OREGON SOILSOREGON SOILSMANUAL FOR JUDGINGMANUAL FOR JUDGING

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Manual 6 • Reprinted September 2007$12.50

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............................................................................................... 1

Factors of Soil Formation ................................................... 4Processes of Soil Formation............................................... 6From Rock to Soil ................................................................ 7

Master Horizons .................................................................. 9Special Kinds of A, B, and C Horizons ............................ 10Transition Horizons .......................................................... 12Subdivisions of Thick Horizons ....................................... 13More Than One Kind of Parent Material......................... 13Typical Horizon Sequences .............................................. 14Locating Boundaries Between Horizons ........................ 14

Color of the Soil Matrix ..................................................... 15Mottling .............................................................................. 17Texture ................................................................................ 18Coarse Fragments .............................................................. 23Soil Structure ..................................................................... 25Soil Consistence ................................................................ 28Horizon Boundaries .......................................................... 29Special Features of Soil Horizons .................................... 30

Effective Depth of Rooting ............................................... 33Available Water-Holding Capacity ................................... 34Permeability ....................................................................... 36Water Erosion Hazard ....................................................... 39Wind Erosion Hazard ........................................................ 41Internal Soil Drainage ........................................................ 42Color Plates ........................................................................ 47

Landform ............................................................................ 51Parent Materials ................................................................ 54Stoniness and Rockiness .................................................. 57Slope .................................................................................... 58Aspect ................................................................................. 59

Feasibility of Artificial Drainage ...................................... 61Suitability for Irrigation .................................................... 63Most Intensive Crop .......................................................... 64Erosion Control Practice .................................................. 70Reaction Correction .......................................................... 73Limitation for Septic Tank Drainfields ............................ 74

A. How to Use Interpretation Guides .............................. 79B. How to Set Up and Run a Soil Judging Contest ......... 81C. Horizon Names Used Before 1983 ............................... 85Glossary .............................................................................. 87Scorecard ............................................................................ 95Interpretation Guide ......................................................... 97

ContentsChapter lWhy Study Soil?

Chapter 2The Soil We Study

Chapter 3Kinds of Soil Horizons

Chapter 4Properties of Soil Horizons

Chapter 5Properties of the Whole Soil

Chapter 6Site Characteristics

Chapter 7Management Interpretations

Appendixes

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Soil judging has been part of thevocational agriculture curriculum inOregon high schools for more than

40 years. Since 1956, the OSU ExtensionService and the Oregon Association ofConservation Districts have workedtogether to organize and operate the StateSoil Judging Contest.

The Extension Service also has pro-vided educational materials for the stu-dents and conducted training sessions forthe instructors. For many years, Exten-sion Bulletin 769, Soil Judging from theGround Up, was used as the primary textfor the program.

As we learned more about Oregon soils,however, and as soil management alterna-tives became more complicated, werealized that just revising EB 769 wouldnot do the job. Instead a completely newmanual was needed.

We began writing a new soil judgingmanual, and developing a new format forjudging soils, in 1978, when we introducedseveral new ideas to a group of about 20Vo-Ag instructors. They liked the newformat, and their comments were veryhelpful as we wrote a draft of a newmanual. We used this draft for 4 years whilewe tested new ideas and revised the text.

Publication of this manual would nothave been possible without numeroussuggestions from many soil scientists andVo-Ag instructors who have been veryclosely associated with the soil judgingprogram for a number of years. To each ofthem, we express our sincere apprecia-tion.

There are several major changes in thisnew Manual.1. We encourage students to study the

whole soil profile and evaluate each ofthe natural soil horizons present.

2. We stress gathering primary data onjust a few important properties thatstudents can determine in the field—color, texture, and structure.

3. We show students how to use thesebasic properties to discover some

Foreword

important aspects of soil behavior,such as effective depth, water-holdingcapacity, susceptibility to erosion, andinternal drainage class.

4. We explain why soil properties and soilbehavior are important to soil manage-ment.

5. We develop keys for students to use torelate soil properties to importantdecisions about irrigation suitability,crop choices, erosion control, andother questions.We don’t expect students to memorize

all the factors and interactions that enterinto a management decision. We do hope,however, that they will learn how to usethe keys to translate their evaluations ofprofile data into solutions to real prob-lems facing modern agriculturalists andenvironmental scientists.

Above all, we view soil judging prima-rily as an educational program. Contestscertainly add a dimension of challengeand fun, but they are not the primaryobjective of the program. Instead, themain objective is to encourage studentsto investigate this fascinating resource wecall soil, to discover how soils are orga-nized into horizons, to learn both how todescribe a few key properties of soilhorizons and to interpret them in terms ofmanagement practices—and to develop asense of stewardship for the soils thatsupport them.

