Soil Sciencefor Archeologists

Stewart ReedNathan Bailey

Oghenekome Onokpise

Edited by: Michael Russo and Virginia Horak

Florida Agricultural and Mechanical University and Southeast Archeological Center, National Park Service

Volume 1 — June 2000

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CONTENTS

FIGURES ............................................................................................................................................................... 4

TABLES ................................................................................................................................................................ 4

INTRODUCTION ..................................................................................................................................................... 5

WHAT IS A SOIL? ................................................................................................................................................. 5

CONSTITUENTS OF SOIL ......................................................................................................................................... 6§ Mineral Matter ...................................................................................................................................... 6§ Organic Substances ............................................................................................................................... 6§ Water ..................................................................................................................................................... 6§ Air .......................................................................................................................................................... 6

SOIL FORMATION .................................................................................................................................................. 7§ Parent Material ...................................................................................................................................... 7§ Topography (Relief) ............................................................................................................................... 9§ Climate ................................................................................................................................................... 9§ Biological Activity .................................................................................................................................. 9§ Time ...................................................................................................................................................... 10

WEATHERING ..................................................................................................................................................... 10§ Physical Weathering ............................................................................................................................ 10§ Chemical Weathering ........................................................................................................................... 11

MINERALOGY AND WEATHERING SEQUENCES ....................................................................................................... 12§ Primary Minerals ................................................................................................................................. 12§ Secondary Minerals ............................................................................................................................. 12§ Building a Soil ...................................................................................................................................... 13

SOIL PHYSICAL PROPERTIES ................................................................................................................................ 14§ Color .................................................................................................................................................... 14§ Texture ................................................................................................................................................. 16§ Structure ............................................................................................................................................... 20§ Bulk Density ......................................................................................................................................... 22

MICROMORPHOLOGY ........................................................................................................................................... 23

CHEMICAL PROPERTIES OF SOIL ........................................................................................................................... 23

SOIL PROFILES AND HORIZONS ............................................................................................................................ 25§ Master Horizons ................................................................................................................................... 25§ Transitional Horizons .......................................................................................................................... 25§ Subordinate Distinctions ...................................................................................................................... 26§ Diagnostic Horizons: Epipedons ......................................................................................................... 28§ Diagnostic Subsurface Horizons .......................................................................................................... 28

SELECTED BIBLIOGRAPHY ................................................................................................................................... 30

FIELD DESCRIPTION CHECK LIST ......................................................................................................................... 31

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FIGURES

TABLES

1. Bowen reaction series ............................................................................................................................... 12

2. Example pages from Munsell color charts ................................................................................................ 15

3. Classification of soil particles by size ....................................................................................................... 16

4. Textural triangle illustrating the twelve USDA soil classifications .......................................................... 17

5. Textural triangle indicating the relationship of texture and size ............................................................... 18

6. Texture-by-feel flow chart ........................................................................................................................ 19

1. Prominent minerals in soil clay fraction by relative degree of soil development ..................................... 13

2. Types and classes of soil structure ............................................................................................................ 21

3. General properties of humus and associated effects in the soil ................................................................ 24

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INTRODUCTION

In December 1999, the Southeast ArcheologicalCenter (SEAC) of the National Park Service and theAgricultural Department of Florida Agricultural andMechanical University (FAMU) in Tallahassee en-tered into an agreement to develop a workshop totrain archeologists in soil classification. The goal wasto acquaint the archeologists with soils typicallyfound at archeological sites in the region.

FAMU agronomist, Dr. Stewart Reed, conductedthe two-day class. Two sites were visited: the Span-ish Mission San Luis located on the red clay hills ofTallahassee, and a small, multicomponent prehistoricshell midden (8Wa29) on the sandy Gulf of Mexico

shoreline south of Tallahassee. Open archeologicalunits were examined at both sites. Methods for de-termining horizonization, particle size, compaction,moisture content, clay content, texture, and colorwere described and practiced by the participants.

The class will be offered annually to students andarcheologists. This manual was written for the par-ticipants and will be updated periodically. It is thefirst of two volumes, the second of which is forth-coming and will feature case studies from classprojects. For more information, please contact MikeRusso at SEAC: 850-580-3011, ext. 238, ormike_russo@nps.gov.

WHAT IS A SOIL?

Soil, in terms of its morphological characteristics,is defined as unconsolidated surface material form-ing �natural bodies� made up of mineral and organicmaterials and the living matter within them. Soil is adynamic entity with material continually and simul-taneously added, removed, and transformed. Its for-mation begins with a parent material derived fromeither the underlying rock or material transportedfrom somewhere else to its present site.

It is mainly the combined effects of climate andliving matter that convert a material to a soil. Forexample, in temperate rainy environments, moistureand dense vegetation may lead to deep, richly or-

ganic soils. In deserts, with the lack of moisture andsubsequent vegetation, soils may be thin and remainhighly mineral. Human disturbances, such as dwell-ings, agricultural practices, grave sites, and garbagedumps, may also affect soils, giving them otherunique characteristics.

This manual reviews the basic genesis, morphol-ogy, and physical, chemical, and mineralogical prop-erties of soil. With careful observation of these prop-erties, archeologists can often identify previous hu-man impacts on sites and gain additional data to helpdetermine the activities that led to the present soilcharacteristics.

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CONSTITUENTS OF SOIL

Soil material has four basic constituents: mineral orinorganic matter, organic substances, water, and air.

MINERAL MATTER

Mineral or inorganic matter can be crystalline oramorphous. A crystal is a chemical compound witha definite chemical formula and a distinct molecularstructure. For example, the mineral gibbsite has thechemical formula Al(OH)3. The small Al3+ ion is inthe center, surrounded by three hydroxyls (OH) thatare equidistant apart. Hydrogen ions can be removed,opening bonds. This allows the crystal to grow lat-erally and vertically forming sheets that stack up likea deck of cards.

Amorphous minerals lack a repeating long-rangestructure, but often atoms appear in a definite ratio.A charge may be associated with the surface of min-eral matter. Predominantly negative, the charge canalso be positive. The type and amount of charge giveminerals certain characteristic properties, such asshrink/swell potential and nutrient retention.

ORGANIC SUBSTANCES

Organic substances are molecules with carbon-to-carbon bonds. In soils, these molecules are formedby biochemical activity. Animals, insects, and soilmicroorganisms act together to decompose deadplant leaves, root tissue, and animal remains in thesoil. Organic matter in soils ranges from leaf litter,where decomposition is minimal and plant speciesare still recognizable, to a highly decomposed sub-stance called humus, which gives soils a dark browncolor.

Organic matter tends to accumulate near the sur-face where high biological activity, such as leaf lit-ter, roots, and insect life, occurs. As a result, soilsnear the surface are normally darker in color thanthe soil horizons a few centimeters below. Organicmatter provides a reserve source of plant nutrientsand buffers soil against pH changes. It forms a very

weak cement that, when acting alone, binds soil par-ticles together in a crumb-like structure.

Living organisms residing within the matrix areconsidered part of the soil. Different fungi, bacteria,protozoa, algae, and actinomycetes play a vital rolein converting parent material into soil. Plant roots,rodents, worms, insects, and other burrowing crea-tures help redistribute matter within a soil profile.

WATER

Soil pores provide an important reservoir for waterand atmospheric gases. Soil water is the mediumthrough which nutrients are transferred to plants.Since water has a great capacity to adsorb heat, itcan insulate soil from rapid temperature changes. Amoist soil is slower to heat in the spring and slowerto freeze as the temperature drops.

Hydrogen ions from both organic and inorganicmatter dissociate in water, resulting in the soil�s pH.A soil�s pH affects the solubility of minerals. Soilwater may be lost in several ways:

§ as transpiration from plant leaves;§ through evaporation from the soil surface;§ by draining through soil pores to groundwater res-

ervoirs;§ through lateral flow; and§ by being held in relatively small pores.

The maximum amount of water a soil holdsagainst gravity is its field capacity. A soil�s field ca-pacity is a function of the volume of pores smallenough to hold water against gravity. The process issimilar to that of a sponge holding water.

AIR

Pore space not filled with water contains gases inconcentrations comparable to those in the atmo-sphere. The soil air is the source of oxygen for rootand microbial respiration. A high respiration rate,

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coupled with the twisting path that a gas must fol-low in order to diffuse out of soil pores, results in acarbon dioxide concentration about one hundredtimes greater than that in the atmosphere. Individualgasses move into and out of soil pores primarily bydiffusion. After a heavy rain, soil pores fill with waterdisplacing the air.

Oxygen diffuses very slowly through water.Therefore, once a soil becomes saturated with wa-

ter, respiration quickly removes oxygen from thepores. If the soil layer remains wet for significantperiods during the year, the low oxygen content willresult in a change in the oxidation/reduction state.Soils so affected become increasingly reduced. Ironoxide minerals in this environment of reduction willchange color from red/yellow to a light gray. Thischange in color can be indicative of a seasonal high-water table.

