SOIL PHYSICAL PROPERTIES

Chapter 4

Color

Texture

Structure

Consistence

Particle density

Bulk density

Pore space

Water content

COLOR

Measured in reference to standard colorchips (Munsell Color Book)

Huedominant spectralcolor

Valuerelative blacknessor whiteness

Chromaamount ofpigment mixedwith gray value

Color

Interpretation color

Brown Organic matterBlack

Red Hematite, Fe2O3

Yellow Goethite, Fe2O3 · H2O

Gray Fe3+ Fe2+, reduced iron

Color

Thus, red = oxidized = aerated but gray = reduced = anoxic, poorly aerated.The oxidation state of Fe in Fe2O3 and Fe2O3 H2O is + 3.

Color

This surface horizonis dark due to enrich-ment in organic matterfrom grass roots andsurface residue.

Common feature ofprairie soils is highorganic matter contentto appreciable depth(see inches).

Good or bad drainage and aeration?

Red

Yellow

Gray

Color

Green blue Gley horizon

EXTREMELY POOR drainage

Color

These colors, certainly, but also gray color generally indicate poor drainageand aeration. Colors of chroma 2 or 1 indicate gley.

Mottling Varied colors

Mottles Drainage and aeration

None Good

Brown ModerateYellowGray

Gray PoorBlue

Color

These secondary colors modify one’s interpretation based on the main(matrix) color –like yellow or gray within an otherwise oxidized matrix saydrainage and aeration are not uniformly good. And visa-versa –red mottlesagainst gray matrix = some veins that are well drained / aerated.

TEXTURE

Proportion of different size particles

USDA classification system

2.0 to 1.0 mm sand very coarse1.0 to 0.5 coarse0.5 to 0.25 medium0.25 to 0.10 fine0.10 to 0.05 very fine

0.05 to 0.002 silt

0.002 to smaller clay

Texture

Illustration of howsurface area increaseswith decreasing size.

If you prefer algebra,for a cube, A = 6s2 andV = s3, so A / V = 6 / s.The smaller the cubes,the greater the surfacearea per volume (or permass, since mass isproportional to volumethrough density).

As surface area increases

Chemical adsorption Increase or DecreaseWeathering rateParticle coherenceWater holding capacity

Tillage ease

Texture

All these are surface phenomena, therefore,increase with increasing particle surface area.

On the other hand, pulling an implement througha clayey soil is much harder than through asandy soil, no?

Textural Classes

Coarse Medium Fine

Sandy soils Loamy soils Clayey soils

Sand Sandy loam Sandy clayLoamy sand Loam Silty clay

Silt loam ClaySiltSandy clay loamSilty clay loamClay loam

What’s thetextural class of a soil that’s

50 % sand10 % silt

Answer,sandy clay.

Determination of Textural Class

Feel methodMechanical analysis

Mechanical analysis

Based on Stokes’ law

Settling rate of spherical particles in a viscous liquid

Texture

FUP = Bouyant + Drag

FDOWN = Gravity

FBouyant = 1/6 π d3 ρL g

FDrag = 3 π d μ V

FGravity = 1/6 π d3 ρS g

At equilibrium, i.e., terminal velocity, forces balance

v = g( s - l)d2/18

Settling velocity decreases as d2 decreases

Texture

Smaller d means smaller v, no? So comparing the largest clay (d = 0.002 mm) to largest sand (d = 2 mm), vclay / vsand = 0.0022 / 22 = 1 / 1,000,000.

Pipette method

v = z / t

t = 18 z / [g ( s - l) d2]

Find t for particlesdiameter d

Texture

Express velocity as distance / time,rearrange and find time need for smallestsand to settle a prescribed distance. So,aliquot taken above this depth contains no sand, only silt and clay. Do samecalculation for smallest silt, take aliquot at that time and it contains only clay.

Separate Minimum size Time to settle 10cm

Sand d > 0.05 mm < 1 minSilt d > 0.002 mm 8 hr

Texture

Your times may vary depending on density and viscosity of the medium (seeequation). Besides any solutes added to aid dispersion of particles, temperatureaffects these parameters.

Must disperse clay particles

Some adsorbed cations like Ca2+ hold clayparticles together (flocculated)

Replace these with Na+ to disperse

Texture

Depending on the nature of the soil, more extensive methods may be necessaryto disperse particles.

Soil texture may be considered a permanentproperty of a soil

Why so? Suggest a few ways by whichtexture can be changed.

Sure, erosion can change texture at a location but barring gross erosion, this is slow.Certainly, internal translocation of clay will make the topsoil more sandy but this isvery, very slow. Unless you move a lot of dirt around, texture is effectively a fixedproperty.