Nothing would make us happier thanfor a student to challenge some of thegeneralized management interpretationswe describe in this manual in terms oftheir relevance to his or her own soilconditions—and for the instructor toseize that opportunity to do some realteaching about the kinds of interactionsone must consider when deciding howbest to use the soil resources available ona given farm. When this happens, theManual for Judging Oregon Soils will haveaccomplished its real purpose.

1Chapter 1

Why Study Soil?

Soil is an essential natural resource.Without a doubt, that is the mostimportant reason for studying the

soil. Nathaniel Shaler put it this way: “Soil,ever slipping away in streams to the sea,is a kind of placenta that enables livingthings to feed upon the earth.”

The sketch in Figure 1 illustrates ourdependence on the soil. Unlike plant life,we human beings can’t manufacture ourown food from the four primary resourcesof soil, air, water, and sunlight. Instead, wedepend completely on green plants, whichtake nutrients and water from the soil andcombine them with air and sunlight toprovide our food supply.

Some of those plants, such as wheatand corn, we eat directly. Others, such as

1Shaler, N.S., Man and the Earth (Chautauqua,NY: The Chautauqua Press, 1915).

alfalfa hay and range grasses, we processthrough livestock first. Even the fish weeat depend on plants that grow in the seausing nutrients that have been washed outof the soil and carried to the sea in riversand streams.

We study the soil, then, to increase ourunderstanding of this resource that sup-ports us.

Another important reason for studyingsoil is that soils are different. Oregon alonehas nearly 2,000 different kinds of soils,ranging from deep to shallow, clayey tosandy, wet to dry, nearly level to steeplysloping. These differences are important,because different soils require differentkinds of management practices.

Wet soils, for example, are essentialcomponents of wetland ecosystems.Proper management of wet soils dependson recognizing the imprint of saturated

Figure 1.—All of our food resources can be traced directly back to the soil.

2 Chapter 1 • Why Study Soil?

conditions on soils and maintaining watertables at desired levels.

Many soils require irrigation for maxi-mum productivity. Both the amount ofirrigation water needed and the propermethod of applying it depend on a soil’spermeability rate and water-holdingcapacity.

Still other soils have a very seriouserosion hazard. The proper choice ofconservation practices depends in part onthe texture of the surface horizon and thesteepness of the slope.

Only by studying soils can we learn howto tailor management practices to thespecific needs of each of the many differ-ent kinds of soils we depend on.

Some of us study soil because we’re justplain curious about this unique andfascinating natural resource. When youdig a hole or scrape off a road cut, youdiscover right away that there’s a wholelot more to the soil than just the top 8 or10 inches.

Soil scientists, in fact, study the soil to adepth of about 5 feet. They see severaldifferent layers, or horizons, in the soil.Together, these horizons make a soilprofile.

We describe the horizons in a soilprofile in terms of their properties, or

morphology. Some properties, such ascolor and root abundance, can be deter-mined by eye. Others, such as texture andstructure, require a keen sense of touch.

You, too, can learn how to determinethe important properties of soil horizons.Then you will be able to make a number ofimportant decisions about drainage,irrigation, crop selection, erosion con-trol—and much, much more.

At the beginning of this chapter, wetalked about how all human life dependson our soil resources. Knowing that, wemust study the soil so that we can learnhow to protect it for others to use. There’splenty of good soil on this planet—as longas we take care of it properly. But if we letit erode, or compact it, or mine it, it willfail to support us.

Farmers are the primary stewards of thesoil, for they are the tillers of the land. Allof us, however, share the responsibility toprotect this valuable resource. If wemanage our soil properly, it will continueto nourish us for generations to come. Ifwe don’t, our very civilization is threat-ened.

So study the soil, learn about its proper-ties and behavior, manage it wisely, anddo your part as a steward of the land.

3

The Soil We Study

Chapter 2

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Soil has been defined by lots of different people in lots of different ways.Here’s a very basic definition:

SOIL—the natural medium inwhich plants grow.

This definition, however, may be a littltoo simple. Here’s a better one:

SOIL—a natural body that devel-ops in profile form from a mix-ture of minerals and organicmatter. It covers the earth in avery thin layer and suppliesplants with air, water, nutrients,and mechanical support.

Our definition is, of course, the one weprefer:

SOIL—a living, dynamic system atthe interface between air androck. Soil forms in response to

ms

the winter to very dry in the summer.Even under irrigation, the amount ofwater in the soil can vary widely.