SOIL FORMATION

The five soil forming factors are: parent material,topography, climate, biological activity, and time.

Soil formation begins with a parent material de-rived from weathering of either the native rock ormaterial transported to the site. The concerted effectof climate and biological activity then transforms par-ent material by producing the physical and chemicalenergy to alter minerals and vertically redistributematerial through the soil profile. The effect of cli-mate and biological activity is modified by topogra-phy. For example, slope affects the amount of waterflowing down through the profile as opposed to run-ning off the surface. Finally, soil forming processeswork slowly over time. The intensity and directionof these processes can also change over time. Duringany given period, one process may dominate; but,with time, another process can become dominant.

PARENT MATERIAL

Parent material is the initial mineral substance thatforms a soil. It may reside at the site of its origin orbe transported from somewhere else to its currentlocation. A soil formed from parent material foundat the site of its origin is called a residual or seden-tary soil. Bedrock weathering in place produces astony, massive material called saprolite. As physi-cal and some chemical weathering occur, the sapro-

lite becomes more dense than the underlying bed-rock. The texture and original rock structure remain,but the material is soft enough to dig with a handshovel. As chemical weathering converts primaryminerals to secondary minerals, particles are redis-tributed vertically. As material is both added andremoved, a soil develops. A residual soil will retainmany of its characteristics from underlying bedrock.Soil texture, mineralogy, pH, and other characteris-tics may be a direct result of the saprolite below.

Material can be eroded from one place and trans-ported to another where it becomes parent materialfor a soil at the new site. Often weathering occursbefore the material is transported to the new site. Inthis case, the soil may have few features in commonwith the underlying rock. Transported material canbury an existing soil at the new site. Once a deposi-tional episode is completed, time zero for the newsoil�s formation begins. Several forces can supplyenergy for the transportation of parent material: ice,wind, water, and gravity.

IceGlacial deposits occur at the front and sides of ad-vancing ice. Normally this material is poorly sortedwith respect to particle size. Because ice melts fromthe bottom, this is also true of material depositedunder a glacier. Also, material can be deposited asoutwash in the glacier�s meltwater.

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Soils formed from glacial deposits vary in com-position depending on the rock type over which theglacier traveled. Since glaciers advance and retreatwith time, the composition and depositional envi-ronment of the parent material can be quite com-plex. Overall, the texture of soil produced in glacialdeposits reflects the mode and distance of transportand the type of rock scoured. Shale and limestonescouring tends to produce a soil with relatively moreclay and silt-sized material. Igneous and metamor-phic rocks produce mostly sandy soils. Deposits be-neath the ice usually result in finer textured, densermaterials, whereas outwash and front and side de-posits are generally coarser.

WindWind deposits two major types of material: eoliansands and loess. Clay-sized material (< 0.002 mm)tends to bind together in aggregates too large to erodeby wind.

Eolian sands are windblown deposits of materialpredominantly greater than 0.05 mm (0.05 to 2 mm)in diameter. Most of this material moves in a seriesof short-distance jumps called saltation. Eolian de-posits may move several kilometers from the source.Material adhering to saltating sand particles andmaterial deposited as an aerosol are the sources ofclay in eolian sand. Normally this material has a nar-row textural range and is deposited on the leewardside of valleys or bodies of water.

Loess, which is windblown silt-sized material(0.002 to 0.05 mm), once airborne, can travel sev-eral hundred kilometers before deposition. The tex-ture of loess usually does not vary in a vertical di-rection, but tends to thin with horizontal distancefrom the source.

Windblown material tends to have sharp edges, aconchoidal shape, and surface etching. In contrast,material deposited by water tends to have roundededges and a polished surface. Careful observationunder a hand lens can shed light on the environmentpresent at deposition.

WaterAn alluvial or stream-borne deposit occurs in flood-plains, fans, and deltas. Because fast-moving waterpicks up debris, a river meandering downstream willundercut the outer bank of each bend. Water movesslower around the inner bank than the outer bank

and therefore loses energy. Thus, coarse materialsettles out, forming a bar over the inner bank. Aswater levels rise during floods, the stream overflowsits channel and spills over onto the floodplain.

Typically, alluvial deposits are characteristic ofthe decrease in energy during deposition. Where thestream overflows its bank, the energy is still rela-tively high; only deposits of coarse material occur,forming a levee. On the far side of a levee, moderateenergy is available, and silty material settles.

On the floodplain, water velocity and its corre-sponding energy is low, and clay settles. Becausebars form under moderate energy, this type of sort-ing does not occur on the plain. However, a flood-plain may surround a bar. As the distance from thechannel increases, the material�s texture becomesfiner, and the thickness of the deposit decreases.

Alluvial fans form where water in a channel, car-rying sediments downhill, experiences an abrupt re-duction in slope. The stream energy is reducedquickly, and material settles. This also occurs wherea narrow valley opens onto a wide flat. Fans have acone shape, widening in the downslope direction.Channels shift easily in fan deposits, and sedimentsare reworked over time. The texture of a fan becomesfiner with distance from its apex. Normally fans inhumid areas are not as steep and cover a much largerarea than those in arid regions.

Marine and lacustrine deposits form in low-energyenvironments under inland seas and lakes. Thesesediments are typically coarse near the shore andfiner toward the middle of the lake or sea.

Several shoreline features can be associated withinland water bodies, including deltas, sand dunes,and beaches. Deltas are essentially alluvial fans withtheir sediments deposited underwater. As lakes dry,evaporite minerals form. Under other conditions, eo-lian sediments can fill in the lakebed. Such soils havea finer texture and occupy lower sections on the land-scape. Soils formed in shoreline deposits have acoarser texture and occupy higher landscape posi-tions. In lakebeds with a very low influx of sedi-ments, organic substances dominate the sediments,and peats form.

GravityColluvium or hillslope sediments result from theforce of gravity and runoff moving downslope. Thismaterial may be deposited in catastrophic events,

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such as mudslides, or by very slow but persistentprocesses, such as slope wash or surface creep. Asviewed from the crest of a hilltop, sediments thicken,and the clay content increases on the downslope.

TOPOGRAPHY (RELIEF)

Topographic relief, or the slope and aspect of theland, has a strong influence on the distribution ofsoils on a landscape. Position on a slope influencesthe soil depth through differences in accumulationof erosional debris. Slope affects the amount of pre-cipitation that infiltrates into soil versus that whichruns off the surface. Aspect, or the direction a slopeis facing, affects soil temperature. In northern hemi-sphere sites, south-facing slopes are warmer thanthose facing north. Differences in moisture and tem-perature regimes create microclimates that result invegetational differences with aspect. Differences inweathering, erosion, leaching, and secondary min-eral formation also can be associated with relief.

CLIMATE

Climate arguably has the greatest effect on soil for-mation. It not only directly affects material translo-cation (leaching or erosion, for example) and trans-formation (weathering), but also indirectly influencesthe type and amount of vegetation supported by asoil. Precipitation is the main force in moving clayand organic matter from the surface to a depth withinthe profile. When a soil is at field capacity, the addi-tion of more water will result in drainage either down-ward or laterally. Drainage water carries with it dis-solved and suspended clay particles that collect at anew location within the soil profile. As a result, soilsoften show an increase in clay with depth as winderosion selectively removes clay (and organic mat-ter) from surface horizons.

Temperature and moisture affect physical andchemical weathering. Diurnal and seasonal changesin temperature cause particles to expand and con-tract unevenly, breaking them apart. Heat and mois-ture are active agents of chemical weathering, theconversion of one mineral into another.

Climate affects the type and amount of vegeta-tion in a region. A warm, humid climate produces

the most vegetative growth; however, microbial de-composition is also rapid. The net effect is that tropi-cal and subtropical soils are generally low in organiccontent. In contrast, organic matter tends to be high-est in a cool damp environment where decomposi-tion is slow.

Temperature and the amount of water movingthrough a profile affects all of the following:

§ the amount and characteristics of organic matter;§ the depth at which clay accumulates;§ the type of minerals present;§ soil pH (humid climates tend to produce more

acidic soil than do arid climates);§ soil color;§ iron, aluminum, and phosphorus distributions

within a soil profile; and§ the depth to calcium carbonate and/or salt accu-

mulation.

BIOLOGICAL ACTIVITY

Biological activity and climate are active forces insoil formation. Soil pedogenesis involves a varietyof animals, plants, and microorganisms. Ants, earth-worms, and burrowing animals, for example, mixmore soil than do humans through plowing and con-struction. Plant roots remove mineral nutrients fromsubsoil and redeposit them at the surface in leaf lit-ter. Growing roots open channels through soil whererainwater can wash clay and organic matter downalong these channels. Soil microbes decompose plantand animal debris, releasing organic acids. This bio-chemical activity is the catalyst for a great deal ofthe oxidation/reduction and other chemical reactionsin soil.

