Learn About Soil

Plain-language guides to what soil is and how it is classified. USDA’s soil survey covers about 2.3 billion acres of mapped US land^[1]; these pages explain what the survey records and how to read it.

• Soil typesThe twelve USDA texture classes, how each behaves, and where it is common.
• Soil ordersThe twelve orders of Soil Taxonomy, from Alfisols to Vertisols.
• Soil horizonsThe O, A, E, B, C and R layers that make up a soil profile.
• Alluvial soilSoils built from river and stream deposits, and how to tell if yours is one.
• Sand, silt & clayThe three particle sizes that set a soil's texture and how it behaves.
• What is a soil survey?How USDA mapped the nation's soils, and what the survey records.
• Read the texture triangleHow to plot sand, silt and clay to name your USDA texture class.
• Texture triangle worksheetA printable worksheet for plotting soil texture by hand.

Start with texture

Texture is a good place to start. It comes from three particle sizes: sand, from 2.0 down to 0.05 mm; silt, from 0.05 to 0.002 mm; and clay, under 0.002 mm^[3]. The share of each puts a soil in one of the twelve USDA texture classes, the soil types this site maps.

Then the layers

Dig down and soil changes with depth. Soil scientists name the layers with capital letters: O for organic matter, A for topsoil, E for a light, leached layer, B for subsoil, C for the material below it and R for bedrock, and not every soil has all of them^[4]. The soil horizons guide shows how deep each one runs across the US.

Then the classification

Texture describes one layer; Soil Taxonomy classifies the whole soil. Its top level is the order, and there are twelve, from Alfisols to Vertisols^[2]. The soil orders guide explains each one and where it is common, and alluvial soil covers the young soils rivers lay down.

Where the information comes from

A soil survey is the systematic description, classification and mapping of soils, and in the US the surveys are published by USDA’s Natural Resources Conservation Service^[5]. Field scientists build it by walking over the land and sending samples to laboratories^[1]. The soil survey guide explains how to read one, and the soil maps show it for any address. NRCS’s own application for the full survey, with every report, is Web Soil Survey^[6].

How it comes together for one address

Search an address on the soil map and the result card puts these ideas side by side: the soil series mapped there, its texture class, its order, the layers USDA describes with their depths, and how the soil drains. Each part links to the guide that explains it, so you can read your own soil one term at a time.

Common mix-ups

• Topsoil isn’t a texture. UConn defines topsoil as the uppermost layer of soil and loam as a textural class, so not every topsoil is a loam^[7].
• Clay names two things: the particle size under 0.002 mm, and the texture class of a soil that is at least 40% clay^[3]. A clay loam has less clay than that, so it isn’t a clay in the texture sense.
• A soil type and a soil series aren’t the same. Soil type on this site is the texture class; a series is a named soil such as Clarion, described in its own Official Series Description^[8].

Key terms

Soil survey

The systematic description, classification and mapping of the soils of an area^[5].

Texture class

The name for a soil’s mix of sand, silt and clay, such as loam or silty clay; USDA’s system has twelve, set on the texture triangle^[3].

Horizon

One layer of a soil profile, named O, A, E, B, C or R; not every soil has all of them^[4].

Order

The top level of Soil Taxonomy; there are twelve^[2].

Series

A named soil, such as Clarion or Blichton, with an Official Series Description kept by NRCS^[8].

Map unit

An area on a soil survey map holding one to three major soils^[1].

Drainage class

A soil’s natural drainage, set by how often and how long it stays wet^[9].

Water table

A saturated zone in the soil that lasts a month or more^[10].

Where to start

Ready to work on your own ground? See how to improve your soil.