Soil organic matter increases when cropresidues are worked in, and decreases asfresh plant materials decay. Soil nutrientsincrease as soil minerals break down.They decrease as water moving throughthe soil carries them away. Even soilacidity, or pH, changes seasonally.

The word system says that all parts ofthe soil work together to make up thedynamic whole. A change in one part maycause changes in many other parts.

Suppose, for example, we add wateruntil the soil is very wet. That reduces theamount of air available to plant roots. Itmakes the soil colder, and the activity ofroots and soil microbes (very small plantsand animals) slows down. The wet soil isstickier and cannot bear as much weight.

Figure 2.—Living, dynamic soil. The microscopic worm shown is called anematode. The lower part of the nematode is being attacked and invadedby a fungus.

forces of climate and organisthat act on parent materialin a specific landscapeover a long period of time.

We like this definitionbecause each key word sayssomething important about thesoil. Why living? Because thesoil is full of living organisms:roots large and small, animalsand insects, millions of micro-scopic fungi and bacteria.

Equally important are thedecaying remains of plants andanimals after they die. Theyform soil organic matter, orhumus, which is vitally impor-tant for good soil tilth andproductivity.

Dynamic says that the soilchanges all the time. Oregonsoils change from very wet in

4 Chapter 2 • The Soil We Study

Now let’s remove the excess water. Thewhole system changes to a warmer anddrier soil that is better for plants to growin and easier for farmers to manage.

The word interface stresses the ideathat soil is indeed a very thin rind at theearth’s surface. When air meets rock,especially if the air is warm and the rockis moist, the rock begins to change. Somechanges are physical. They break rocksdown into smaller pieces. Other changesare chemical. They destroy some of theoriginal minerals and create new ones.

These physical and chemical changesare called weathering. Weathering occursonly within the first few feet of the earth’ssurface. Plate 5 illustrates a stronglyweathered soil. The light-colored parts ofthe BC horizon are highly weatheredbedrock remnants not yet fully changedinto soil.

Now consider the size of the earth. Thedistance from the surface to the center ofthe earth is about 4,000 miles. Thus,10 feet of weathered rock out of4,000 miles is something less than.00005 percent. Soil does indeed occur atthe point of contact between earth andatmosphere!

Factors of Soil FormationThe rest of the key words in our defini-

tion of soil tell us something about howsoil forms. We think in terms of fivesoil-forming factors. Two of them, climateand organisms, are called active factors.They provide the forces that cause soil toform. The other three, parent material,

Five Soil-forming FactorsClimate

OrganismsParent Material

TopographyTime

topography, and time, are called passivefactors. They respond to the forcesexerted by climate and organisms.

Together, the interactions between theforce factors and the response factorsresult in a new product, a unique naturalresource, which we call soil.

ClimateClimate affects soil most directly

through temperature and rainfall. Inwarm, moist climates, rocks and mineralsweather very quickly. The soil that formsoften has a reddish color. Most of the redsoils in western Oregon are red becausethey formed when the climate waswarmer than it is now.

High rainfall also causes leaching—theremoval of soil materials by water flowingthrough the soil. Free lime is completelyleached from western Oregon soils. Thesesoils are acid. Free lime is still present inmany eastern Oregon soils because thereisn’t enough rain to leach the soil com-pletely.

Warm, moist climates encourage lots ofplant growth, and that means lots of soilorganic matter. The opposite is true inhot, dry climates. Soils in the WillametteValley are dark-colored because they haveplenty of organic matter. Soils in theOntario area are light-colored becausethey have very little organic matter.

OrganismsOrganisms are of three general types:

large plants, tiny plants (microbes), andanimals. Roots of large plants help breakrocks apart and mix soil particles. Rootchannels provide pathways for water andair movement through the soil. Above-ground plant parts die and decay, therebybuilding up the organic matter in the soil.

Microscopic organisms, or microbes,are an extremely important part of thesoil. They are the primary decomposers.They change raw plant material into acomplex black substance called humus. Atthe same time they release soil nitrogen,

5Factors of Soil Formation

an essential nutrient that plants need inlarge quantities. Thus, rich, fertile topsoilsare rich and fertile because they are wellsupplied with humus (see Plates 3, 4, and8). Even the earthy smell of moist, rich,topsoil is caused by a microorganism.

Microbes and the humus they producealso act as a kind of glue to hold soilparticles together in aggregates. Well-aggregated soil is ideal for providing theright combination of air and water toplant roots.

Without microbes, therefore, soil wouldbe a virtually inert (lifeless) body. Withthem, soil is truly a living, dynamic sys-tem.

