greasy, and cannot form a ribbon the way clay does.

Soil Texture

Soils are usually a mixture of particle sizes. After a soil scientist determines the relative percentages of sand, silt and clay in a sample, the texture can be determined using a textural triangle (see figure below). The textural triangle is a triangular graph. Each side represents the percentage of one of the three particle sizes. When the percentage of two of the particle sizes are plotted on this graph, the soil’s texture is the area on the graph where the two lines cross.

Soil Profile

If you dig a hole in soil, and then look at the wall of the hole, you will see a series of layers, more or less parallel to the surface, called horizons.

Horizons of soil (described from surface to depth)

The horizons listed above are found in many of our soils here in South Carolina. Some of our soils no longer have A horizons, due to erosion caused by poor soil management. Young soils, like those found near the coast or in a floodplain, may not have E or B horizons. Often, some experience is needed to properly interpret what is seen in the field.

It is the characteristics of the different horizons that tell a soil scientist the soil type.

After the deposition of the parent material, the first horizons that form are the O and A horizons. It only takes a few years for the O horizon to begin building up. After one or two hundred years, the A horizon will be noticeable.
The B horizon forms next. With time, the B horizon deepens, and sometimes extends to depths greater than 6.5 feet in South Carolina. Some time after the B horizon begins to form, the E horizon forms.

The B horizon is commonly referred to as red clay (especially in the piedmont). When a homeowner or farmer is dealing with red clay at the surface of the land, it is a sign that the A and E horizons have been removed. Erosion is often responsible, but sometimes topsoil is removed in housing developments by the developer, to be sold elsewhere.

Different Depositional Environments

You may note that some soils on the coastal plain are very sandy. This is expected since the parent materials were deposited by the ocean. What about the soils on the coastal plain that are high in clay?

Feel the texture of the soil materials on the table marked Duplin and Chipley. These two soils were collected at sites that were only about a mile apart. What differences do you notice?

Imagine driving to the beach. What sort of environments do you cross? As you get closer to the beach, dune environments are very obvious. Old beach ridges can be identified several miles inland. It is understandable that the soils on these beach ridges are sandy.

Another important depositional environment of the coast is the salt marsh. The bottom of these quiet backwaters are covered with finer particles (silts and clays). When we find clayey soils in the lower coastal plain, they are often on sites that at one time were marshes.

Both sandy and clayey materials are deposited by water. When water is moving, it retains smaller particles in suspension. If it’s moving fast enough, it can carry sand, gravel, and even move pebbles and rocks. When it slows down, it drops the larger heavier particles first. When water sits quietly, the finest particles are deposited. Texture can tell us a lot about the origins of young soils.

Soil Color

Mineral soil particles are almost always white or gray in color. The reason we see black browns, yellows and reds in soil is because the soil particles are coated with a colored material. Typically, the staining agents that coat soil particles are iron oxides and organic matter (humus). Iron oxides are responsible for the yellow and red colors that we see. Organic matter is responsible for the black and some of the brown colors that we see.

Look at the display of soil materials on the paper at the front of the room. Do you notice any trends?

Color is an indicator of the relative age of a soil. As one moves from the coast to the upstate, subsoil (B horizon) color changes from yellow to red. There are a number of different iron oxides, and the redder ones take the longest time to form.

Color is an indicator of the amount of water in a soil. Another trend in soil color that you will find is that wet soils are not as brightly colored as dryer, upland soils. In addition, wet soils often have darker A horizons (topsoil) than dry soils. Both of these facts are used in identifying wetlands.

The reason for the color change we see when we move from a well drained soil to a poorly drained soil is from the types of bacteria that live in these soils. Most living creatures need oxygen in order to respirate.

In order to get energy from food, we transfer electrons from molecule to molecule. Each time an electron is transferred, our bodies get some energy out of it. Ultimately, the electron is transferred to oxygen (O2). If no oxygen is available, the electron tranfers stop, and we can get no energy from our food. Within minutes, the cells in our bodies start to die.

In wet soils, the pore space is filled with water—little or no oxygen is present. Air breathing bacteria (aerobes) cannot survive. Some species of bacteria have the ability to use other substances as their electron acceptor. These bacteria are called anaerobes. Anaerobic bacteria will use nitrates, iron oxides and sulfates for respiration in wet soils. When they use iron oxides, they reduce Fe3+ to Fe2+. Fe3+ compounds are yellow and red, and very insoluble; Fe2+ compounds are colorless and highly soluble.

Drawing Typical Soils of South Carolina

Drawing profiles is a good way to develop an appreciation of the variety of soils in the state, and to help recall the differences we see in these soils. After considerable study, the following colors (found in a standard 64 color box of Crayola crayons) were found to satisfactorily depict South Carolina’s soils:

Draw at least two profiles. Include all horizons you’d expect to see, the position on the landscape and/or the region of the state. Don’t forget to include the plants or activities occurring on the surface!