Soil propertiesA. TextureB. Adhesive-Cohesive properties

(Plasticity/Stickiness)C. StructureD. ColorE. Density

A. Texture Relative proportion of sand, silt, clay sized

particles in a soil

Does not change (in human lifetime)

Most important property for agricultural and engineering uses

Fine earth fraction only

Does not include coarse fragments:

Boulders: > 600 mm Stones: 250 – 600 Cobbles: 75 – 250 Gravels : 2 - 75

USDA fine earth fraction (“soil separates”):

Sand 0.05 – 2.0 mm Very coarse 1.0 – 2.0 Coarse 0.5 – 1.0 Medium 0.25 – 0.5 Fine 0.1 – 0.25 Very fine 0.05 – 0.1

Silt 0.05 – 0.002 Clay <0.002

sand Naked eye Gritty Predominantly quartz Round

silt Light microscope Cannot feel individual grains; slippery Predominantly quartz and other primary

minerals In between round and flat

clay Electron microscope Wide variety of minerals Flat

Properties that vary with particle size:

Surface area Geometry of pore spaces Adhesive / Cohesive properties;

Plasticity / Stickiness

Surface area (site of water adsorption, gas adsorption, mineral

weathering, nutrients)

Very coarse sand: Particles per gram = 90 Surface area = 11 cm2 / gm

Clay : Particles per gram = 90,260,853,000 Surface area = 8,000,000 cm2 /gm

Pore space geometry Sand has large pores between grains

Highly permeable

Silt has relatively small pores Less permeable

Clay has very small pore spaces Least permeable

B.Adhesive/Cohesive properties Adhesion: force with which something

clings to other surfaces Soil and water

Cohesion: force with which something clings to itself Soil particles

Plasticity/Stickiness

Plasticity is ability to be molded; force required to deform soil in wet pliable

condition

Make a “worm” of soil; see how thin the worm can be and still support its own weight on end

Indicates cohesiveness

Stickiness is force required to pull soil apart when wetted (beyond plastic)

Press moist soil between thumb and forefinger and see how much sticks to fingers

Indicates adhesiveness

Shape governs extent of contact between adhering and cohering surfaces

Greatest contact occurs when flat surfaces lie parallel to one another (as in clay)

e.g. cohesiveness makes some clays turn into hard clods when dry and become very sticky when wet

Sand has a large particle size and round shape

Limited contact with other surfaces not sticky, not plastic

Silt is more cohesive and adhesive than sand, but has only limited plasticity and stickiness

Can be crushed when dry

C. Structure Way in which soil particles are assembled in

aggregate form

Results from pedogenic processes

Structural unit is ped e.g., blocky soil has blocks as peds Ped: < cm to several cm

Structures:

1. Platy: flat horizontal units; diverse sizes

2. Prismlike: tall peds with flat sides

Prismatic: flat tops

Columnar: rounded tops

3. Blocky

Angular: flat faces, sharp corners

Subangular: faces and corners are rounded

Angular blockySubangular blocky

columnarprismatic

4. Granular: roughly spherical; porous

5. wedge-shaped peds

form in clays where cracking and swelling cause soils to slide along planes

6. Structureless

single-grained

massive

Importance of structure

Movement of air and water

Root penetration

What gives structure to soil? Organic gums (HUMUS!)

Decay products Shrink and crack on drying

Shrink-swell clays Roots Freeze/thaw cycles Soil animals

Pan structures Dense layers, diverse origins

1. Clay panClay accumulation; usually B

2. Duripancemented by ppt silica , iron oxides, and/or CaCO3

3. Fragipanhard, brittledense and compact, but breaks apart when taken out

4. Calichewhite layer of CaCO3 (soft or hard);

aridnear surface

5. Plinthite (laterite)

sesquioxides, usually B

tropical, weathered

soft when wet; brick hard when dry

6. Plowpan

compaction from weight of implements

Clay pan

duripan

fragipan

caliche

plinthite

Plow pan

Plow pan

Structural stability Ability of soil to resist physical breakdown

Maintaining structure is desirable for soil health Keeps surface well-granulated Aeration, water penetration, seedling emergence

Destroyed by machinery, animals, mountain bikes, etc.

puddling Soil loses structure; becomes massive Causes:

Compaction Cultivation Rain on exposed soil

Type of ions is important High valence cations (Ca+2 Mg+2 Al +3 )

Best bonding Single valence (Na+)

Weak bonding

Can improve puddled soils by replacing Na with Ca

D. Soil Color Munsell chart

Hue: spectral color (red, yellow, blue) Value: lightness or darkness Chroma: strength/purity

Color indicates:

Extent of weathering Amount & distribution of OM State of aeration

Extent of weathering

Secondary iron oxides, manganese oxides Red, yellow, brown Coatings of iron oxides around other particles:

light brown, buff

Organic matter

dark

State of aeration Poor aeration

Iron and manganese assume reduced forms Bluish, grey “REDOX DEPLETIONS”

Good aeration Iron and manganese oxidize

Bright colored oxide coating on minerals “REDOX CONCENTRATIONS”

“mottling” is old term

Poorly aerated soils reduced forms of iron and manganese

Fe+2, Mn+2

Reduced iron is soluble; moves through soil, removing red, leaving gray, low chroma colors (redox depletions)

Reduced manganese : hard black concretions

Well-aerated soils Oxidized forms of iron and manganese

Fe+3 Mn+4

Fe precipitates as Fe+3 in aerobic zones or during dry periods

Reddish brown to orange (redox concentrations)

Plate 26  Redox concentrations (red) and depletions (gray) in a Btg horizon from an Aquic Paleudalf.

Plate 16  A soil catena or toposequence in central Zimbabwe. Redder colors indicate better internal drainage. Inset: B-horizon clods from each soil in the catena.

Plate 21  Effect of poor drainage on soil color. Gray colors and red redox concentrations in the B horizons of a Plinthaquic Paleudalf.

Manganese concretions

E. Density

(Mass / volume)

Particle density Bulk Density

Particle Density Weight/volume of soil particles

Depends on minerals present

Average = 2.65 g/ml for soils from silicate minerals

Particle density of iron oxides = 4 g/ml

Procedure: Weigh soil; pour into known volume of water; record volume change

Bulk Density Wgt / vol of whole soil

Volume includes pore space

Depends on particle density and proportionate volume of solid particles and pore space

Procedure: Use bulk density sampler

Cylindrical core sampler of known volume Does not compress soil Oven dry and weigh soil B.D. = dry wgt soil / volume of sampler

Used to gauge effects of machinery, etc. on soil COMPACTION

Engineers need compacted soils for road fill and earthen dams

Porosity amount of total pore space

If mineral comp. of two soils is similar, bulk densities vary because of porosity differences

Texture affects porosity:

Coarse texture Larger but FEWER pores Low porosity High bulk density

Fine texture Smaller but MORE pores High porosity Low bulk density