Soil animals include large burrowinganimals, small earthworms and insects,and microscopic worms called nema-todes. All are important because they helpmix soil. Animal mixing carries raw plantdebris that lies on the soil surface downinto the topsoil. Only then can themicrobes do their job of changing plantmaterial to humus.

Parent materialParent material is the original geologic

material that has been changed into thesoil we see today. Parent material ispassive because it simply responds to thechanges brought about by weathering andbiological activity.

Many parent materials are some kind ofbedrock, like sandstone or basalt. Othersare deposits of sediments carried bywater, wind, or ice. Volcanic ash, lake-laidsilts, dune sand, and glacial gravels all areexamples of transported parent materials.

TimeTime is the great equalizer. Young soils

inherit the properties of their parentmaterials. They tend to have the samecolor, texture, and chemical compositionas their parent materials. Later on, the

influence of parent material is not asevident.

As soils age, many original minerals aredestroyed. Many new ones are formed.Soils become more leached, more acid,and more clayey. In short, the soilbecomes more strongly developed withthe passage of time.

TopographyTopography, or landscape position,

causes localized changes in moisture,temperature, and parent material. Whenrain falls on a hillslope, for example, waterruns away from the top of the hill. Excesswater collects at the bottom of the hill.Soils at the top of the hill are relativelydry and often show the effects of erosionon the soil profile. Soils at the bottom ofthe hill not only are wetter but often areformed in materials transported downslope and deposited in lower landscapepositions.

Another effect of topography is due tothe direction that a slope faces. Soils onnorth-facing slopes, for example, tend tobe cooler and wetter than soils on south-facing slopes.

Figure 3.—Humus formation. Strands of fungus surround afreshly clipped blade of grass. Decomposition is beginningand will soon convert the clipping to soil organic matter.

6 Chapter 2 • The Soil We Study

Processes of Soil FormationOur definition of soil has identified five

factors of soil formation. We also can thinkin terms of four major processes thatchange parent material into life-sustainingsoil. They are additions, losses, transloca-tions, and transformations.

AdditionsThe most obvious addition is organic

matter. As soon as plant life begins togrow in fresh parent material, organicmatter begins to accumulate. Organicmatter gives a black or dark brown colorto surface soils. This is why even veryyoung soils usually have a dark-coloredsurface layer.

Other additions come with rainfall. Onthe average, rainfall adds about fivepounds of nitrogen each year to everyacre of soil. Rainfall also can be acid,especially downwind from industrialareas. Acid rain may change the rate ofsome soil processes. Rainfall, by causingrivers to flood, is indirectly responsiblefor the addition of new sediments to thesoil on a river’s floodplain.

LossesMost losses occur by leaching. Water

moving through the soil dissolves certainminerals and carries them out of the soil.Some minerals, especially salt and lime,are readily soluble. They are removedvery early in the history of a soil’s forma-tion. That’s why soils in humid regionsdon’t contain free lime.

Processes of Soil FormationAdditions

LossesTranslocations

Transformations

Many fertilizers, especially nitrogenfertilizers, also are quite soluble. They,too, are readily lost by leaching, either bynatural rainfall or by irrigation water.

Other minerals, such as iron oxides andsand grains, dissolve very slowly. Theyremain in the soil until it is very old andhighly weathered.

Losses also occur as gases or solids.Oxygen and water vapor are lost from soilas fresh organic matter decays. And whensoils are very wet, nitrogen can bechanged to a gas and lost to the atmo-sphere. Solids are lost by erosion, whichremoves both mineral and organic soilparticles. Such losses are very serious, forthe soil lost by erosion usually is the mostproductive part of the soil profile.

TranslocationsTranslocation means movement from

one place to another. Usually we think ofmovement out of a horizon near the soilsurface and into another horizon that isdeeper in the soil.

In low rainfall areas, leaching often isincomplete. Water starts moving downthrough the soil, dissolving soluble miner-als as it goes. But there isn’t enough waterto move all the way through the soil.When the water stops moving, thenevaporates, the salts are left behind.That’s how subsoil accumulations of freelime are formed (Plate 9). Many hardpansin soils of dry areas form this way, too(Plate 10).

Upward translocation also is possible.Even in the dry areas of eastern Oregon,there are some wet soils that have highwater tables. Evaporation at the surfacecauses water to move upward continu-ously. Salts are dissolved on the way, andleft behind as the water evaporates(Figure 4). Salty soils are difficult tomanage, and they are not very productive.

Another kind of translocation involvesvery thin clay particles. Water movingthrough the soil can carry these particlesfrom one horizon to another, or fromplace to place within a horizon. When the

7From Rock to Soil

water stops moving, clay particles aredeposited on the surface of soil aggre-gates. We call these coatings clay skins,and they have a dark, waxy appearance(Plate 15).

