Unified Soil Classification System

Compiled by B. W. Pipkin, University of Southern California

Major Divisions	Group Symbols Typical Names
Coarse-Grained Soils More than half of material is larger than no. 200 sieve size	Gravels more than half of coarse fraction is larger than no. 4 sieve size Clean Gravels GW Well-graded gravels, gravel-sand mixtures, little or no fines GP Poorly graded gravels, gravel-sand mixtures, little or no fines Gravels with Fines GM Silty gravels, gravel-sand silt mixtures GC Clayey gravels, gravel-sand-clay mixtures Sands more than half of coarse fraction is smaller than no. 4 sieve size Clean Sands SW Well-graded sands, gravelly sands, little or no fines SP Poorly graded sands, gravelly sands, little or no fines Sands with Fines SM Silty sands, sand-silt mixtures SC Clayey sands, sand-clay mixtures
Fine-Grained Soils More than half of material is smaller than no. 200 sieve size	Silts and Clays Low Liquid Limits ML Inorganic silts and very fine sands, rock flour, silty or clayey fine sands, or clayey silts, with slight plasticity CL Inorganic clays or low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays. OL Organic siltes and organic silty clays of low plasticity High Liquid Limits MH Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts CH Inorganic clays of high plasticity, fat clays OH Organic clays of medium to high plasticity, organic silts
Highly organic soils	Pt Peat and other highly organic silts

Checklist for Field Descriptions of Soils

Roy W. Simonson. Principal sources are U.S. Department of Agriculture Handbooks 18 and 436.

GENERAL INFORMATION AND SETTING
IDENTIFICATION: Name of soil series or broader class, as specific as feasible.
PHYSIOGRAPHY: Such as till plain, high terrace, flood plain.
UNDERLYING MATERIALS: General nature, such as calcareous clayey till or residuum from granite.
SLOPE: Approximate gradient.
PLANT COVER: Vegetation at site, such as oak-hickory forest, corn, pasture.
MOISTURE STATUS: Conditions at the time, such as wet, moist, dry.
REMARKS: Other features such as stoniness, salinity or depth to ground water; not applicable or observable everywhere.

DESCRIPTIONS OF INDIVIDUAL HORIZONS
DESIGNATION: See hypothetical soil profile, Data Sheet 36.
DEPTH: cm (or inches) from top of a horizon and from surface of organic soil.
THICKNESS: Average, such as 15 cm, plus range, such as 10-20 cm.
BOUNDARY: Lower one, as to distinctness: abrupt, clear, gradual, or diffuse; and as to topography: smooth, wavy, irregular or broken.
COLOR: Record colors of both wet and dry specimens if possible, but always for wet conditions. Use number-letter notations from Munsell Soil Color charts, e.g., lOYR 5/4. Record mottles (patches of one color in matrix of another color) as to abundance: few, common, many; as to size: fine, medium, coarse; and as to contrast: faint, distinct, prominent.
TEXTURE: Classes should show relative proportions of the separates sand, silt, and clay. See triangular graph showing textures, Data Sheet 37.2.
STRUCTURE: Describe natural units as to grade (distinctness): weak, moderate, strong; as to size: very fine, fine, medium, coarse, very coarse; and as to type: platy, prismatic, blocky, granular. Without peds, horizon can be either single-grained or massive.
CONSISTENCE: Cohesion, adhesion, and resistance of specimens to deformation and rupture. When wet: nonsticky, slightly sticky, sticky, or very sticky; also: nonplastic, slightly plastic, plastic, or very plastic. When moist: loose, very friable, friable, firm, very firm, or extremely firm.
When dry: loose, soft, slightly hard, hard, very hard or extremely hard.
ROOTS: Numbers of observable roots: few, common, or many; and dimensions: fine, medium, or coarse.
PORES: Numbers of field-observable pores: few, common or many; dimensions: very fine, fine, medium, or coarse; and shapes: irregular, tubular or vesicular.
REACTION: pH as measured with field kit.
ADDITIONAL FEATURES: Other features if present, such as iron or carbonate concretions (use same abundance and dimension classes as for roots), effervescence with dilute HCI, krotovinas (filled animal burrows), cementation (weakly, strongly, indurated), and stone lines.

U.S. Department of Agriculture Soil Conservation Service May 1, 1950
Names and sizes of classes of soil separates or “fine earth” forming bases for texture determinations.

Particle Size Descriptions

Size Term	Particle Diameter
Sedimentary Units:
Boulder	> 256 mm
Cobble	64 to 256 mm
Pebble	4 to 64 mm
Granule	2 to 4 mm
Very Coarse Sand	1 to 2 mm
Coarse Sand	1/2 to 1 mm
Medium Sand	1/4 to 1/2 mm
Fine Sand	1/8 to 1/4 mm
Very Fine Sand	1/16 to 1/8
Silt	1/256 to 1/16 mm
Clay	< 1/256 mm
Pyroclastic Units:
Bomb or block	> 32 mm
Lapilli	4 to 32 mm
Coarse Ash	1/4 to 4 mm
Fine Ash	< 1/4
Igneous Rocks:
Pegmatitic	> 30 mm
Coarse Grained	5 to 30 mm
Medium Grained	1 to 5 mm
Fine Grained	< 1 mm