The distribution of organic matter in a forest soilis different from that in a grassland. The surface soilsof forests tend to have concentrated organic matter,which quickly decreases with depth. Grassland soilstend to accumulate organic matter to a greater depththan do forest soils. It is important for archeologiststo note that the dark staining from the humic frac-tion of organic matter can persist in a buried soil.Thus, ancient buried surface soils may be recognizedin the field by color alone.

The distribution of iron and aluminum through-out a profile also differs between forest and grass-

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land soils. In forests, due to the greater rainfall, claysand organics drain downward, leaving behind resis-tant minerals. As a result, iron and aluminum in Bhorizons in forest soils are found in higher concen-trations than in grassland soils.

TIME

Soils develop over time. Soil formation is a dynamicprocess, where a steady state is slowly approachedbut only rarely reached. The rate at which a soil formsis related more to the intensity of other soil formingfactors than to chronological age.

Soil development begins with a parent materialthat has a surface layer altered by vegetation andweathering. For example, a young Coastal Plain soilhas relatively uniform material throughout, and isaltered only by a dark-stained surface layer that hasbeen formed by vegetation. A more mature soil, on

WEATHERING

the other hand, shows evidence of the removal andtransport of surface-layer clay to a subsurface layercalled the B horizon. In an even older soil, chemicalweathering and leaching have removed silicon, caus-ing a change in the suite of clay minerals. A senilesoil is excessively weathered and dominated by veryresistant iron and aluminum oxide minerals. The ratethat a young Coastal Plain soil becomes a senile soildepends not on its chronological age but on how rap-idly minerals are transported and transformed withinthe profile.

Human activity frequently alters the process ofpedogenesis. Once human activity ends, soil forma-tion can continue as before�if no radical change inthe soil-forming factors occurred in the interim. Be-cause fine material leaches selectively faster thancoarse material, differences between human-alteredand undisturbed soils in the ratio of fine to coarseclay may be apparent in a relatively short span oftime (one hundred years in a humid environment).

Weathering is the physical and chemical processesby which rocks and minerals are disintegrated, de-composed, and resynthesized into new compounds.(Here rocks refer to unconsolidated material and soilat the surface [regolith], while minerals are inor-ganic substances with a definite chemical structureand formula.)

Weathering encompasses both physical and bio-geochemical processes, which generally occur si-multaneously. At different times, however, one pro-cess may dominate. In a soil forming from sapro-lite, for example, physical weathering dominates ini-tially. As more surface area is exposed with smallerparticles, and as biological activity increases, chemi-cal weathering takes over.

PHYSICAL WEATHERING

Physical weathering is the mechanical disintegrationof rocks and minerals into smaller sizes. Some ofthe several mechanisms that work to break apartrocks include: temperature, water, ice, glaciers, ero-sion, wind, and plants and animals.

TemperatureSeasonal and even day-to-night temperature changescan cause rocks to heat and cool unevenly. As rocksheat up, they expand; as they cool down, they con-tract. The outer surface expands and contracts fasterthan the interior, causing the outer surface to sepa-rate and peel off.

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WaterThe force of raindrops beating down on soft rocks,and the scouring effect of suspended material inwater flowing over rocks can wear the rocks awaywith time.

IceWater can infiltrate the cracks and pores of rocksand freeze. As the ice expands and thaws, the rocksbreak up.

GlaciersGlaciers weather rocks in several ways. The weightof a glacier can crush rocks. As it moves over anarea, a glacier can grind and pulverize rocks. As itrecedes, the pressure release can cause rocks to ex-pand and crack.

ErosionErosion causes pressure-release related weathering.

WindWind suspends fine particles. As the particles arepushed and bounced over one another, they abradethe rock surfaces over which they pass, slowly wear-ing the rocks down. Over time, the material removedresults in pressure-release weathering similar to thatof retreating glaciers.

Plants and AnimalsThe expansion and decomposition of roots growingin soil can alter the density and coherence of par-ticles. The digging and burrowing of animals canhave the same effect.

CHEMICAL WEATHERING

The process of chemical weathering changes theatomic makeup of a mineral. Near the surface, wa-ter and biological activity play important roles inchemical weathering. Given time and enough watermoving through a profile, even seemingly insolubleminerals will slowly dissolve. These minerals loosea portion of their atomic makeup and reprecipitateas new minerals in a leachate.

Water hydrates minerals, weakening them as itexpands the size of their crystals. Hydrolysis removesatoms (ions) from certain minerals and, in the pro-cess, splits water molecules affecting the soil pH.Carbon dioxide mixed with water causes a form ofacid hydrolysis called carbonation.

Another mechanism of chemical weathering isoxidation/reduction, or the transfer of electrons fromone substance to another. Oxidation/reduction affectsboth the solubility and stability of minerals. Somemechanisms of chemical weathering include:

Solution.....CaCl2 + H2O 6 Ca2+ + 2Cl- + H2O

Hydration.....2Fe2O3 + 3H2O 6 2Fe2O3@H2O

Hydrolysis.....KAlSi3O8 + H2O 6 HalSi3O8 + KOH

Carbonation.....CaCO3 + H2O W HCO3- + Ca2+ + H+

oxidation W reduction

Oxidation/reduction.....4FeO + O2 W 2Fe2O3

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Based on their formation, minerals are grouped intotwo broad classes: primary minerals and secondaryminerals.

PRIMARY MINERALS

Primary minerals have not been altered chemicallysince the time of their crystallization from moltenlava and their subsequent deposition. The Bowenreaction series chart (Figure 1) lists several primaryminerals in sequence based on resistance to weath-ering. The lower the minerals fall on the chart, themore they resist weathering.

SECONDARY MINERALS

Secondary minerals form from the decompositionof primary minerals and a subsequent reprecipitationinto a new, chemically distinct mineral. Layer alumino-silicates are the dominant minerals formed in most

temperate region soils. These layer silicates are com-posed of various arrangements of silicon/oxygensheets in tetrahedral coordination and aluminum/oxygen sheets in octahedral coordination.

Kaolinite is composed of one silicon/oxygen tet-rahedral sheet and one aluminum/oxygen octahedralsheet and therefore is called a 1:1 mineral. Kaoliniteforms in warm to hot, subhumid to humid climates.This mineral crystallizes in acid soil where basiccations (positive ions) and some silicon have beenleached. Vermiculite is a 2:1 mineral with two sili-con tetrahedral sheets surrounding one aluminum oc-tahedral sheet. It forms in subhumid to humid soilshigh in mica. Hydrous mica (illite) forms in sub-humid cool areas as mica dissolves and recrystal-lizes. Smectites, including montmorillonite, form inarid to humid soils with low permeability and mini-mal leaching. As primary minerals dissolve, leach-ing does not remove their constituents and they areavailable for recrystallization as smectites. Illites andsmectites are 2:1 minerals. The mineral chlorite (2:2)forms in marine sediments exposed to weathering.

MINERALOGY AND WEATHERING SEQUENCES

olivine Ca-feldspar

pyroxene

amphibole Na-feldspar

biotite

K-feldspar

muscovite

quartz

ÕÕ

Õ

Õ

Õ

Õ

Õ

Õ

Figure 1 � Bowen reaction series.

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Within a warm to hot, subhumid to humid climate,well-drained soil containing parent minerals high inmagnesium fosters chlorite formation.

Iron and aluminum oxides and hydrous oxides,collectively called sesquioxides, dominate soils inthe humid tropics. Sesquioxides form in hot wet re-gions where soils are subject to excessive weather-ing. High precipitation is necessary to leach siliconand basic cations from the soil leaving the relativelyinsoluble iron and aluminum compounds.

A difference in the ratio of secondary mineralspresent in two soils with the same parent materialindicates a difference in weathering intensity. As asoil becomes more intensely weathered, mineralswith a ratio of two silicon tetrahedral sheets to onealuminum octahedral sheet (2:1) are converted to 1:1minerals (one tetrahedral to one octahedral sheet).Still greater weathering converts 1:1 minerals to ses-quioxides. Table 1 shows the sequence of clay min-eral distribution as weathering increases.

BUILDING A SOIL

Soil formation is a dynamic process with materialcontinually added, transformed, and/or removed.Beginning with soil from a relatively uniform par-ent material, windblown sediments and annualfloods, for example, add new material to the sur-face. Physically weathered saprolite adds materialto the bottom of the profile. Dissolved and suspendedmaterial can be deposited or redistributed within asoil profile by water flowing below the surface.Evaporite minerals commonly accumulate in thesubsoil at the top of a water table. Developmentally,a high annual sediment input often characterizesyoung soils.

At the other extreme, material is continually re-moved from a soil. Erosion by wind and water, evenunder dense vegetation, can remove five tons of soilper acre, per year. Whenever precipitation exceedsthe field capacity of a soil, material can be leachedbelow the soil solum (the root zone or an area activein soil pedogenesis). Biochemical degradation canremove organic matter. This can lead to a signifi-cant reduction in soil volume.