STRUCTURE

Particles adhering together into aggregates or peds

Some soils are structureless

Single grain noncoherent

Massive cohesive mass

Circle correct answer.

Some clay / sandy soils are singlegrain structureless soils.

Some clay / sandy soils are massivestructureless soils.

Types of structural units

Spherical Crumb Granular

Common in surface soils

Platy

Surface and subsurface

Prism Columnar (tops of prismlike unitsrounded)

Prismatic (tops angular)

Subsoil

Blocky Angular blocky(edges distinct)

Subangular blocky (rounded)

Usually subsoil

Description of structure includes

Relative size class

Strength of cohesion grade

Structure

Munsell book give guide.

Structure affects water movement

Easiest in soil with which structure?

A) massive, B) blocky, or C) platy

Probably blocky because the large pores between aggregates aremore vertically oriented than in soil with platy structure. Obviously,the tiny pores in a massive clay conduct water very slowly.

How does structure develop?

Adhesive effect of organic substances

Flocculating cations

Shrink-swell clays

The organics jointly adhere to surfacesof adjacent mineral particles. Cations likeCa2+ tend to be closely associated withparticle surfaces, bridging adjacent minerals.Shrink-swell clays tend to give persistentinternal fissures.

REVIEW QUESTIONS

The B horizon of soil X is red andthe B horizon of soil Y is yellowwith gray mottles.

Which one is better drained?

What’s the gray color due to?

X is better drained. You know this because the color indicates oxidized Fe, Fe3+,which exists under a prevailing oxidized (oxic, aerated) environment. On theother hand, gray color is due to chemically reduced Fe, Fe2+, that occurs underanoxic conditions.

The capacity to adsorb chemicals isgreatest in soils that have whichtexture?

ClayLoamSandy loamSand

And why? The one with the greatest surface area, clay, of course.

Name the 4 general types of soilstructure (geometric shape types).

The four types of geometric structure are spherical (all dimensions about thesame with radial orientation, one dimension short compared to the other two, i.e., flat = platy, one dimension long compared to the other two, i.e., elongated =prism-like and all dimensions about the same but with x-y-z orientation, i.e.,cubic = blocky.

Name the 2 types of prism-likestructure.

Name the 2 types of blocky structure.

It couldn’t hurt to remember these and the subtypes of spherical structure.

Surface Structure Management

Aggregate stability favored by

Organic matter

Flocculating cations

May be destroyed by rain or tillage andtraffic, leading to a crust

Infiltration slow and runoff fast

Structure

Crust = dense, i.e., low porosity and pores aresmall so that infiltration is greatly reduced. Bad.

Puddled

Tillage when soil is too wet can puddle it

Especially clay soil

Structure

Sure, there are large pores between clods but the large pores that previously existed throughout are gone.Bad.

Minimize tillage

Keep residues on soil

Use cover crops

Even add organic matter

Protect surface and addorganic matter

Structure

Protecting an otherwise bare surface from raindrop impact willpreserve surface aggregation, thus, preserve good infiltration, soilwater conservation and limit runoff, erosion and downhill degradation of surface water quality. Adding organic matter aids structure stabilityand redevelopment if degraded.

SOIL CONSISTENCE

Resistance of soil to mechanical stresses

Judged at different moisture contents

Besides color, texture and structure, this is another feature included in horizondescriptions.

Moist soils

LooseVery friableFriableFirmVery firm

PARTICLE DENSITY

ρs = mass soil solids / volume solids(g / cm3)

Depends on mineralogy to some extent

So, how would you go aboutdetermining the particle density of asoil sample?

Use what is given on the next pageand pencil and paper.

Doesn’t vary much

Average 2.65 g cm-3

Well, it can vary depending on whether a very low density mineral predominates,however, this is usually not the case and common soil minerals have densitiesthat are about 2.65 g cm-3.

BULK DENSITY

ρb = mass of soil solids / total volumeoccupied by solids (g / cm3)

Is ρb greater than or less than ρs?

Remove a core of known volume

Mass of solids is determined afterwater is evaporated (105 C)

Why 105 C? The boiling point ofwater is 100 C, isn’t it?

You are dealing with a solution. Recall that solutes elevate the boiling point.

Bulk density depends on

TextureDepth in profileTillage

Small porous aggregates

Solid grains

Higher in coarse textured soils?

This is the usual explanation –internal porosityof small aggregates that exist in finer texturesoil compared with sand, which tends to lackmuch aggregation.

Clayey soils are considered heavy by virtueof their typically higher water content. Strictlybased on bulk density (oven-dry mass), theyshould be considered light.