TransformationsThese are changes that take place in the

soil. Microorganisms that live in the soilfeed on fresh organic matter and change itinto humus. Chemical weathering changesthe original minerals of parent materials.Some minerals are destroyed completely.Others are changed into new minerals.Many of the clay particles in soils areactually new minerals that form duringsoil development.

Still other transformations change theform of certain elements. Iron oxideusually gives soils a yellowish-brown orreddish-brown color. In waterlogged soils,however, iron oxide changes to a differentform that we call reduced. Reduced ironoxide is lost quite easily from the soil byleaching. After the iron is gone, the soilhas a gray or white color.

Repeated cycles of wetting and dryingcreate mottled soil (Plates 7 and 16). Part

of the soil is gray because of loss of iron,and part is yellow-brown where the ironoxides have accumulated in localizedareas.

From Rock to SoilHow do all these processes work

together to form soil? Let’s start with afresh parent material. Climate starts actingon it immediately. Weathering begins tochange minerals. Leaching removes salts,then free lime.

As soon as plants begin growing, theyadd organic matter to the soil. Biologicalactivity increases, and humus forms. Soona dark-colored surface horizon is present.

Weathering and leaching continue tochange soil minerals and remove solublecomponents. More horizons developbeneath the surface. The soil becomesmore acid. Clay minerals begin to form.Clay is translocated and clay skins becomevisible.

As the amount of clay in subsoil hori-zons increases, the rate of water move-ment through the soil decreases. Weather-ing continues, but leaching isn’t as rapid.After a while, further change is very slow,

and the whole soil-plant-landscape system is in a kindof steady state.

How do we know that all thishas happened? First, we dug ahole to reveal a soil profile.Next, we studied the soilcarefully, located the horizons,and determined their proper-ties. Then we interpreted theinformation from the profile interms of the factors andprocesses of soil formation.

You, too, can learn how todescribe and interpret soilprofiles. That’s what the rest ofthis Manual is all about.

Figure 4.—Upward translocation. The white areas, calledslick spots, are accumulations of salts left on the soilsurface after water moving up through the soil evaporatedat the surface.

9

Kinds of Soil Horizons

Chapter 3

Asoil horizon is a layer of soil paral-lel to the earth’s surface. It has aunique set of physical, chemical,

and biological properties. The propertiesof soil horizons are the results of soil-forming processes, and they distinguisheach horizon from other horizons aboveand below.

Soil horizons are named using combina-tions of letters and numbers. Six generalkinds of horizons may occur in soil pro-files (Figure 5), and they are named withcapital letters: O, A, E, B, C, R. These arecalled master horizons.

Figure 5.—Generalized soil profile. Eachmaster horizon is shown in the relativeposition in which it occurs in a soil profile. Allsix master horizons are shown, even thoughany one soil usually has only three or four.

Litter layer

Mineral surface horizon, dark-colored, granular structure

Strongly leached horizon,light-colored, platy structure

Subsoil horizon of maximumdevelopment, “brown,” blockystructure

Weathered “parent material,”“brown,” massive structure

Hard bedrock

Gradual changes from one master hori-zon to another give rise to transitionhorizons. These are named with twoletters, for example, AB, BA, BC. Specialkinds of master horizons are named byadding lower case letters—for example Ap,Bt, Cr. Thick horizons may be subdividedusing Arabic numbers, as in Al, A2, or Bw1,Bw2, Bw3.

A single soil profile never has all thehorizons that are possible. Most Oregonsoils have A, B, C, and one or two transi-tion horizons. Other Oregon soils mayhave an A horizon resting directly on aC horizon, or an A-E-B-C horizon sequence,or even an O-E-B-C sequence.

Because all six master horizons occursomewhere in Oregon, we need to knowwhat each one is and how it differs fromthe others.

Master HorizonsEach master horizon has a distinct set of

properties.

O horizonThe O stands for organic. O horizons

don’t have to be 100 percent organicmaterial, but most are nearly so. Forestsoils usually have thin organic horizons atthe surface. They consist of leaves andtwigs in various stages of decay (Plate 1).

Wet soils in bogs or drained swampsoften have O horizons of peat or muck.Soils in Lake Labish, Waldo Lake, andLower Klamath Lake all have O horizons ofthis kind. Other than these, very fewagricultural or rangeland soils in Oregonhave O horizons.

10 Chapter 3 • Kinds of Soil Horizons

A horizonThe A horizon is the surface horizon of

mineral soil. Its unique characteristic is adark color formed by addition of humus(Plates 3, 4, 5, 6, 8, and 12). Granular orfine blocky structure (aggregate shape)and friable consistence (ease of crushing)also are typical.