Changes also occur within a soil profile. Mate-rial is converted from one form to another and trans-located within a profile. Clay, organic matter, andiron/aluminum ions typically migrate out from sur-face horizons especially in humid-region soils. Theroot zone provides a good environment for biogeo-chemical activity. Two examples of this are mela-nization and gleization. Melanization is the darken-ing of a soil layer by organic matter. This processgives the surface (A horizon) its brownish color. Glei-zation is the reduction of mostly iron-bearing min-erals. It produces a gray to greenish color in soil.Saturation of a soil layer for long periods within ayear usually causes this condition.

Table 1 � Prominent minerals in soil clay fraction byrelative degree of soil development from least developed(1) to most developed (13) (adapted from Jackson andSherman 1953).

Gypsum, sulfides, and soluble salts ............................. 1Calcite, dolomite, and apatite ...................................... 2Olivine, amphiboles, and pyroxenes ........................... 3Micas and chlorite ....................................................... 4Feldspars ...................................................................... 5Quartz .......................................................................... 6Muscovite .................................................................... 7Vermiculite and hydrous micas ................................... 8Montmorillonites ......................................................... 9Kaolinite and halloysite ............................................. 10Gibbsite and allophane .............................................. 11Goethite, limonite, and hematite ............................... 12Titanium oxides, zircon, and corundum .................... 13

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SOIL PHYSICAL PROPERTIES

Soils are categorized by certain physical and chemi-cal characteristics. Many physical characteristics,including color, texture, and structure, can be deter-mined in the field through careful observation andhand manipulations. Others, such as bulk density,require simple laboratory procedures.

COLOR

The most obvious soil characteristic is color. Al-though color is not used as a quantitative measure, itdoes give a good indication of certain conditions. Ablack to dark brown color usually suggests stainingwith organic matter. Red indicates the presence ofoxidized iron and is normally found in well-drainedsoils. In soil saturated for long periods during a year,oxides become reduced, yielding a gray or bluishgray color. Soil color is described by three attributes:hue, value, and chroma.

HueHue is the dominant spectral color. It is related tothe wavelength of light reflected by soil particles.Common soil colors are white, gray, black, yellow,brown, red, and their various mixtures.

ValueValue is the lightness or darkness of the color. It is ameasure of the amount of light reflected. Since mois-ture affects how light is reflected, normally soil colordeterminations are reported at three different mois-ture contents.

ChromaChroma is the strength or purity of color. It indicatesthe degree of difference between white, black, orneutral color.

§§§

Munsell Soil Color ChartsSoil color is characterized by comparison to theMunsell Soil Color Charts, which contain severalseries of distinctively colored chips (Figure 2). Each

page represents a different hue. The Munsell booknormally has 15 pages, each with a number (10, 7.5,5, or 2.5) followed by a letter or letters indicatingred (R), yellow (Y), green (G), blue (B), or combi-nations of these. For example, the 10 Y/R page con-tains color chips yellow-red (Y/R) with more yel-low than red (10).

Value units range between 0 and 10. The num-bers ascend vertically on the page from the lowestto highest numbers, indicating dark to light values.Thus, a 0 value is black with no light reflected, while10 is white with maximum light reflected. Chromaunits are arranged horizontally across the page from0 to 10, increasing in numbers from left to right. Lownumbers indicate an increase in grayness, while highnumbers signify a pure color with little mixing withother hues. Hence, a designation of 10R 6/4 indi-cates a hue of 10R, a value of 6, and a chroma of 4.

On careful observation, most soils contain morethan one color. Therefore, the matrix or dominantbackground color and mottles or colors differentfrom the background must be described. While thematrix is simply described by a Munsell number,the mottles must be described by their abundance,size, and contrast to the background.

§ AbundanceAbundance is the relative amount of mottling. Itis described by three classes. Mottles that occupyless than 2 percent of the exposed horizon areclassified as few; 2 to 20 percent as common; andmore than 20 percent as many.

§ SizeSize is a measure of the estimated average diam-eter of individual mottles along their greatest di-mension. Mottles less than 5 mm in diameter areclassified as fine; 5 to 15 mm as medium; andgreater than 15 mm as coarse.

§ ContrastContrast is an indication of the relative differencein color between the matrix and mottles. If thecontrast in color is only recognizable after close

15 Figure 2 � Example pages from Munsell color charts. (For illustrative purposes only. Colors should not be used for soil comparisons.)

16

examination, it is classified as faint. A distinctpattern is readily seen although not striking. Itmay vary one or two hues or several value orchroma units. Mottles are considered prominentwhen they are the outstanding feature of the ho-rizon. The colors of the matrix and mottles areseparated by several units of hue, value, andchroma.

TEXTURE

Texture is the relative percentages of sand-, silt-, andclay-sized particles in a soil. It is a soil�s single mostinfluential physical property. Texture influences soilpermeability, water infiltration rate, porosity, andfertility. Soil particles are classified into one of threegroups based on size (diameter): clay (<0.002 mm);silt (0.002 to 0.05 mm); and sand (>0.05 mm) (Fig-ure 3). In addition, larger objects may be describedas pebbles (2 to 75 mm); cobbles (75 to 250 mm);stones (250 to 600 mm); and boulders (>600 mm).

These soil particle size boundaries are not totallyarbitrary, as they roughly match changes in proper-ties associated with the differing size fractions.Chemically, sand- and silt-sized particles are rela-tively inert. They differ in that sand is large enoughto resist erosion by wind. Sand-sized particles arepredominantly quartz (SiO2) with small amounts ofsilicate-based primary minerals. Feldspars, horn-blende, and micas may total up to 20 percent of thesand fraction in soil. Sand tends to have angularrough surfaces, whereas silt is spherical and morepolished. Silt also is predominantly quartz withslightly larger amounts of primary minerals and ironand aluminum oxides. Wind easily erodes the smallersilt grains.

Clay particles are chemically active and stick to-gether in aggregates that resist wind erosion and in-crease soil porosity. The clay fraction in most tem-

perate region soils is dominated by layer alumino-silicate minerals. In the humid tropics, where weath-ering is more intense, iron and aluminum oxides andhydrous oxides are the dominant minerals present.

The USDA has specified twelve different texturalclasses of soil based on particle-size distribution. Thetextural class can be determined with any two par-ticle size groupings. For example, using the triangleillustrated in Figure 4, the classification of a soil with30 percent clay and 10 percent silt would be deter-mined in the following way:

1. Find the mark labeled 30 on the left side of thetriangle, which indicates the percent of clay.

2. Find the mark labeled 10 on the right side of thetriangle, which indicates the percent of silt.

3. Trace a line from the left mark (clay) horizon-tally and from the right mark (silt) diagonallydownward until the two lines intersect. The pointof intersection indicates that the soil classifica-tion is �Sandy Clay Loam.�

Note that if a line is drawn diagonally upwardfrom the mark labeled 60 at the bottom of the tri-angle, which indicates the percent of sand, it willalso intersect with the other two lines in the arealabeled �Sandy Clay Loam.� Hence the classifica-tion could also have been determined with the per-cents of clay and sand, or sand and silt.

The triangle in Figure 5 illustrates the relation-ship of texture and size, which is further explainedin the following paragraphs.

SandSand is the largest textural class. Sandy soils aredominated by the properties of sand: weak structure,rapid infiltration rate, slight erosion potential, looseconsistence, and low fertility. When the soil is moist

0.05 2.00.002 0.1 0.25 0.5 1.0

Fine MediumVery Fine Coarse Very CoarseCLAY SILT GRAVEL

SAND

Figure 3 � Classification of soil particles by size (mm).

17

Figure 4 � Textural triangle illustrating the twelve USDA soil classifications.

and molded into a ball, it will easily crumble whentouched (Figure 6). Sands contain 85 to 100 percentsand, 0 to 15 percent silt, and 0 to 10 percent clay.Sand is further divided into the following four cat-egories.

§ Coarse SandMore than 25 percent of sand particles are 0.50mm diameter in size or larger, and less than 50percent are between 0.05 and 0.50 mm.

§ Medium SandTwenty-five percent of the particles are largerthan 0.25 mm. Less than 50 percent measure be-tween 0.25 and 0.05 mm.

§ Fine SandMore than 50 percent of the particles are between0.10 and 0.25 mm or less than 25 percent aregreater than 0.25 mm and less than 50 percentrange between 0.05 and 0.10 mm.

§ Very Fine SandMore than 50 percent of the particles are between0.10 and 0.05 mm.

Loamy SandThis category contains 70 to 85 percent sand, 0 to30 percent silt, and 10 to 15 percent clay. Becauseloamy sand contains more clay than does sand, it isslightly cohesive and can be molded into a ball that

Perc

ent C

lay

Percent Sand

Percent Silt

Clay

Clay Loam

Loam

Silt Loam

Silty ClayLoam

Sandy Loam

Silt

SiltyClay

Sandy Clay Loam

SandyClay40

50

60

70

80

90

0

10

20

30

0102030405060708090

0

10

20

30

40

50

60

70

80

90

SandLoamy Sand

18

Figure 5� Textural triangle indicating the relationship of texture and size.

will maintain its form under gentle pressure. Soilsqueezed between the thumb and forefinger, how-ever, will not form a ribbon (Figure 6).

