The Typical Progression of a Soil Profile

Surface Soil Layers

• The surface of the soil is where new organic materials from plants and animals accumulate. This is the O layer, which has fresh litter and unrecognizable decayed matter. This is the most nutrient-rich layer of the soil. There is more acidic material here, so the pH is lower than the pH of the rest of the soil. This soil level is also darker. Below the O horizon is the beginning of the true mineral portion of the soil, the A horizon. This layer is where organic matter and clay, silt and sand are the most mixed together. Farther down in the soil, the organic matter becomes sparse, and the soil is dominated by inorganic materials.

Middle Layers

• In a well-developed soil, the middle layers contain most of the clay and smaller mineral materials. The middle layers are lighter in color than the surface soil. The E horizon is where clay is leached and has an especially pale color. Deeper down from the E are the B horizons. A soil will often have several B layers because of different soil-forming factors at work in them. The B horizon is where clay accumulates from being washed downward with water. Clay is also carried upward into the B horizon as water moves toward the surface when the soil is drying.

Lower Layers

• In the typical progression of soil formation, there are different kinds of B layers all the way down to the C and R soil profile layers. The C layer is the soil parent material, which can be unconsolidated rock breaking off from the bedrock or some other material that the soil is forming from. Soil can form on an old river or lake bed. The material that a soil comes from may also have been blown into an area. The C horizon need not necessarily be rock. Bedrock, layer R, is always solid rock material.

Soil-forming Factors

• There are five soil-forming factors that work to develop the soil profile of a specific soil. The factors are parent material, climate, topography, organisms and time. The first four all depend on the fifth for their ability to structure the way the soil looks. The type of climate, temperature and moisture all determine how chemical weathering will change the mineral content of the soil. The type of material on which the soil forms will determine many of its qualities. The kinds of plants and animals that are able to survive on the soil will determine how much and what kinds of organic materials are added to the soil. A steep mountain will develop very different soil layers from a flat plain. The topography changes how the soil erodes and how deep it is able to reach.