The thickness of A horizons rangesfrom a few inches in dry, rangeland soilsto over 20 inches (50 cm) in someWillamette Valley soils. Every cultivatedagricultural soil has an A horizon.

A horizons are extremely important inmaintaining soil fertility and providing afavorable environment for root growth.They should be protected from damage byerosion or compaction.

E horizonThis horizon has a light gray or white

color. It’s not present in all Oregon soils,but when it is, it usually occurs immedi-ately beneath an O or an A horizon(Plates 1, 5, 6, and 11).

E horizons are light-colored becausenearly all the iron and organic matter hasbeen removed. You can think of the E asmeaning exit or leaching.

E horizons occur in several of the sandysoils along the Oregon coast. They alsooccur in some wet, silty soils that havedense, clayey subsoils. In the wet soils,the E horizon also has noticeably less claythan the B horizon beneath it.

B horizonThe B horizon is the subsoil layer that

changes the most because of soil-formingprocesses. Several kinds of changes arepossible.

In some soils, the B horizon has thebrightest yellowish-brown or reddish-brown color (Plates 1, 2, 4, 5, 9, and 12). Inothers, it has the most evident blocky orprismatic structure (Plates 2 and 3). ManyB horizons have more clay than any other

horizon, and you may be able to see clayskins (Plates 3, 5, 6, and 9). Each of thesemajor kinds of B horizons is discussedmore fully in the next section, “SpecialKinds of A, B, and C Horizons.”

C horizonThe C horizon is weathered geologic

material below the A or B horizon. Any-thing that you can dig with a spade butwhich has not been changed very muchby soil-forming processes is consideredC horizon (Plates 1, 2, 3, 4, 9, and 13).

R horizonR stands for rock. It refers to hard

bedrock that you can’t easily dig with aspade. Depending on the depth to bed-rock, the R horizon may occur directlybeneath any of the other master horizons(Plate 12).

To judge an R horizon, mark its color,and check none for mottles and NA fortexture. Bedrock essentially is 100 percentcoarse fragments, so check more than60 percent. The structure type is massiveand the grade is structureless.

Special Kindsof A, B, and C Horizons

Many horizons are the result of uniqueprocesses that leave a distinct mark onthe horizon. We identify these horizonswith a lower-case letter immediatelyfollowing the master horizon symbol.Over 25 letters and combinations ofletters are possible. We’ll discuss onlynine that you are most likely to encounterin Oregon soils.

Ap horizonThe surface horizon of any soil that has

been plowed or cultivated is called theplow layer (Plates 2, 6, 9, and 12). That’swhat the p stands for. Cultivation thor-oughly mixes the upper 8 to 12 inches

11Special Kinds of A, B, and C Horizons

(20 to 30 cm) of the soil and destroys anynatural horizons that may have beenpresent.

If the original A was very thick, plowingconverts the upper part into an Ap, andthe lower part remains simply as a secondA horizon. If the original A was very thin,then the Ap could rest on a B, a C, or atransition horizon.

Even when a soil has been severelyeroded, such that all the original A is gone,plowing an exposed B or C horizon wouldautomatically make the surface horizon an Ap.

Bt horizonThe t stands for texture. Textural B

horizons have distinctly more clay inthem than the horizons above or below.You can feel the difference.

Some of the clay comes from the soilabove the Bt. Water moving down throughthe soil carries very fine clay particleswith it. When the downward movementstops, the clays are deposited, building upthe waxy coatings we call clay skins. Someof the clay also comes from the weather-ing of original minerals in the Bt.

Bt horizons are quite common inOregon soils. They usually have well-developed blocky or prismatic structure(Plates 3, 5, 6, and 9).

Bg horizonThis horizon is gleyed. That means it’s

very wet for long periods of time. Iron inthe soil is chemically reduced, and muchof it has been leached out of the soil. As aresult, gleyed horizons usually are darkgray in color (Plates 7 and 8). They alsomay be mottled (see page 17), but notnecessarily so.

Gleyed horizons almost always tell usthat the soil is poorly or very poorlydrained. Gleying is not restricted to theBg; other gleyed horizons include Ag, BAg,BCg, Cg.

Bs horizonWe call this horizon a spodic horizon.

It’s common only in some of the sandysoils on marine terraces along the Oregoncoast. A few soils at very high elevationsin the Cascades and the Blue Mountainsalso have spodic horizons.

