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Introduction to Soil Descriptions

Part 2 of 3

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Acknowledgement

This work was supported [in part] by the National Decentralized Water Resources Capacity Development Project with funding

provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR827881-01-0) with

Washington University in St. Louis. The results have not been reviewed by EPA or Washington University in St. Louis. The

views expressed in this presentation are solely those of NCSU, and University of Arkansas and EPA and Washington University in St.

Louis do not endorse any products or commercial services mentioned in the presentation.

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Soil Consistence

Rule .1941 (a3)

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Definition of Consistence

Consistence is the degree and kind of cohesion and adhesion that soil exhibits, and/or the resistance of soil to deformation or rupture under an applied stress.

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ConsistenceRupture ResistanceStickinessPlasticityManner of Failure Penetration Resistance

Consistence is the degree and kind of cohesion and adherence that soil exhibits, and/or the resistance of soil to deformation or rupture under applied stress. Moisture content strongly influences soil’s consistence. There are 5 ways to record consistence in the field; Rupture Resistance, Manner of Failure, Stickiness, Plasticity, and Penetration Resistance. Each type is recorded at specific moisture contents or within given moisture content ranges.

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The Field Book for Describing and Sampling

Soils

ConsistenceMoist consistenceWet consistence

Soil Taxonomy has changed since the last rule rewrite.

Wet Consistence:Historically, moderately sticky was called sticky.Historically, moderately plastic was called plastic.So technically the regulations have not kept up with the changes in soil taxonomy.

Also, Soil Taxonomy is not the reference we use since “The Field Book for Describing and Sampling Soils” was published

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Moist Consistence

Moist Consistence provides a measure of the tendency for peds or masses of soil material to break when pressure is applied between the thumb and forefinger.

Categories: loose, very friable, friable, firm, very firm, or extremely firm

Loose: Non-coherent.

Very friable: Soil material crushes under very gentle pressure but coheres when pressed together.

Friable: Soil material crushes easily under gentle to moderate pressurebetween thumb and forefinger, and coheres when pressed together.

Firm: Soil material crushes under moderate pressure between thumb and forefinger but resistance is distinctly noticeable.

Very Firm: Soil material crushes under strong pressure; barely crushable between thumb and forefinger.

Extremely Firm: Soil material crushes only under very strong pressure; cannot be crushed between thumb and forefinger and must be broken apart bit by bit.

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Rupture Resistance Classes: Blocks, Peds, and Clods

Strong BlowsInduratedV. RigidV. Rigid

Light BlowsV. Str. cemRigidRigid

Foot pressureStr. cemSl. RigidEx. Hard

Moderate HFMod. cemEx. FirmV. Hard

Str. FFW. cemV. FirmHard

Moderate FFV. w. cemFirmMod. Hard

Slight FFEx. w. cemFriableSl. Hard

Very slight FFNon-cemV. FriableSoft

Cannot get samp.n.a.LooseLoose

DescriptionCementationMoistDry

The Field Book for Describing and Sampling Soils describes the specific force that corresponds with each class. A 2.5 to 3.1 cm cube should be used or a plate 1.0 –1.5 cm long by 0.5 cm thick.

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Slight FF (Finger Force)

Fails under slight force applied slowly between thumb and forefingeri.e. very rich cake or biscuits

FRIABLE

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Moderate FF (Finger Force)

Fails under moderate force applied slowly between thumb and forefingeri.e. well done, dry toast Skin on fingers is depressed while pressure is appliedTendons of fingers will barely flex

FIRM

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Strong FF (Finger Force)

Fails under strong force applied slowly between thumb and forefingeri.e. very stale bread, biscottiSkin on fingers is depressed to bone, may almost be painfulTendons of finger will flex

VERY FIRM

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Wet Consistence

Wet consistence measures the stickiness or plasticity of wet soil material.Stickiness - The capacity of soil to adhere to other objects.Plasticity - The degree to which “puddled” or reworked soil can be permanently deformed without rupturing.

Use criteria set forth in “The Field Book for Describing and Sampling Soils”.

Stickiness and plasticity are estimated at the moisture content that displays the greatest adherence and maximum plasticity.

See page 2-50

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Stickiness

The capacity of soil to adhere to other objectsEstimated at moisture content that displays maximum adherence between thumb and fore finger

The capacity of soil to adhere to other objects such as your fingers. Estimated at moisture content that displays maximum adherence between thumb and fore finger.

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Stickiness ClassesNon-Sticky – little or no soil adheres to fingers after release of pressureSlightly Sticky – soil adheres to both fingers after release of pressure with little stretching on separation of fingersModerately Sticky – soil adheres to both fingers after release of pressure with some stretching on separation of fingersVery Sticky - soil adheres firmly to both fingers after release of pressure with stretches greatly on separation of fingers

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Non-Sticky

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Moderately Sticky

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Very Sticky

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Plasticity ClassNon-Plastic – will not form a 6 mm dia, 4 cm long wire, or if formed , can not support itself if held on endSlightly Plastic – 6 mm dia, 4 cm long wire wire supports itself, 4 mm dia, 4 cm long wire wire does notModerately Plastic – 4 mm dia, 4 cm long wire wire supports itself, 2 mm dia, 4 cm long wire wire does notVery Plastic – 2 mm dia, 4 cm long wire wire supports itself

