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

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

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

15. 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

16. 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

17. 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

18. 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

19. Structures:
Platy: flat horizontal units; diverse sizes

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

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

28. Subangular blocky
Angular blocky
columnar
prismatic

30. 4. Granular: roughly spherical; porous

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

35. 6. Structureless
single-grained
massive

36. Importance of structure
Movement of air and water
Root penetration

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

38. Pan structures
Dense layers, diverse origins

39. 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

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

41. Clay pan

42. duripan

43. fragipan

44. caliche

45. plinthite

46. Plow pan
Plow pan

47. 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.

48. 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

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

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

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

54. Organic matter
dark

57. 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

58. 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

59. 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)

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

66. 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.

67. 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.

68. Manganese concretions

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

70. 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

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

72. 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

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

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

75. 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