The color of a spodic horizon is quitedistinctive. It’s usually bright yellowish-brown or reddish-brown, and it fades withdepth (Plate 1). Often there’s a thin blacklayer at the top. The spodic horizon formswhen iron, aluminum, and organic matterall are leached out of surface horizons,carried downward, and deposited in thesubsoil.

Bw horizonThink of the w as meaning weathered.

Bw horizons have been changed by weath-ering, but not enough to form a Bt, Bg, orBs. In Oregon soils, the Bw differs from theC by having weak or moderate blockystructure (see page 25). The Bw also mayhave a little brighter color (Plates 2, 4, 11,and 12), and it may be more leached thanthe C.

Bw horizons are common in soils of theCascade and Coast Range Mountains, inyoung soils on river floodplains and lowterraces, and in many soils of easternOregon.

Bx horizonThis refers to a special feature called a

fragipan. It is a massive, dense, but notcemented, soil horizon. The fragipan isoften mottled and has streaks of gray siltscattered throughout (Plate 11). Thefragipan is so dense that neither plantroots nor water can penetrate, except inthe gray silt streaks. In Oregon, fragipansoccur only in some of the upland soils ofColumbia, Washington, Multnomah, andClackamas counties.

12 Chapter 3 • Kinds of Soil Horizons

Bk horizonThis horizon has an accumulation of

calcium carbonate, or free lime. Carbon-ates leached from upper horizons havebeen redeposited in the Bk (Plate 9). Youshould be able to see white streaks ornodules of lime. These will bubble vio-lently when a drop of hydrochloric acid(HCl) is placed on them.

Be careful though. Some soils in easternOregon have had free lime in them rightfrom the beginning. They will react to theacid, too. We use the k only to indicate ahorizon enriched in carbonates by trans-location. A Bk horizon may very well havean ordinary C horizon beneath it thatcontains only its original amount of lime.

Bkqm horizonThis horizon is called a duripan

(Plate 10). It is enriched with calciumcarbonate (k) and silica (q), and it isstrongly cemented (m).

Duripans are common in several soils ofeastern Oregon. Limited rainfall hasleached lime and silica from the upper 10to 20 inches of soil and redeposited themin the Bkqm. Thin, pinkish coatings ofopal may be present on the upper sur-faces of duripan fragments. The duripanusually is only 6 to 10 inches thick, but it isso cemented that plant roots can’t gothrough it.

A dense mat of roots spreading horizon-tally is a good indicator of a duripan.Sometimes, however, there are fracturesin the duripan that will allow some plantroots to find a way down. And if the pan isonly weakly cemented, you can break itup even more by ripping.

For soil judging contests, mark the mostappropriate color. Check none for mot-tling, NA for texture, and more than60 percent coarse fragments. The struc-ture type is massive and the grade is struc-tureless.

Cr horizonWeathered bedrock, or rock that is soft

enough to slice with a knife or a spade, iscalled Cr (Plate 13). It’s rock material, andyou often can see original rock structure,but it’s not hard enough to be desig-nated R.

When judging a Cr, first record its color,then check none for mottles, NA fortexture, and more than 60 percent coarsefragments. The structure type is massiveand the grade is structureless.

Transition HorizonsMaster horizons rarely change abruptly

from one to another. Instead, the changesoccur gradually throughout a zone thatmay be 5 or 10 inches thick. These zonesare called transition horizons. There arethree common ones in Oregon soils.

AB horizonThis transition horizon occurs between

the A and the B. It’s dominated by proper-ties of the A, but some of the properties ofthe B are evident. Dark colors associatedwith organic matter are fading becauseorganic matter is decreasing (Plate 4). Thestructure often changes from granular tosubangular blocky (see Chapter 4, “SoilStructure”).

BA horizonThis horizon also occurs between the A

and the B, but it has more of the charac-teristics of the B. Generally, the structurewill be the same type as the B, but lessstrongly expressed. The color may be alittle darker than the B (Plate 3), or theclay content may be less than the maxi-mum in the B.

BC horizonThis is a transition from B to C. Proper-

ties of the B are dominant, but someinfluence of the C horizon is evident

13Subdivisions of Thick Horizons

(Plates 2 and 3). Often the clay contentwill be less than the maximum in the B,but more than in the C. Or the color willbe fading. If the C is massive, the BC hasstructure, but it may have larger units andbe more weakly expressed than in the B.

Subdivisionsof Thick Horizons

Sometimes one or two of the horizonsin a soil are so thick that they need to besubdivided. Small changes in texture,color, or structure commonly are used tomake the subdivision.