Higher in subsoil?

Compaction greater there?

Less organic matter there?If ‘yes,’ then is aggregation less there?

Yes to all, no?

Higher in tilled soils?

Is compaction a possibility?

What effect does tillage have on theamount of organic matter in soil?

Well, the answer depends on your time of reference. Obviously, if you till asoil, you fluff it up, therefore, instantaneously reduce the bulk density. However,the fluff goes away and the soil reconsolidates. The long-term effect pertains todestruction of organic matter with consequent loss of porosity, especially the larger pores. So, in the long-term density increases. Heavy equipment tendsto compact the soil. Plowing also tends to increase subsurface density (lifting thesurface soil up, pushes the soil below it down).

This is the classic observation. Many studies, many places.However, this can be, to various extent, reversed by reducingtillage and adding organic matter.

High bulk density is a good thingbecause water, air and roots movemore easily through a very dense,low porosity soil than through a lessdense, high porosity soil.

True / False

Related terms

Hectare-furrow slice mass of hectare to 15cm

2200 Mg

Acre-furrow slice mass of acre to6 in

1000 tons

Obviously, these are expressions for bulk density.Let’s see what density is implied on a g cm-3 basis.

ρB = 2,200 Mg x 1000 kg Mg-1 x 1000 g kg-1 /10,000 m2 ha-1 x 0.15 m x 1,000,000 cm3 m-3

1.47 g cm-3

These density units are useful in production-scale calculations. By the way,what is the bulk density (g cm-3) assumed when saying an AFS weighs 1000 tons?

PORE SPACE

Occupied by air or soil water

Vp = Vt - Vs

If you know bulk density and particledensity you can calculate pore space.Show how.

If this volume wascompletely filled withsolid particles it wouldhave a mass = Vt s

But it has a mass = Vt b

So this volume is only b / s filled with solids

So

ρs

ρb

ρb / ρs

Vp = Vt - Vt ( b / s)

or, as a fraction,

Vp / Vt = 1 - b / s

Pore space and bulk density are inverselyrelated

(ρb / ρs)

(ρb / ρs)

If you don’t think this is an intuitive approach, try algebra. The volume of solidsVs = ms / ρS (from definition of ρS) so Vp = Vt - ρB x ms. The mass of solids,ms = ρB x Vt (from definition of ρB), so Vp = Vt – (ρB / ρS) x Vt. Now divide by Vt.

OK, you’ve got a core of soil that has abulk density of 1.30 g / cm3 and aparticle density of 2.60 g / cm3.

What fraction of it is pore space?

How much water would be in 1 m3 ofthis soil if it was completely saturatedwith water?

Are all pores in a soil the same size?

True or False?

Macropores rapid water flow and goodaeration

Micropores slow and poor

Distribution of pore sizes is affected by

TextureStructureManagement

Sandy soils mostly have _____pores

Clay soils have macropores (between / inside) aggregates

Massive clays mostly have _____pores

macro-

micro-

What effect does tillage have on porespace?

Increase it or decrease it?

Decrease it?

Why?Native soil

This is directly related toeffect of tillage on bulk density.Tillage increases density inthe long-term because of loss oforganic matter that results in lossof inter-aggregate porosity. Again,the effect is primarily on macro-porosity.

Water Content

gravimetric water content

mass of water / mass of oven-dry soil

volumetric water content

volume of water / volume of soil

Know these two definitions. By the way,which one of these can you always measure?

Note on air-dry moisture content

mair-dry soil = msoil solids + mwater

let mwater / msoil solids = gw

mair-dry soil = msoil solids (1 + gw )

The basis for most soil chemical data is dry soil solid mass (i.e., oven-dry), however, you can’t maintain soil in an oven-dry state unless it’s in a dessicatorand when you remove it to do something with it, it starts to immediately adsorbwater from the atmosphere. So, you correct to oven-dry moisture. Here, gwis gravimetric water content .

What You Now Know AboutSoil Physical Properties

And With Such Amazing Ease

Color

Munsell chart and parametersWhat soil colors usually indicate

Texture Definitions of separates How to use a textural triangle Basis and method of mechanical analysis Importance of texture to other soil properties

Structure TypesEffects on aeration and water movementFormation and stability of surface structureManagement ideas for preserving

Particle density DefinitionHow to measure it

Bulk density DefinitionHow to measure itInfluences of texture, structure and management on it

Porosity How to calculate itInfluence of texture, structure and management on it

Water content GravimetricVolumetricAir-dry soil contains adsorbed water and that’s a nasty little inconvenience