SiltSilts are highly erodible, relatively infertile soils.They contain 80 to 100 percent silt, 0 to 20 percentsand, and 12 percent or less clay. They can be moldedinto a ball that keeps its shape under gentle pres-sure. The low percentage of clay precludes the for-mation of a ribbon. Silts are distinguished fromloamy sands by placing a small amount of exces-sively wet material in the palm of your hand andrubbing the wet soil. Silt feels floury, whereas loamysand feels gritty (Figure 6).

ClayClayey soils have a very slow infiltration rate, drainslowly, are very sticky and plastic when wet, andform hard clods when dry (Figure 6).

§ ClayThese soils contain 40 to 100 percent clay, 0 to45 percent sand, and 0 to 40 percent silt. The highclay content makes these soils extremely stickyand plastic. They are readily shaped and, whenmolded, resist deformation if squeezed with mod-erate pressure. Pressure between the thumb andforefinger will create a ribbon longer than 5 cm.Clay feels non-gritty but not very slippery whenexcessively wet.

Clayey(Very Fine)

Clayey(Fine)

Fine Loamy

Coarse LoamySandy

CoarseSilty

Perc

ent C

lay Percent Silt

40

50

60

70

80

90

0

10

20

30

0102030405060708090

0

10

20

30

40

50

60

70

80

90

Percent Sand

FineSilty

19

Figure 6 � Texture-by-feel flow chart.

LOAM CLAYLOAM CLAY

SILT

SANDYLOAM

SILTLOAM

SANDYCLAYLOAM

SANDYCLAY

SILTYCLAY

No

YesYes

Yes

Yes

Yes

YesYesYes

No

No

No

NoNo

YesYes

YesYesYes

Yes

NoNoNo

NoNoNo

Place 25�50 g of soil in palm. Add waterslowly and knead soil to wet all aggregates.Soil is at proper consistency when plasticand moldable, like moist putty.

START

Does soil make aribbon 2.5 cm or lessbefore breaking?

Does soil make aribbon 2.5�5 cmbefore breaking?

Does soil make aribbon 5 cm or longerbefore breaking?

Does soil form a ribbon?

Is soil too dry? Is soil too wet?Does soil remain in aball when squeezed?

Add more dry soil.

Gritty

Excessively wet a small pinch of soil in palm and rub with forefinger.

Excessivelywet a smallpinch of soilin palm andrub withforefinger.

Does grittyfeelingpredominate?

Does grittyfeelingpredominate?

Does grittyfeelingpredominate?

Does smoothfeelingpredominate?

Does smoothfeelingpredominate?

Does smoothfeelingpredominate?

Place ball of soil between thumb andforefinger gently pushing soil withthumb and squeezing it upward intoa ribbon. Form a ribbon of uniformthickness and width. Allow ribbon toemerge and extend over forefingeruntil it breaks from its own weight.

SILTYCLAYLOAM

LOAMYSAND

SAND

No

20

§ Silty ClaySilty clays are similar to clays. They contain 40to 60 percent clay, 0 to 20 percent sand, and 40 to60 percent silt. They form a ribbon greater than 5cm in length and are very smooth when exces-sively wet.

§ Sandy ClayThis category contains 35 to 55 percent clay, 45to 65 percent sand, and 0 to 20 percent silt. Likethe other clayey soils, sandy clays form long rib-bons. When excessively wet, however, the highersand content gives them a gritty feel.

LoamLoamy soils have characteristics intermediate be-tween those of sandy and clayey soils. These soilscan be molded, and, as clay content increases, themold becomes firm and resists deformation undermoderate to strong hand pressure. Also, as the claycontent increases, the infiltration rate slows and thesoil forms hard clods when dry (Figure 6).

§ Sandy LoamThese loams contain 85 to 43 percent sand, 0 to50 percent silt, and 0 to 20 percent clay. They areslightly cohesive and can form ribbons less than2.5 cm in length. When wet, they have a verygritty feel. Sandy loams are further divided intothe following categories:

� Coarse Sandy LoamThis group contains more than 25 percentsand-sized particles greater than 0.50 mm indiameter and less than 50 percent between 0.05and 0.50 mm.

� Medium Sandy LoamMore than 30 percent of this group is made ofparticles greater than 0.25 mm in diameter; lessthan 25 percent measures between 1 and 2 mm;and less than 30 percent falls between 0.05and 0.25 mm.

� Fine Sandy LoamMore than 30 percent of the fine sandy loamshave particles that range in size between 0.05and 0.10 mm; 15 to 30 percent are greater than0.25 mm.

� Very Fine Sandy LoamMore than 30 percent of these loam particlesrange between 0.05 and 0.10 mm in diameteror more than 40 percent range between 0.05and 0.25 mm (half of which are less than 0.10mm) and less than 15 percent are greater than0.25 mm.

§ Silt LoamSilt loams contain 0 to 50 percent sand, 50 to 88percent silt, and 0 to 27 percent clay. They areslightly cohesive when wet and form soft clodswhen dry. Silt loams feel smooth when wet andcan form a ribbon less than 2.5 cm in length.

§ LoamLoams contain 23 to 52 percent sand, 28 to 50percent silt, and 7 to 27 percent clay. Slightly co-hesive, they form ribbons less than 2.5 cm long,and feel moderately smooth when wet.

§ Sandy Clay LoamContaining 45 to 80 percent sand, 0 to 28 percentsilt, and 20 to 35 percent clay, these loams aremoderately cohesive, forming ribbons between2.5 and 5.0 cm in length. When wet, they have agritty feel.

§ Silty Clay LoamThis group contains 0 to 20 percent sand, 60 to73 percent silt, and 27 to 40 percent clay. Rib-bons 2.5 to 5.0 cm long can be formed. Whenwet, the soil has a moderately gritty feel.

§ Clay LoamClay loams contain 20 to 45 percent sand, 15 to53 percent silt, and 27 to 40 percent clay. Thesesoils are sticky and plastic when wet and hardwhen dry. They form ribbons 2.5 to 5.0 cm inlength and are moderately gritty when wet.

STRUCTURE

Soil structure is the aggregation of primary particlesinto secondary shapes or forms called peds. Shrink/swell, freeze/thaw, and other forces in soil bringparticles into close proximity, where they can be ce-mented together. Organic matter forms a weak ce-

21

menting agent that may eventually give way to stron-ger bonding by humus. Silica, metal oxides, and car-bonates also cement peds. Structure is described bygrade, class, and type.

GradeGrade represents the stability or distinctiveness ofthe ped. Because it is moisture dependent, the gradeis normally described when the soil is slightly moist.Structural grades are classified as follows:

§ WeakPeds can be seen in place with careful observa-tion, however, they cannot be removed intact.

§ ModeratePeds can be readily seen in place and, once re-moved, will remain intact with gentle handling.

§ StrongPeds are distinctive in place and will withstandconsiderable handling.

ClassClass refers to the size of the ped. Since some struc-tural types are inherently larger than others, a sizerange for each structural type has been determined,as illustrated in Table 2. The class designations are:very fine or very thin, fine or thin, medium, coarse

Table 2 � Types and classes of soil structure (Soil Conservation Service 1975).

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22

or thick, and very coarse or very thick. These range,respectively, from the smallest to the largest ped sizefor each type.

TypeType refers to the shape of an individual ped (Table2). Structural types are classified as follows:

§ Single GrainIndividual soil particles do not form aggregates;soil tends to have a sandy texture very low in or-ganic matter.

§ GranularThese spheroids or polyhedrons are of roughlyequal size in all dimensions and have plane orcurved surfaces with slight or no accommoda-tion to the faces of surrounding peds. Nonporouspeds are generally found in sandy, low-organic-matter soils.

§ CrumbThese soil particles are similar to the granularclass, however, the peds are porous.

§ PlatyThese particles are much longer and wider thantall. The flat peds are arranged around a horizon-tal plane.

§ Angular BlockyAngular blocky peds are of roughly equal size inall dimensions; blocks or polyhedrons have planeor curved surfaces that are casts of the moldsformed by the faces of the surrounding peds.Faces are flattened, and most vertices are sharplyangular. These particles tend to occur in B hori-zons or where moderate amounts of clay arepresent.

§ Subangular BlockyBasically the same as the angular blocky particles,

the subangular blocky faces are mixed, rounded,and flattened with many rounded vertices.

§ PrismaticThese particles, with two horizontal dimensions,are smaller than the vertical and taller than longor wide. They are arranged around a vertical linewith vertical faces well defined and angular ver-tices without rounded caps. They are generallyfound in arid regions below the surface in hori-zons with moderate to high clay content.

§ ColumnarColumnar particles are like the prismatic particlesbut with rounded caps.

§ Massive or StructurelessThe shape of these particles cannot be determined;they cling together in huge masses with no defi-nite arrangement along lines of weakness. Theyare normally very hard.