Subdivisions, or vertical sequenceswithin any single kind of horizon, alwaysare indicated by a number immediatelyfollowing the letter symbol(s). Here are afew examples of some thick soil horizonsthat could be subdivided:

Thick A horizon—Al, A2 (Plate 8)Thick Bg horizon—Bg1, Bg2 (Plate 7)Thick Bt horizon—Bt1, Bt2 (Plate 5)Thick Bw horizon—Bw1, Bw2 (Plate 2)Thick C horizon—C1, C2 (Plates 3

and 4)

More Than One Kindof Parent Material

Parent material is the geologic stufffrom which soils form. It may be a riverdeposit, volcanic ash, clays weatheredfrom rock in place, or one of many otherkinds of materials. When all the horizonsof a soil have formed in a single kind ofparent material, we simply use the ordi-nary A, B, and C designations.

Some soils, however, have formed inmore that one kind of parent material. Aflooding river, for example, may depositfresh silts on top of older sands andgravels. Or volcanic ash may be depositedon top of weathered bedrock.

If soil horizons are developed in morethan one material, we place a number infront of the horizon name to indicate itsposition from the top down.

The geologic material at the surface isalways assumed to be the first one, andthe number 1 is never used.

The second geologic material is indi-cated by a 2, the third by a 3, and so on.Thus a soil developed in silt loam overgravel could have the following set ofhorizons: A-AB-B-2BC-2C.

Figure 6.—Multiple parent materials. The gray material isrecent volcanic ash resting on older, more weathered soilmaterial. Notice the abrupt contact between the two contrast-ing materials.

14 Chapter 3 • Kinds of Soil Horizons

Typical Horizon SequencesSeveral common Oregon soils are listed

below, followed by the names of thehorizons in a typical profile. Nearly allgeographic areas of Oregon are repre-sented by the soils on this list.

Alicel Ap-A-BA-Bw-2CBandon O-E-Bs1-Bs2-Bs3-Bs4-CBrenner Ap-A-Bg1-Bg2-BCg-CgCarney A1-A2-AB-C-2RCascade A-Bw1-Bw2-2Bx1-2Bx2-2Bx3Dayton Ap-E-2Bt-2BCt-3CDeschutes A-BA-Bw-2C-3RFordney Ap-C1-C2Gem Al-A2-BAt-Bt1-Bt2-BCt-BCtk-RHoopal A1-A2-Bw-Bkqm-CJosephine O-A-BA-Bt1-Bt2-Bt3-C-CrMalabon Ap-AB-Bt1-Bt2-BCt-2CNehalem Ap-A-Bw-CNyssa Ap-Bw-Bkq-Bkqm1-Bkqm2Oakland A1-A2-Bt1-Bt2-Bt3-BCt-CrQuincy C1-C2Ritzville Ap-BA-Bw-Bk-C1-C2Salem Ap-Bt-BCt-2CSimas A-2Bt1-2Bt2-2BCk1-2BCk2Walla Walla Ap-A-BA-Bw-BCk1-BCk2

Locating BoundariesBetween Horizons

The most useful thing you can do tohelp yourself find and name soil horizonsis to prepare a good exposure of the soilprofile. Either a good pit or a road cut willdo, but in either case you should clean upthe face of the vertical cut. Use your knifeto pick off any soil that may have fallendown from the surface or that wassmeared by a shovel or a backhoe. Afteryou have a good, fresh surface, try each ofthe following techniques.

1. Look for color changes. Where there isan obvious color change there also is ahorizon change. Color alone, however, isnot sufficient to separate all horizons.Several soils in Oregon have nearlyuniform colors extending all the waythrough the B and into the C.

2. Take a knife and gently poke the soilevery few inches from the surface downto the lower part of the pit. Often youcan “feel” the firmer consistence ofsubsoils and restrictive layers. You mayeven be able to locate a contactbetween B and C this way.

3. Starting at the top, check the soil tex-ture with your fingers every 2 to4 inches. If there is a marked increase inclay from A to B, you should detect itthis way. A decrease in clay from B to Calso should be evident.

4. With your knife, remove a handful of soilfrom the upper 4 inches of soil. Carefullybreak it apart and observe the size,shape, and strength of structural aggre-gates. Repeat this process every 4 to6 inches down through the profile.Structural changes may be a good clueto boundaries between horizons and tothe presence of transition horizons.

5. Each time you locate a tentative bound-ary, mark it with a nail, twig, or someother convenient marker. As you con-sider more and more characteristics,you may want to adjust some of theboundaries up or down.

6. When you have settled on an initial setof horizon boundaries, start lookingmore carefully at the color, texture,structure, pores, clay skins, etc. of eachhorizon. With a complete set of informa-tion, you may wish to make a finaladjustment in your horizon boundaries.