Soil Science: Composition, Horizons, and Soil Types in the US

Soil is not dirt — a distinction pedologists make with some feeling. It is a living system, built over thousands of years, that sustains roughly 95 percent of the world's food supply (Food and Agriculture Organization of the United Nations, Soils and Biodiversity). This page covers how soil is composed, how it organizes itself into distinct vertical layers called horizons, and what the major soil classification groups across the United States look like in practice — including where they form and why they behave so differently from one another.

Definition and scope

Pedology — the scientific study of soils in their natural setting — treats soil as a product of five interacting factors: parent material, climate, topography, biological organisms, and time. That framework was formalized by Russian geologist Vasily Dokuchaev in the late 19th century and remains the conceptual backbone of the USDA Natural Resources Conservation Service (NRCS), which maintains the official United States soil classification system known as Soil Taxonomy.

Soil Taxonomy, published by the USDA and last comprehensively revised in the 12th edition (2014), organizes soils into 12 orders at the broadest level — think of them as the phyla of the soil world — and subdivides those orders into suborders, great groups, subgroups, families, and series. The United States alone contains approximately 20,000 named soil series (NRCS Web Soil Survey), each mapped to specific geographic locations with enough precision that a farmer, an engineer, or a land-use planner can pull data on the exact profile beneath a given field.

The scope of soil science extends well beyond agriculture. It intersects with hydrology and the water cycle, carbon sequestration, land-use policy, and the kind of foundational earth systems thinking covered across earthscienceauthority.com.

How it works

Soil forms through the physical and chemical breakdown of parent material — bedrock, glacial till, river sediment, volcanic ash — combined with the slow accumulation of organic matter from decomposing plants, fungi, bacteria, and soil fauna. This process, called pedogenesis, operates across timescales of hundreds to hundreds of thousands of years depending on climate and parent rock type.

The internal architecture that results is called a soil profile: a vertical cross-section from surface to bedrock revealing distinct horizontal bands called horizons. Understanding the profile is how pedologists read soil the way a physician reads an X-ray.

Standard horizon sequence (USDA nomenclature):

1. O horizon — Organic layer of decomposing leaf litter and plant material; most prominent in forested soils.
2. A horizon — Topsoil; rich in organic matter (humus), darkened in color, biologically active. This is where most root activity and microbial life concentrate.
3. E horizon — Eluviation zone; minerals and clays leach downward, leaving a lighter, sandier layer. Common in humid forest soils.
4. B horizon — Subsoil; accumulation zone where clays, iron oxides, and organic compounds illuviated from above collect. Often redder or more structured than the A horizon.
5. C horizon — Weathered parent material; less altered than upper horizons, minimal biological activity.
6. R horizon — Unweathered bedrock.

Not every soil has all six horizons. A young volcanic soil might show only an A and a C. A well-developed Midwestern Mollisol might display a thick A horizon stretching 60 centimeters deep before the B horizon begins — a profile that took roughly 10,000 post-glacial years to build.

The conceptual logic behind horizon formation is the same logic behind understanding how layered earth systems interact — a theme explored in the how-science-works conceptual overview on this network.

Common scenarios

Across the contiguous United States, 5 of the 12 soil orders dominate the landscape and represent the scenarios most relevant to agriculture, construction, and conservation.

Mollisols cover the Great Plains — Kansas, Nebraska, the Dakotas — where deep, dark, humus-rich A horizons formed under tallgrass prairie. The NRCS estimates Mollisols underlie approximately 22 percent of U.S. cropland, making them the agricultural backbone of the country (NRCS, Soil Orders).

Ultisols dominate the Southeast, from the Piedmont of Georgia through the Carolinas. These are ancient, heavily weathered soils — low in base saturation (often below 35 percent), clay-rich in the B horizon, and prone to crusting under intensive rainfall. Their red and yellow coloration comes from iron oxide accumulation.

Spodosols appear in the sandy, acidic soils of the Northeast and upper Great Lakes — the soils beneath New England's pine and spruce forests. Their defining characteristic is a rust-colored or black spodic horizon just below the E horizon, formed by the illuviation of iron and aluminum complexes.

Aridisols cover roughly 35 percent of the U.S. land area (NRCS Soil Survey Division) across the desert Southwest and Great Basin — soils defined by aridity rather than richness, often containing caliche (calcium carbonate) layers and very low organic matter.

Inceptisols are the generalists — young, minimally developed soils found in disturbed or rapidly eroding landscapes, river floodplains, and high-elevation mountain zones. They show horizon development but lack the mature features of Mollisols or Ultisols.

Decision boundaries

Choosing between soil types — for crop selection, foundation engineering, or ecological restoration — depends on three diagnostic properties that soil scientists weight against each other:

Texture vs. structure. Texture (the ratio of sand, silt, and clay particles) is fixed by parent material and cannot be practically altered at scale. Structure (how those particles aggregate) can be improved through organic matter additions, cover cropping, or tillage management. A clay-heavy Ultisol and a sandy Spodosol may have similar drainage problems but require opposite interventions.

pH and base saturation. Mollisols typically maintain pH values between 6.0 and 7.5, supporting a wide range of crops without amendment. Ultisols often fall below pH 5.5, requiring lime applications before most commercial crops are viable. The FAO's World Reference Base for Soil Resources notes that acid soil conditions affect an estimated 30 percent of the world's ice-free land area (FAO, World Reference Base, 2014).

Drainage class. NRCS classifies soils on a seven-point drainage scale from "excessively drained" (Spodosols on coastal dunes) to "very poorly drained" (Histosols in peat bogs and wetlands). Engineering decisions — septic system placement, road base design, building foundations — hinge almost entirely on drainage class and the depth to seasonal water table, both of which are mapped at the series level in the NRCS Web Soil Survey.

The soil beneath a parking lot and the soil beneath a prairie that has never been plowed may occupy the same map unit. How those soils diverge — and what that divergence means for restoration, land use, or natural hazard risk — is the central practical question of applied pedology.