BULK DENSITY

Bulk density is a measure of a soil�s compactness,defined as a soil�s oven-dry mass divided by its vol-ume including the pore space. Soil is sampled bydriving a metal cylinder of known volume into thesoil. The cylinder is removed with a soil core intact.With a straight edge, the soil is leveled to the edgesof the cylinder. In the lab, the intact core is ovendried at 100 ºC until there is no more weight changewith additional drying. The oven-dry weight is de-termined and divided by the cylinder�s volume.

The bulk density of soil in good physical condi-tion ranges from 0.8 to 1.6 g cm-3. Roots tend to pro-liferate more in soil with low bulk density. Soil withhigh organic matter content tends to have good struc-ture and lower bulk density than similar soil withlow organic matter. Cultivation destroys structure,reducing organic matter and increasing bulk density.

23

Soil micromorphology is the study of size, shape,aggregation, etching, coating, accumulation, anddepletion of minerals associated with various soilprocesses. Soil formation is a dynamic process withmaterial continually being added, removed, andtransformed. For example, water moving through asoil profile will pick up fine-textured material anddeposit it as a coating along channels formed by thefaces of adjacent peds.

Human activity can interrupt these processes andresult in subtle differences in morphology. Waterinfiltration in the soil around a structure would beless than that in adjacent soil, because the structurewould divert water away from it and compact thesoil beneath it. Consequently, coatings on soil pedsbeneath the structure would be thinner, less well ori-ented, and have a different ratio of fine to coarsematerial than in the adjacent undisturbed soil.

MICROMORPHOLOGY

CHEMICAL PROPERTIES OF SOIL

Soil minerals and organic matter have weak electro-static charge sites associated with their surface. Whilethese sites may be positively or negatively charged,the predominant charge is negative. The magnitudeof the charge associated with clay minerals may bevery large. Thus, soils with a high clay content tendto have a larger negative charge. This charge attractscations dissolved in soil solution. A soil�s capabilityto replace cations on the surface with those in a soilsolution at a given pH is called cation exchange ca-pacity (CEC). We report CEC in centimoles ofcharge per kilogram of soil (cmolc kg-1 soil). Ex-change occurs on a charge-for-charge basis.

While the total amount of exchange depends oncharge, a cation�s affinity for the surface is a func-tion of its charge and hydrated radius. Cations witha high charge and small hydrated radius have agreater affinity for the surface. The attraction to thesurface of cations commonly found in soil solutionis in the order of Al3+>Ca2+>Mg2+>K+~NH4

+>Na+.Because human habitation often selectively enrichesor depletes ions, a comparison of ion ratios may helpexplain land-use patterns.

In soil classification it is helpful to know the pro-portion of a soil�s CEC occupied by basic and acidic

cations. Basic cations are Ca2+, Mg2+, K+, and Na+.The sum of these four basic cations divided by theCEC is called the percent base saturation. The ratioof Ca2+ to Mg2+ is an indication of the degree ofweathering. The relative depletion of Ca2+ versusMg2+ is an indication of advanced weathering. Ex-change acidity is a term given to the sum of Al3+ andH+ extracted in solution buffered at pH 8.2. Ex-changeable acidity increases with leaching of basiccations and weathering.

Soil pH is a measure of the ability of soil miner-als and organic matter to act as dilute acids and do-nate hydrogen ions into solution. A soil pH of <3.5is normally associated with sulfur oxidation. This iscommon in coastal marshes and mine spoils, whereburied sulfide minerals are exposed to oxygen. Inforest soils, the acid-forming litter tends to keep soilpH below 5.5. Soil pH between 3.5 and 6.5 indi-cates free iron at the lower pH levels, and then alu-minum hydrolysis at higher values controls pH. AtpH values between 6.5 and 8.5, free CaCO3 controlspH. Soil pH >8.5 indicates a high sodium content.These soils tend to be hard and very impermeable.

Soil organic matter is the partially decomposedresidue of plants and animals. As it breaks down, it

24

coats soil particles giving them a dark brown to blackcolor. Organic matter can be categorized into threefractions:

1. Plant litter and animal remainsThese tend to be new additions to the soil. De-pending on the climate, they decay relativelyquickly, with a turnover time of some five yearsor less.

2. Microbial metabolites and stable cellular debrisThis fraction includes humus, which gives soilmany beneficial characteristics. It has a turnovertime of fifty years or more. Table 3 details someof the effects humus has on soil.

3. Highly resistant fractionThese compounds may last in soil for 2,500 yearsor longer.

Organic matter is strongly adsorbed by certainclay minerals. Organomineral complexes protectsubstance from microbial decay. As a result, soilorganic matter stabilizes quickly�100 years in soilswith a high clay content, and it may not reach a steadystate in sandy soils for 1,500 years or more. Withtime, substances become increasingly decomposed,and the carbon to nitrogen ratio of soil organic mat-ter increases.

Organic matter accumulates near the surfacewhere there is a high root density (A horizon). Vari-ous vegetation types have different effects on soilorganic matter. Grasslands have a dense rooting pat-tern resulting in a thick dark colored A horizon. For-est soils have a thinner layer of organic staining. Inareas of high rainfall, soluble organic matter can bewashed out of a subsurface horizon (E horizon). Itaccumulates in a lower horizon and appears as a darkstaining on particles.

Table 3 � General properties of humus and associated effects in the soil (after Stevenson 1982).

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25

SOIL PROFILES AND HORIZONS

MASTER HORIZONS

Horizontal layers of soil called horizons can be de-scribed by their different morphological character-istics. Capital letters designate master horizons,which are further subdivided by Arabic numerals.Master horizons are used to describe similar appear-ing soil layers and should not be confused with di-agnostic horizons used to classify soils.

O HorizonThe O horizon is a surface layer dominated by or-ganic material. An O horizon may be found belowthe surface if it has been buried. Predominantly foundin forested regions, the O horizon is composed ofleaf litter in various stages of decay.

A HorizonThe A horizon is the uppermost mineral layer. It maylie below the O horizon. An A horizon has a highconcentration of humus and is not dominated by themigration of clay, humus, aluminum, or iron into orout of the horizon. The humus content gives it adarker color than the horizon below.

E HorizonThe E horizon is a layer of eluviation where clayorganic matter and iron and aluminum oxides havebeen leached out. Remaining material tends to belight colored and coarse textured. The E horizon isnormally found below an O or an A horizon andabove a B horizon. However, it may separate sec-tions of a B horizon.

B HorizonThe B horizon is a subsurface layer showing evi-dence of one or more of the following processes:

1. illuvial accumulation of alumino-silicate clay,iron, aluminum, gypsum, or silica;

2. carbonate removal;

3. residual concentration of sesquioxides;

4. coating of sesquioxides, which makes the hori-zon conspicuously lower in color value, higherin chroma, or redder in hue without apparent il-luviation of iron than that found in the overlyingand underlying horizons;

5. alteration that forms silicate clay or liberates ox-ides, or both, and that forms a granular, blocky,or prismatic structure if volume changes accom-pany changes in moisture context; or

6. brittleness.

C HorizonThe C horizon is a layer of minimal alteration. Ma-terial may be similar to or unlike that from whichthe other horizons formed. C horizons lack the prop-erties of O, A, E or B horizons, and can includecoprogenous earth (sedimentary peat), diatomaceousearth, saprolite, unconsolidated bedrock, and otheruncemented geologic materials or materials softenough for excavation with moderate difficulty.

R LayerAn R layer refers to hard bedrock. Material is ce-mented and manual excavation is impossible. Intru-sive soils can be found in rare cracks in the bedrock.Examples of R layer material include: granite, ba-salt, quartzite, indurated limestone, or sandstone.

TRANSITIONAL HORIZONS

Transitional horizons are dominated by propertiesof one master horizon but have the subordinate prop-erties of another. These are designated by two capi-tal letters, for example, AB, EB, BE, or BC. Thefirst letter represents the dominant horizon charac-teristics, the second indicates the weaker expressedcharacteristics.

A second type of transitional horizon has two dis-tinct parts with recognizable properties of the twomaster horizons indicated by the capital letters. Partsof one surround the other. This type of transitional

26

horizon is designated by a capital letter for the partwith the greatest volume, followed by a slash andanother capital letter for the secondary part (for ex-ample, E/B, B/E, or B/C).

SUBORDINATE DISTINCTIONS

Master horizons are further divided by subordinatecharacteristics, which usually do not apply to transi-tional horizons. Subordinate distinctions are identi-fied by lower-case letters, called suffix symbols. Insome cases, they describe an accumulation of mate-rial. This means that the so-designated horizons con-tain more of the material in question than is presumedto have been present in the parent material. For ex-ample, Bt refers to a B horizon with more clay thannormal. The symbols and their meanings follow.

§ a � highly decomposed organic materialUsed with O to indicate the most highly decom-posed organic materials, which have rubbed fibercontent of less than 17 percent of the volume.

