Soils Exam 2

The 1° soil particels

Defined on the basis of EFFECTIVE DIAMETER and include sand, silt, and clay.

The three types of soil particles are

Sand, Silt, and Clay

Sand

• 2 - 0.05 mm diameter

• Irregular in size and shape

• Primary mineral is quartz (siliceous)

• Big pores

• Low soil organic matter which means little material holding it together. This leads to high erosion rate

• Does not compact well

Silt

• 0.05 - 0.002 mm diameter

• Irregular size and shape

• high pH resistance and water holding

• Somewhat good compatibility (depending on how much weight there is)

  • Susceptible to wind and water erosion 

Clay

• < 0.002mm diameter

• < 0.001mm diameter is “colloidal”

• Aluminosilicates base = crystalline structure

• Fe & Al oxide base = between crystalline to amorphous structure

• Slow drainage rate with poor aeration. This causes high water holding capacity

• High soil organic matter and fertility

• Very compactable

• LARGE SURFACE AREA

When soil is aggregated (clumped)

The soil is not susceptible to wind/water erosion

When soil is dispersed (not clumped)

The soil is susceptible to wind/water erosion

Is Poorly Drained soil ALWAYS poorly aerated?

NO. Poorly Drained soil is NOT always poorly aerated

The 2° soil particles

Defined as the aggregation of 1° particles

Soil Structure is characterized by

Shape, Grade, and Size

What are the different shapes of soil?

• Granular (porous)

  • O horizon

• Crumb (very porous)

  • O horizon

• Platy

  • Horizontal plates

  • Not very porous

  • B horizon

• Angular Blocky

  • Block shape

  • Subsoil

• Sub-angular blocky

• Similar to angular block but not as sharp edges

• Columnar

  • Columns shaped

  • Found in dry, calcified regions

• Prismatic

  • Vertical orientation

  • Bx horizon

  • Example: Fragipan

What are the different grades of soil?

• Weak

  • Poorly formed, in distinct peds, barely observable in place

• Moderate

  • Well formed peds evident in undisturbed soil

• Strong

  • Well formed peds distinct in undisturbed soil

What are the different sizes of soil?

• Very Fine

• Fine

• Medium

• Coarse

• Very Coarse

Aggregated Formation (Soil Structure Genesis)

Physical Processes

• Wetting and drying

• Freezing and thawing

• Physical effects of roots and other organisms

Aggregated Stability (Soil Structure Genesis)

1. Soil organic Matter

• Microbial decomposition products

• root exudates

• fungal hyphae exudates (glomalin)

2. Inorganic Material (interacts with organic material)

• Silicate Clay

• Fe and Al oxides

• Cations (polyvalent = flocculation)

  • Ex: anything with more than one cation (+2, +3, +4)

  • Some are monovalent = dispersion (anything with one cation (+1))

Bulk Density (Db)

Mass per unit volume of DRY UNDISTURBED soil