Soil properties A. Texture B. Adhesive-Cohesive properties (Plasticity/Stickiness) C. Structure D. Color E. 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 :

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 – 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 cm 2 / gm Clay : Particles per gram = 90,260,853,000 Surface area = 8,000,000 cm 2 /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 blocky Subangular blocky columnar prismatic

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

compaction

Pan structures Dense layers, diverse origins

1. Clay pan Clay accumulation; usually B 2. Duripan cemented by ppt silica, iron oxides, and/or CaCO 3 3. Fragipan hard, brittle dense and compact, but breaks apart when taken out 4. Caliche white layer of CaCO 3 (soft or hard); arid near 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

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

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