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

Particle Density (Dp) always equals

2.65 Mg/m³

• This is the general particle density of things around Central New York

Another term for Dp is Specific Gravity

Total Pore Space (TPS) %

• Volume solids = (Db/Dp)*100

• Volume pores = 100 - volume solids

• TPS = 100 - [(Db/Dp)*100]

  • TPS = (1 - Db/Dp)*100

Does Db have pores? What about Dp?

Db = WITH pores

Dp = WITHOUT pores

Structure = fn (Clay, SOM)

• Aggregated has stability and holds things together

• When SOM comes together = HUMIFIED

  • Horizons Oe and Oi are NOT humified

What must you have in order have water stable aggregates?

You must have CLAY

• Sands do not form water stable aggregates

• Smaller particles come together to form large particles

Water Stable aggregates

Clusters of soil particles bound together that resist falling apart when wet

What is the most important thing to remember when it comes to Structure and texture?

You can always change or alter soil structure but you can NOT change texture

Relation between Site Quality and Soil Texture

What’s up with the red line?

• The max quality of undisturbed, not touched soil

• Structural development can improve this

• Structure can work with texture in order to improve this

Limitations on Sand:

• Nutrients and Minerals

• Water retention

Limitations on Clay:

• Lack of drainage and large pores

• Little to no aeration

What is an easy way to measure volume

Displacement by water

Specific Gravity

AKA Dp (Particle Density)

Usually a property of crystal structure and chemical composition

• Example: Quartz and feldspar is the bulk of the minerals for the central New York region. The average specific gravity for these minerals is 2.65 Mg/m³

That is where the 2.65 comes from

Bulk Density (Db) vs Particle Density (Dp)

As Bulk Density increases, Total Pore Space decreases

Example Db and Dp Problem. Given the Db = 2.0 Mg/m³, compute the TPS

1. TPS = (1 - Db/Dp)*100

2. TPS = (1 - 2.0/2.65)*100

3. TPS = (1 - 0.755)*100

4. TPS = 0.245 ×100

5. TPS = 24.5%

Factors that influence TPS and Db

1. Texture

2. Structure

3. Organic Matter

4. Depth

5. Management Practices

Texture (Factors that Influence TPS and Db)

• Bulk Density decreases with finer textured surfaces

• This means Total Pore Space INCREASES

• Example

  • Cultivated Surface

    • Sand & Sandy loam: 1.2-1.8 Mg/m³ Density; 55-32% TPS

    • Silt & Clay loam: 0.9-1.5 Mg/m³ Density; 66-43% TPS

Structure (Factors that influence TPS and Db)

Bulk density is lower for well developed granular structure