§ b � buried genetic horizonUsed in mineral soils to indicate identifiable bur-ied horizons with major genetic features that weredeveloped before burial. Genetic horizons mayor may not have formed in the overlying mate-rial, which may be either like or unlike the as-sumed parent material of the buried soil. Thissymbol is not used in organic soils or to separatean organic from a mineral layer.

§ c � concretions or nodulesIndicates a significant accumulation of concre-tions or nodules. Cementation is required, but thecementing agent is not specific, except that it can-not be silica. The symbol is not used if the con-cretions or nodules consist of dolomite or cal-cite, or more soluble salts. It is used if the nod-ules or concretions are enriched with minerals thatcontain iron aluminum, manganese, or titanium.

§ d � physical root restrictionIndicates root-restricting layers in naturally oc-curring or man-made unconsolidated sedimentsor materials, such as dense basal till, plow pans,and other mechanically compacted zones.

§ e � organic material of intermediate compositionUsed with O to indicate organic materials of in-termediate composition with rubbed fiber con-tent between 17 and 40 percent (by volume).

§ f � frozen soilIndicates permanent ice content in a horizon orlayer. The symbol is not used for seasonally fro-zen layers or for so-called dry permafrost (mate-rial that is colder than 0º but does not contain ice).

§ g � strong gleyingIndicates either that iron has been reduced andremoved during soil formation, or that saturationwith stagnant water has preserved it in a reducedstate. Most of the affected layers have a chromaof 2 or less, and many have redox concentrations.The low chroma can represent either the color ofreduced iron or the color of uncoated sand andsilt particles from which the iron has been re-moved. The symbol g is not used for materials oflow chroma that have no history of wetness, suchas some shales or E horizons. If g is used with B,pedogenic change in addition to gleying is im-plied. The horizon is designated Cg if no otherpedogenic change besides gleying has occurred.

§ h � illuvial accumulation of organic matterUsed with B to indicate the accumulation of illu-vial, amorphous, dispersible organic-matter-sesquioxide complexes if the sesquioxide com-ponent is dominated by aluminum but is presentonly in small quantities. The organo-sesquioxidematerial coats sand and silt particles. In some ho-rizons, these coatings have coalesced, filled pores,and cemented the horizon. The symbol h is alsoused in combination with s, as in Bhs, if theamount of sesquioxide component is significantbut the color value and chroma of the horizonwhen moist is 3 or less.

§ i � slightly decomposed organic matterUsed with O to indicate the least decomposed ofthe organic materials. Its rubbed fiber content is40 percent or more (by volume).

§ k � accumulation of carbonatesIndicates an accumulation of alkaline-earth car-bonates, commonly calcium carbonate.

27

§ m � cementation or indurationIndicates continuous or nearly continuous cemen-tation. The symbol m is used for horizons thatare more than 90 percent cemented, although theymay be fractured. The cemented layer is physi-cally root-restrictive. The predominant cement-ing agent (or the two dominant cementing agents)may be indicated by using defined letter suffixes,singly or in pairs. Following are some suffix com-binations and what they indicate:

km � cementation by carbonates;qm � cementation by silica;sm � cementation by iron;ym � cementation by gypsum;kqm � cementation by lime and silica; andzm � cementation by salts more soluble than

gypsum.

§ n � accumulation of sodiumIndicates an accumulation of exchangeable so-dium.

§ o � residual accumulation of sesquioxides

§ p � tillage or other disturbanceIndicates a disturbance of the surface layer bymechanical means, pasturing, or similar uses. Adisturbed organic horizon is designated Op. Adisturbed mineral horizon is designated Ap, eventhough it is clearly a former E, B, or C horizon.

§ q � accumulation of silicaIndicates an accumulation of secondary silica.

§ r � weathered or soft bedrockUsed with C to indicate root-restrictive layers ofsaprolite, such as weathered igneous rock, or ofsoft bedrock, such as partly consolidated sand-stone, siltstone, and shale. Excavation difficultyis low to high.

§ s � illuvial accumulation of sesquioxides andorganic matter

Used with B to indicate an accumulation of illu-vial, amorphous, dispersible, organic-matter-sesquioxide complexes if both organic-matter andsesquioxide components are significant, and if

color value and chroma of the horizon when moistis 4 or more. The symbol is also used in combi-nation with the symbol h, as in Bhs, if both theorganic-matter and sesquioxide components aresignificant, and if the color value and chroma,moist, is 3 or less.

§ ss � presence of slickensidesIndicates the presence of slickensides. Slicken-sides result directly from the swelling of clayminerals and shear failure, commonly at anglesof 20 to 60 degrees above horizontal. They areindicators that other vertic characteristics, suchas wedge-shaped peds and surface cracks, maybe present.

§ t � accumulation of silicate clayIndicates an accumulation of silicate clay that haseither formed and subsequently been translocatedwithin the horizon or has been moved into thehorizon by illuviation, or both. At least some partof the horizon should show evidence of clay ac-cumulation either as coatings on surfaces of pedsor in pores, or as lamellae or bridges betweenmineral grains.

§ v � plinthiteIndicates the presence of iron-rich humus-poorreddish material that is firm or very firm when moistand hardens irreversibly when exposed to the at-mosphere and to repeated wetting and drying.

§ w � development of color or structureUsed with B to indicate the development of colorand structure, or both, with little or no apparentilluvial accumulation of material. It should notbe used to indicate a transitional horizon.

§ x � fragipan characterIndicates a genetically developed layer with a com-bination of firmness, brittleness, and commonlya higher bulk density than adjacent layers. Somepart of the layer is physically root-restrictive.

§ y � accumulation of gypsum

§ z � accumulation of salts more soluble than gyp-sum

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DIAGNOSTIC HORIZONS: EPIPEDONS

Master horizons describe a soil profile, while diag-nostic horizons are used to classify soils. Whereasmaster horizons are based on appearance, diagnos-tic horizons are based on soil formation processes.These two classification schemes are not comple-mentary. Diagnostic horizons can contain all or partof more than one master horizon.

An epipedon is the surface, or uppermost soil hori-zon. It may be thinner than the soil profile A hori-zon, or include the E or part or all of the B horizon.Epipedons derived from bedrock lack rock structureand are normally darkened by organic matter.

Anthropic epipedonWhile similar to the mollic epipedon, the anthropicepipedon contains greater than 250 ppm citric acidsoluble P2O5 with or without a 50 percent base satu-ration and requires that the soil is moist three monthsor more over 8 to 10 years. It is commonly found infields cultivated over long periods of time.

Histic epipedonThis organic horizon is water saturated long enoughfor reduced conditions to occur unless artificiallydrained. It is 20 to 60 cm thick and has a low bulkdensity often less than 1 g cm-3. The actual organicmatter content is dependent on the percent clay. Ifthe soil has not been plowed, it must contain between12 percent or more organic carbon with no clay and18 percent or more organic carbon with 60 percentor more clay. When the soil has been plowed, theorganic carbon content is from 8 percent with noclay to 16 percent with 60 percent or more clay.

Melanic epipedonThis thick, black surface horizon with a high organicmatter content formed in volcanic ejecta. It has aminimum thickness of 30 cm, contains 6 percent ormore organic carbon, and has volcanic mineral-likeallophane throughout.

Mollic epipedonThis epipedon is a soft dark grassland soil. Its or-ganic carbon content is 0.6 percent or more result-ing in a color value of 3 or less moist, 5 or less dry.Its base saturation is 50 percent or more. It mea-sures a minimum of 18 cm thick if not directly above

a petrocalcic horizon, duripan, or a lithic or paralithiccontact, and contains less than 250 ppm P2O5. Moistthree months or more each year, it cannot have bothhard consistence and massive structure.

Ochric epipedonThis epipedon does not meet the definitions of anyother surface horizon. It does not have the thickness,percent organic carbon, or color to be a mollic orumbric epipedon. The ochric epipedon extends tothe first illuvial (B) horizon.

Plaggen epipedonThis man-made horizon is 50 cm or more thick andhas resulted from centuries of accumulation of sod,straw, and manure, for example. It commonly con-tains artifacts such as pottery and bricks.

Umbric epipedonMollic-like in thickness, organic carbon content,color, P2O5 content, consistence, and structure, thisepipedon has less than 50 percent base saturation.

DIAGNOSTIC SUBSURFACE HORIZONS

Diagnostic subsurface horizons can be categorizedas weakly developed horizons, as horizons featur-ing an accumulation of clay, organic matter, or inor-ganic salts, as cemented horizons, or as stronglyacidic horizons.

AgricThis horizon forms under a plow layer. It normallyhas lamellae (finger-shaped concentrations of mate-rial) of illuvial humus, silt, and clay.

AlbicClay, humus, and other coatings have been leachedfrom this eluvial horizon, leaving light-colored sandand silt particles.

