As soils sit they are constantly being transformed. These transformations vary by the depth, amount of moisture, biological activity levels, and amount of nutrient in the soil. Because these transformations vary by depth the soil develops clearly distinguishable layers called horizons. A core of soil from a location will show all the layers of soil in a given area. This is called a soil profile.

 

Soil Horizons

Soils consist of organic material (derived from living organisms at the surface) and inorganic material (derived from minerals in underlying rocks and from chemical reactions within the soil). Because of the different sources of these materials, soils can often be divided into different layers according to their appearance, composition, and origin. These layers (called horizons) often end up in very similar arrangements regardless of the environment where the soil forms, so soil geologists have come up with standard classifications of these horizons. Here they are:

O
 The O Horizon is a layer of organic matter on the surface of a mineral soil. The actual source of the organic matter (e.g. dead leaves, roots, etc.) can be seen in the soil.
A
 The A Horizons are on the surface or just below an O Horizon. Living organisms are most active and are marked by the accumulation of humus. It may contain soluble salts and clay. This is also known as the topsoil and is typically very dark in color. The soil is still almost entirely organic, but you can't tell what the organic material came from.
B
 The B Horizon is below the A. The difference between A and B horizons is that A horizons lose material to dissolution, while B horizons accumulate this material from above. There is often the accumulation of clay and humus, a blocky structure, redder or stronger colors than the A horizon. This is known as the subsoil.
C
 The C Horizon is below the B and is composed of the weathered rock immediately below it. The C horizons lie below the part of the soil penetrated by plant roots. C horizons can be distinguished from those above by the fact that they still contain the original minerals from the parent rock, often in small weathered chunks surrounded by clay and other secondary minerals.
D
 The D (R) Layer is the consolidated rock beneath the soil. This rock usually underlies the C horizon, but may be immediately beneath the A or B horizon. The D horizon is the bedrock of the region -- the actual rock from which the soils above have been weathered.

Physical Properties of Soils

During a survey of diversity of an ecosystem it is important to describe both the abiotic and biotic factors in the ecosystem. A description of the soil is important to describing the complete ecosystem. To describe a soil it is important to clearly describe the soil profile and some of the soil's key physical properties.

For our study you will measure and report the following properties.

Texture, dry density (bulk), porosity, organisms, pH, sedimentation, and slope.

Texture

Add water to the soil and rub it between your fingers

  • sand = gritty
  • silt = smooth but not sticky
  • clay = smooth, plastic-like, sticky
Dry density Collect a known volume of soil in a metal container. Place the soil on a warm surface for 24 hours. Calculate density as mass/ volume.
Porosity Record how quickly water passes through the soil. Design a process to record the rate of passage of water under the force of gravity through your sample.
pH Using de-ionized water mix enough water with layers O-A of your sample to produce an excess of water. Stir or shake the sample and measure the pH of the water.
Sedimentation Fill a clear container 1/3 full using layers O-C of a soil PROFILE. Nearly fill the container with water. Turn and shake the container to mix the soil and water. Allow the soil to settle. Record the height and color of the layers.
Slope Calculate the slope of the land at the point you took your sample.

 

soils | formation | transportation | characteristics | horizons | layers