PHYSICAL PROPERTIES OF SOIL

PHYSICAL PROPERTIES OF SOIL

INTRO TO SOILSCH 4

SOIL TEXTUREDescribes the proportion of soil particle sizes:

SandSiltClay

Soil Texture influences other traits such as:Water Holding CapacityAeration

EFFECT OF PARTICLE SIZESmaller particles –

larger internal surface area

EFFECT OF PARTICLE SIZEMore pores –

micropores

Larger pores – macropores

SOIL SEPARATESSAND

2.00 – 0.05 mmSILT

0.05 – 0.002 mmCLAY

< 0.002 mm

TEXTURAL CLASSIFICATION

Twelve Textural

Classes

Textural Triangle

SOIL DENSITY AND PERMEABILITY

DENSITY:

MASS per VOLUME

D = M V


Two densities in Soil:

Particle Density – PD

Bulk Density - BD


PD average soils ~ 2.65 gm/cu cm

BD average range from 1.0 – 1.8 gm/cu cm

Depends on amount of pore space

SOIL POROSITYUsually expressed as a percentage; e.g. 50%

Two means determining porosity:1) Calculate ratio water volume to total core

volume2) Calculate from bulk density and particle

density

SOIL POROSITYExamples

Water Volume to Core Volume

Porosity = wet weight (g) – dry weight (g) soil volume (cu cm) x

100%


An oven-dry soil core, volume 500 cu cm, weighs 650g. When wet, it weighs 900g. Find it’s % porosity.

Porosity = 900g – 650g x 100% = 250g x100% = 50% 500 cu cm 500 cu cm

Note: the unit cancellation is made possible by the metric system which defines 1 cu cm of water as weighing 1g.


Bulk Density to Particle DensityDefines the percentage of the soil that is solid matterThe percent solid matter is subtracted from 100% to give percent porosity:

Porosity = 100% - (BD/PD x 100%)


An undisturbed oven-dry soil, BD of 1.3 g/cu cm, consists of average mineral composition(PD 2.65 g/cu cm). Find its % Porosity:

Porosity = 100% - (1.3 g/cu cm/2.65 g/cu cm x 100%)

Porosity = 100% - (0.49 x 100%) = 100% - 49% = 51%

SOIL POROSITYQuestion:Which has greater porosity, Sand or Clay?Answer:Clay at about 50%; Sand is lower at about 30%

Why?

TEXTURE AND SOIL PORES

Why, then, does water move through sandy soil very rapidly, but moves s l o w l y into clay ?

This is explained by the physical property Permeability

PERMEABILITYPermeability – the ease with which air, water, and

roots move through soil- Depends on number, size, and continuity of pores- Liken to a maze

Fine-textured soils would be impermeable if not for:

Soil Structure

STRUCTUREStructure – the way

soil particles clump together into large units called aggregates or peds

STRUCTUREStructure can alter the effects of texture- e.g. a fine-textured silty clay with good structure can

be permeable!

STRUCTURE- Structure is classified by three groups of traits:1) Type – refers to shape of aggregates

e.g. Granular, Platy, Blocky, Prismatic, Columnar

2) Class – refers to size of pedse.g. very fine, fine, medium, coarse, very coarse

3) Grade – refers to strength and distinction of peds

e.g. weak/not visible vs. strong/easily distinguished

FORMATION OF SOIL STRUCTURE

Two-step formation:1) Individual soil particles loosely aggregate2) Weak aggregates are cemented to strengthen

- clay- iron oxides- organic matter- microorganism gums

ADDNL. SOIL PHYSICAL PROPERTIESSoil Consistence – the behavior of soil when

pressure is applied; measured at three different moisture levels: Wet, Moist, Dry (fig 4-15, p. 59)

Soil Tilth – ease of tillage, seedbed preparation, and seedling/root movement

Compaction – results from pressure applied at the soil surface

ADDNL. SOIL PHYSICAL PROPERTIES

Puddling, Clods, and Crusts:Puddling occurs when pressure is applied to very wet soils (esp. plowing)Crusts occur when bare soil is struck by raindrops; disperses soil then dries to a hardened crust


Improving Tilth:Best accomplished by improving structure- tilth relates to texture, structure, permeability,

and consistence; however, texture and consistence cannot be altered easilytherefore, improve tilth by improving structure and avoiding compaction


Soil Channels – continuous macropores leading from surface to deep subsoil

Soil Pans – any layer of hardened soil; includes:- claypans

- fragipans (clays)- plinthite (tropics)

- caliche (Ca cemented)

ADDNL. SOIL PHYSICAL PROPERTIESSoil Temperature (varies w/color, texture, O.M.)Soil Color

Munsell soil color chartHue, Value, Chromae.g. 10YR 3/6

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PHYSICAL PROPERTIES OF SOIL