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Soil properties A. Texture

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Presentation on theme: "Soil properties A. Texture"— Presentation transcript:

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

2 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

3 Fine earth fraction only
Does not include coarse fragments: Boulders: > 600 mm Stones: 250 – 600 Cobbles: 75 – 250 Gravels :

4 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 – 0.1 Silt – 0.002 Clay <0.002

5 sand Naked eye Gritty Predominantly quartz Round

6 silt Light microscope Cannot feel individual grains; slippery
Predominantly quartz and other primary minerals In between round and flat

7 clay Electron microscope Wide variety of minerals Flat

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9 Properties that vary with particle size:
Surface area Geometry of pore spaces Adhesive / Cohesive properties; Plasticity / Stickiness

10 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

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12 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

13 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

14 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

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16 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

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18 Non-sticky

19 Slightly sticky

20 Very sticky

21 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

22 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

23 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

24 Structures: Platy: flat horizontal units; diverse sizes

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27 2. Prismlike: tall peds with flat sides
Prismatic: flat tops Columnar: rounded tops

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31 3. Blocky Angular: flat faces, sharp corners Subangular: faces and corners are rounded

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33 Subangular blocky Angular blocky columnar prismatic

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35 4. Granular: roughly spherical; porous

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38 5. wedge-shaped peds form in clays where cracking and swelling cause soils to slide along planes

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40 6. Structureless single-grained massive

41 Importance of structure
Movement of air and water Root penetration

42 What gives structure to soil?
Organic gums (HUMUS!) Decay products Shrink and crack on drying Shrink-swell clays Roots Freeze/thaw cycles Soil animals

43 compaction

44 Pan structures Dense layers, diverse origins

45 white layer of CaCO3 (soft or hard); arid
Clay pan Clay accumulation; usually B Duripan cemented by ppt silica , iron oxides, and/or CaCO3 Fragipan hard, brittle dense and compact, but breaks apart when taken out Caliche white layer of CaCO3 (soft or hard); arid near surface

46 5. Plinthite (laterite) sesquioxides, usually B tropical, weathered soft when wet; brick hard when dry 6. Plowpan compaction from weight of implements

47 Clay pan

48 duripan

49 fragipan

50 caliche

51 plinthite

52 Plow pan Plow pan

53 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.

54 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

55 D. Soil Color Munsell chart Hue: spectral color (red, yellow, blue)
Value: lightness or darkness Chroma: strength/purity

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57 Color indicates: Extent of weathering Amount & distribution of OM
State of aeration

58 Extent of weathering Secondary iron oxides, manganese oxides
Red, yellow, brown Coatings of iron oxides around other particles: light brown, buff

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60 Organic matter dark

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63 State of aeration Poor aeration Good 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

64 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

65 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)

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

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72 Plate 16 A soil catena or toposequence in central Zimbabwe
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.

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

74 Manganese concretions

75 E. Density (Mass / volume) Particle density Bulk Density

76 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

77 Bulk Density Wgt / vol of whole soil
Volume includes pore space Depends on particle density and proportionate volume of solid particles and pore space

78 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

79 Used to gauge effects of machinery, etc. on soil COMPACTION
Engineers need compacted soils for road fill and earthen dams

80 Porosity amount of total pore space
If mineral comp. of two soils is similar, bulk densities vary because of porosity differences

81 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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