PHYSICAL PROPERTIES OF SOIL
PHYSICAL PROPERTIES OF SOIL
INTRO TO SOILSCH 4
SOIL TEXTURESOIL TEXTURE
Describes the proportion of soil particle sizes:SandSiltClay
Soil Texture influences other traits such as:Water Holding CapacityAeration
EFFECT OF PARTICLE SIZEEFFECT OF PARTICLE SIZE
Smaller particles – larger internal surface area
EFFECT OF PARTICLE SIZEEFFECT OF PARTICLE SIZE
More pores – micropores
Larger pores – macropores
SOIL SEPARATESSOIL SEPARATES
SAND2.00 – 0.05 mm
SILT0.05 – 0.002 mm
CLAY< 0.002 mm
TEXTURAL CLASSIFICATION
TEXTURAL CLASSIFICATION
Twelve Textural
Classes
Textural Triangle
SOIL DENSITY AND PERMEABILITY
SOIL DENSITY AND PERMEABILITY
DENSITY:
MASS per VOLUME
D = M V
SOIL DENSITY AND PERMEABILITY
SOIL DENSITY AND PERMEABILITY
Two densities in Soil:
Particle Density – PD
Bulk Density - BD
SOIL DENSITY AND PERMEABILITY
SOIL DENSITY AND PERMEABILITY
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
BD = wt. dry soil = _g_
vol. dry soil cu cm
e.g. BD = 650 g = 1.3 g/cu cm
500 cu cme.g.
SOIL POROSITYSOIL POROSITY
Usually 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
SOIL POROSITYExamples
Water Volume to Core Volume
Porosity = wet weight (g) – dry weight (g) soil volume (cu cm) x
100%
SOIL POROSITYExamples
SOIL POROSITYExamples
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.
SOIL POROSITYExamples
SOIL POROSITYExamples
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%)
SOIL POROSITYExamples
SOIL POROSITYExamples
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 POROSITYSOIL POROSITY
Question:Which has greater porosity, Sand or Clay?Answer:Clay at about 50%; Sand is lower at about 30%
Why?
TEXTURE AND SOIL PORESTEXTURE 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
Permeability – 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
Structure – the way soil particles clump together into large units called aggregates or peds
STRUCTURESTRUCTURE
Structure can alter the effects of texture- e.g. a fine-textured silty clay with good structure can
be permeable!
STRUCTURESTRUCTURE
- 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
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 PROPERTIESADDNL. SOIL PHYSICAL PROPERTIES
Soil 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
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
ADDNL. SOIL PHYSICAL PROPERTIES
ADDNL. SOIL PHYSICAL PROPERTIES
Improving Tilth:Best accomplished by improving structure- tilth relates to texture, structure, permeability,
and consistence; however, texture and consistence cannot be altered easily
therefore, improve tilth by improving structure and avoiding compaction
ADDNL. SOIL PHYSICAL PROPERTIES
ADDNL. SOIL PHYSICAL PROPERTIES
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 PROPERTIESADDNL. SOIL PHYSICAL PROPERTIES
Soil Temperature (varies w/color, texture, O.M.)Soil Color
Munsell soil color chartHue, Value, Chromae.g. 10YR 3/6