A soil profile is divided into distinct layers called horizons, each defined by its composition, color, and how it formed. Most well-developed soils have up to six major horizons, labeled O, A, E, B, C, and R from top to bottom. Not every soil contains all six, but together they represent the full picture of how soil changes from the surface down to solid bedrock.
O Horizon: The Organic Layer
The O horizon sits at the very top of the soil profile and is made almost entirely of organic material. To qualify as an O horizon, a layer must contain at least 20% organic matter by mass. This material exists in three stages of decomposition. Fibric material is minimally broken down, and you can still identify individual leaves or plant parts. Hemic material is moderately decomposed, with only barely recognizable plant fragments. Sapric material is so thoroughly broken down that it looks like fine, dark matter with no identifiable plant structures at all.
Not all soils have an O horizon. It’s most common in forests, where fallen leaves, needles, and woody debris accumulate faster than they decompose. In grasslands or agricultural fields, organic matter tends to get mixed directly into the mineral soil below rather than forming a separate layer on top.
A Horizon: The Topsoil
The A horizon is what most people think of as topsoil. It’s a well-weathered, fertile layer dominated by mineral particles but still rich in organic matter. When an O horizon sits above it, decomposed organic material leaches downward into the A horizon, darkening its color and boosting its nutrient content. This is the layer where most plant roots concentrate and where biological activity is highest. Its dark color, a result of organic matter mixing with minerals, makes it visually distinct from the layers below.
E Horizon: The Leached Layer
The E stands for eluviation, which means leaching. In this layer, water moving downward through the soil carries away clay, iron, and aluminum oxides, depositing them in lower layers. Because so much material has been stripped out, the E horizon tends to be noticeably lighter in color than both the A horizon above and the B horizon below. It often looks pale, sandy, or ashy.
Like the O horizon, the E horizon isn’t always present. It develops most commonly under forests, where acidic water from decomposing leaves accelerates leaching. Grassland soils rarely have one because organic acids are less concentrated and biological mixing blends the upper layers together.
B Horizon: The Subsoil
The B horizon is where everything washed out of the upper layers ends up. This process, the opposite of eluviation, is called illuviation. Clay, iron, aluminum, humus, carbonates, gypsum, and silica can all accumulate here, alone or in combination. The specific mix depends on the local climate and the composition of the soil above.
Because of this buildup, the B horizon is typically denser and more clay-rich than the layers above it. It often has a reddish or yellowish tint from iron deposits, and its structure is noticeably firmer. In well-developed temperate soils, the B horizon can be quite thick and is a key indicator of how mature a soil profile is. Plant roots can extend into this layer, but it holds fewer nutrients than the A horizon and can restrict drainage when clay content is high.
C Horizon: The Parent Material
Below the B horizon sits the C horizon, which consists of partially weathered parent material. This is the loose rock, sediment, or mineral matter from which the upper soil layers originally formed. Unlike the A, E, and B horizons, the C horizon has been relatively unchanged by biological activity or the movement of water through the profile. It lacks the structure and organic content of true soil but is soft enough that roots can sometimes penetrate it.
The type of parent material in the C horizon varies enormously by location. It could be glacial till, river-deposited sediment, volcanic ash, or crumbled rock fragments. Its composition directly influences the chemistry and texture of the soil horizons above it.
R Horizon: Bedrock
The R horizon is solid, unweathered bedrock at the base of the profile. More than 80% of the material in this layer is hard enough that roots cannot grow into it. Unlike the loose fragments in the C horizon, bedrock is continuous rock, and its fragments typically have sharp edges and corners because they haven’t been transported or worn down by water or glaciers. In some landscapes, bedrock is exposed at the surface. In others, it lies many feet below ground, accessible only through drilling.
How Horizons Are Identified in the Field
Soil scientists distinguish one horizon from another using a combination of color, texture, structure, and consistency. Color is one of the quickest indicators. Dark brown or black layers are rich in organic matter. Pale, ashy layers suggest heavy leaching. Reddish or yellowish tones point to iron accumulation. Standardized color charts allow scientists to assign precise color values to each layer, making comparisons consistent across different locations and observers.
Texture refers to the proportion of sand, silt, and clay particles in a given layer. The B horizon, for instance, feels stickier and denser than the sandy E horizon above it. Structure describes how soil particles clump together into aggregates, which changes from loose and granular near the surface to blocky or prismatic in the subsoil.
What Creates These Layers
Five environmental factors drive the formation and distinctness of soil horizons: parent material, climate, living organisms, topography, and time. Parent material provides the raw ingredients. Climate controls how fast weathering and leaching occur, with wetter climates producing more distinct E and B horizons. Organisms, from earthworms to tree roots to bacteria, mix organic matter into the soil and accelerate decomposition. Topography affects drainage: soils on slopes lose water quickly and develop differently than soils in low-lying areas where water pools. Time ties it all together. Young soils may show only a thin A horizon over unaltered parent material, while soils that have developed over thousands of years display the full sequence of well-defined horizons from O through R.