Plasticity classes;

Non-Plastic – will not form a 6 mm wire, or if formed , can not support itself if held on endSlightly Plastic – 6 mm wire supports itself, 4 mm wire does notModerately Plastic – 4 mm wire supports itself, 2 mm wire does notVery Plastic – 2 mm wire supports itself

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Moderately Plastic

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Very Plastic

4 cm long 2 mm diameter

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Inferences

Clayey textures (clay, silty clay, sandy clay, some clay loam and silty clay loams) with very firm moist consistence and very sticky, very plastic wet consistence are likely to contain 2:1 shrink-swell clayClayey textures that are friable with only moderately sticky and plastic wet consistence contain less amounts of active shrink-swell clay or are kaolinitc

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Estimating Clay Mineralogy from Consistence and Color

Mixed, 1:1 and 2:1

5YR7.5YR10YR

FirmModerately Sticky

Moderately Plastic

1:1 Kaolinite2.5YR or Redder (10R,

7.5R)

Friable toFirm

Slightly toModerately

StickySlightly to

Moderately Plastic

2:1 Smectite10YR2.5Y5Y

Firm toVery Firm

Very StickyVery Plastic

MineralogyMoist ColorMoist ConsistenceWet Consistence

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Soil ColorRule .1942

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Coloring agents in the soil

Organic matter darkens the soil.

Iron (Fe) is the primary coloring agent in

the subsoil.

Manganese (Mn) is also common in some

soils.

What causes the colors we see in the soil. Organic matter darkens the soil and is typically associated with surface layers. Organic matter will mask all other coloring agents. Iron (Fe) is the primary coloring agent in the subsoil. The orange brown colors associated with well drained soils are the result of Fe oxide stains coating individual particles. Manganese (Mn) is common in some soils resulting in a very dark black or purplish black color.

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It is the most obvious and easily determined soil characteristic. Important characteristics can be inferred from soil color.

Well drained (aerobic) soils have uniform bright colors.Soils with a seasonal saturation are mottled.Wet soil have more gray colors

Color helps us determine the treatment capacity of the soil

Why do we care about soil color?

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Color changesCoating of Fe2O3 (rust)

Remove Fe

Brown Soil Gray Soil

Adapted from Mike Vepraskas

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Munsell Color System

Hue refers to the dominant wavelength of light (red, yellow, green, etc.).Value refers to the lightness and darkness of a color in relation to a neutral gray scale.Chroma is the relative purity or strength of the Hue.Notation

Hue Value/Chroma10YR 5/6

Putting all this together the Munsell Color System was created. It was initially designed for manufacturing but soon made its way into any field that needed to record and communicate color. Hue refers to the dominant wavelength of light (red, yellow, green, etc.). Value refers to the lightness and darkness of a color in relation to a neutral gray scale.Chroma is the relative purity or strength of the Hue. Notation: HueValue/Chroma 10YR 5/6.

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Munsell Soil Color Chart

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What’s mostimportant?

Chroma is most important to describe correctly.

Chromasof 2 or less

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Color Patterns

Matrix color is the dominant color in the soil.Mottling is spots or blotches of color in the soil that differ from the matrix color. Redoximorphic features are mottles that relate to the aeration or drainage of the soil.

In describing colors it is important to determine the variation in color throughout the soil. Matrix color is the dominant color in the soil. Mottling is spots or blotches of color in the soil that differ from the matrix color. The pattern may relate to the aeration or drainage of the soil. Gley colors are low chroma matrix colors with or without mottles. If the soil has a gley color it is likely to be reduced and wet for much of the year.

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By Rule .1942, soil color tells me the water table is here

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Percent

<2% - few2-20% - common>20% - many

Likewise the percent of the given feature should be recorded. Although NRCS does use this chart to indicate abundance it is better to record the actual percent using the charts in the Munsell book to estimate.

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Percent figure seen in the Munsell Book. Note the each square can be subdivided in to quarters. There is the same amount of black in each quarter. This helps in determining % when the features are of different size.

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Each 1/4th

of any one square has

the same amount of

black

Each 1/4th of any one square has the same amount of black, thus features of differing size can be estimated.

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NC STATE UNIVERSITYDEPARTMENT of SOIL SCIENCE

NC STATE UNIVERSITYDEPARTMENT of SOIL SCIENCE

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25%

≈1.5%

6.25%

NC STATE UNIVERSITYDEPARTMENT of SOIL SCIENCE

NC STATE UNIVERSITYDEPARTMENT of SOIL SCIENCE

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Recording Soil Colors

Soil should be moist...This is the most common way soil colors are recorded.Always use sunlight. Do not use artificial light. Do not wear sunglasses or tinted glasses.Always use a freshly exposed face or ped. Do not crush or rub the soil before getting a color.Determine the matrix and all subordinate colors (mottles).

Colors should be recorded in a specific fashion. Soil should be moist...This is the most common way soil colors are recorded.Always use direct sunlight. Do not use artificial light. Do not wear sunglasses or tinted glasses.Always use a freshly exposed face or ped. Do not crush or rub the soil before getting a color.Determine the matrix and all subordinate colors (mottles).

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Summary

Internal drainage can be related to consistenceMineralogy is inferred from consistenceColor is used to determine soil wetnessSome inferences to soil mineralogy can also be made from consistence combined with color (Hue)