ArgillicThis illuvial horizon of mostly high-charged layersilicate clay has clay films on the faces of peds orsome indication of clay movement. It is at least one-tenth the thickness of all overlying horizons. If theoverlying horizon has less than 15 percent clay, theargillic has 3 percent more clay than the eluvial ho-

29

rizon above. If the overlying horizon has 15 to 40percent clay, the argillic has 1.2 times that amount.If the overlying horizon has over 40 percent clay,the argillic has 8 percent more clay.

CalcicMeasuring 15 cm or more thick, this horizon is notindurated or cemented, and has evidence of calciumcarbonate movement. It has a 15 percent or moreCaCO3 equivalent unless there is below 18 percentclay, then the requirement is a 5 percent or moreCaCO3 equivalent. If the horizon is cemented, it isclassified as petrocalcic.

CambicThis horizon shows some evidence of alterations butis very weakly developed between A and C hori-zons. The cambic horizon has less illuviation evi-dence than found in the argillic and spodic horizons.

DuripanThis subsurface horizon is cemented by silica in morethan 50 percent of its volume. It dissolves in con-centrated basic solution or alterating acid and thenbasic solutions, but does not slake in HCl.

FragipanA fragipan is a brittle horizon situated at some depthbelow an eluvial horizon. It has a low organic mat-ter content, lower bulk density than overlying hori-zons, and hard or very hard consistence when dry.

GlossicThis transitional horizon has parts of an eluvial ho-rizon and the remnants of a degrading argillic, kandic,or natric horizon.

GypsicAn illuvial horizon, the gypsic is 15 cm or more thickwith 5 percent or more gypsum and at least 1 per-cent by volume visible gypsum. If the horizon iscemented, it is classified as petrogypsic.

KandicThe kandic is a horizon with an illuvial accumula-tion of 1:1 (kaolinite-like) clay that has a CEC ofless than 16 cmolc kg-1 clay. A clay increase within15 cm of the overlying horizon is 4 percent or moreif the surface has less than 20 percent clay; 20 per-

cent or more if the surface has 20 to 40 percent clay;or 8 percent or more if the surface has greater than40 percent clay. The horizon is 30 cm thick unlessthere is a lithic, paralithic, or petroferric contact, inwhich case minimum thickness is 15 cm. Organiccarbon constantly decreases with increasing depth.

NatricThe natric horizon is similar to the argillic horizonwith the additional characteristics of columnar struc-ture. It has an exchangeable sodium percentage of15 percent or more.

OxicThe oxic horizon contains highly weathered clays.It is 30 cm or more thick and has a CEC of less than16 cmolc kg-1 clay. Less than 10 percent of the min-erals are weatherable. Within a distance of 15 cm,there is an increase in clay of 4 percent or less if thesurface horizon contains less than 20 percent clay;less than 20 percent if the surface contains 20 to 40percent clay; or 8 percent or less if the surface con-tains 40 percent or more clay.

PlacicThis subsurface horizon is cemented by iron, ironand manganese, or iron and organic matter.

SalicMeasuring 15 cm or more thick, the salic horizoncontains at least 2 percent soluble salt. A 1:1 soil towater extract has an electrical conductivity of 30 dS/m-1 (decisiemens per meter) or more.

SombricThe sombric horizon has an illuvial accumulation ofhumus that is not associated with aluminum (spodic)or sodium (natric).

SpodicThis illuvial horizon contains high pH dependentcharge material. A sandy-textured horizon, it has anaccumulation of humus with aluminum and/or iron.

SulfuricThe sulfuric horizon forms as a result of drainingsoil with a high sulfide content that is oxidized tosulfates, drastically reducing the pH. It is at least 15cm thick and has a pH of 3.5 or less.

30

Birkeland, Peter W.1984 Soils and Geomorphology. Oxford University Press, New York.

Bohn, Hinrich L., Brian Lester McNeal, and George A. O�Connor1985 Soil Chemistry. 2nd ed. Wiley, New York.

Bullock, P., and Michael L. Thompson1985 Micromorphology of Alfisols. In Soil Micromorphology and Soil Classification: Proceedings of a Symposium

Sponsored by Divisions S-5 and S-9 of the Soil Science Society of America in Anaheim, California, 28 Nov. � 3 Dec.1982, edited by Lowell A. Douglas and Michael L. Thompson, pp. 17�47. SSSA, Madison.

Buol, S. W., F. D. Hole, R. J. McCracken, and R. J. Southard1997 Soil Genesis and Classification. 4th ed. Iowa State University Press, Ames.

Daniels, Raymond B., and Richard D. Hammer1992 Soil Geomorphology. Wiley, New York.

Jackson, M. L., and G. D. Sherman1953 Chemical Weathering of Minerals in Soils. Advances in Agronomy 5:219�318.

Miller, R. W., and R. L. Donahue1990 Soils: An Introduction to Soils and Plant Growth. 6th ed. Prentice-Hall, Englewood Cliffs, New Jersey.

1995 Soils in Our Environment. 7th ed. Prentice-Hall, Englewood Cliffs, New Jersey.

Munsell1990 Munsell Soil Color Charts. Macbeth Division of Kollmorgen Instruments, Baltimore.

Natural Resources Conservation Service (Soil Survey Staff)1997 Keys to Soil Taxonomy. 7th ed. Natural Resources Conservation Service, U.S. Dept. of Agriculture, Washington.

Scudder, Sylvia J., John E. Foss, and Mary E. Collins1996 Soil Science and Archaeology. Advances in Agronomy 57:1�76.

Soil Conservation Service (Soil Survey Staff)1975 Soil Taxonomy. Soil Conservation Service, U.S. Dept. of Agriculture, Washington.

Stevenson, F. J.1982 Humus Chemistry: Genesis, Composition, Reactions. Wiley, New York.

Thien, S. J., and J. G. Graveel1997 Laboratory Manual for Soil Science: Agricultural and Environmental Principle. 7th ed. Brown, Chicago.

Torrent, J., and V. Barrow1990 Laboratory Measurements of Soil Color: Theory and Practice. In Soil Color: Proceedings of a Symposium Spon-

sored by Divisions S-5 and S-9 of the Soil Science Society of America in San Antonio, Texas, 21�26 Oct. 1990,edited by J. M. Bigham and E. J. Ciolkosz, pp. 21�23. SSSA, Madison.

SELECTED BIBLIOGRAPHY

31

FIELD DESCRIPTION CHECK LIST

TEXTURE:

(Complete entry or circle/highlight appropriate description.)

MASTER HORIZON O A E B C R

SUBORDINATE DISTINCTION (a�z) ____________________________________

DEPTH from ________________ to________________

SLOPE percent

COLOR Hue Value/Chroma

Matrix _________________________________ _________________________________

Mottles _________________________________ _________________________________

Abundance Few (<2 percent) Common (2-20 percent) Many (>20 percent)

Size Fine (<5 mm) Medium (5-15 mm) Coarse (>15 mm)

Contrast Faint (mottles recognizable with close examination)

Distinct (mottles readily seen but not striking; one or two hues or several value or chroma units apart)

Prominent (mottles an outstanding feature; hue, value, and chroma several units apart)

5LEERQ�6L]H'RPLQDQW�)HHO�:KHQ�:HW

1RQH 6KRUW�����LQ�� 0HGLXP���±��LQ�� /RQJ��!��LQ��

6PRRWK 6LOW 6LOW�ORDP 6LOW\�FOD\�ORDP 6LOW\�FOD\

1HLWKHU�VPRRWK�QRU�JULWW\ /RDP &OD\�ORDP �&OD\

*ULWW\��YHU\�VOLJKWO\�FRKHVLYH� /RDP\�VDQG

*ULWW\��QRQFRKHVLYH� 6DQG 6DQG\�ORDP 6DQG\�FOD\�ORDP 6DQG\�FOD\

32

CONSISTENCE:

Moist: 0. Loose ..................... Soil material noncoherent

1. Very friable ........... Aggregates crush easily

2. Friable .................... Gentle pressure required to crush aggregates

3. Firm ....................... Moderate pressure required to crush aggregates

4. Very firm ............... Strong pressure required to crush aggregates

5. Extremely firm ...... Aggregates cannot be crushed by hand

Dry: 0. Loose ..................... Soil material noncoherent

1. Soft ........................ Aggregates easily break to single grains

2. Slightly hard .......... Gentle pressure required to crush material

3. Hard ....................... Aggregates barely breakable by thumb and finger

4. Very hard ............... Aggregates barely breakable in both hands

5. Extremely hard ...... Aggregates cannot be broken by hand

ROOTS:

Abundance: Few (<2 percent) Common (2�20 percent) Many (>20 percent)

Size: Fine (<5 mm) Medium (5�15 mm) Coarse (>15 mm)

HORIZON BOUNDARY:

Thickness: Abrupt (<1 in.) Clear (1�2.5 in.) Gradual (2.5�5 in.) Diffuse ( >5 in.)

Shape: Smooth (boundary is nearly a plane)

Wavy (undulating, greater across the slope)

Irregular (undulating, greater in the direction of the slope)

Broken (discontinuous)