LEC 3: Soil Texture

MODULE 2

Which of the 12 textural classes have the following:

• Particle size: large/coarse

• Pore Size: large

• Reactivity: weak

sandy, loam sand, sandy loam

Which of the 12 textural classes have the following:

• Particle size: medium

• Pore Size: medium

• Reactivity: moderate

Sandy clay loam, silty clay loam, silt clay loam, loam

Which of the 12 textural classes have the following:

• Particle size: fine/small

• Pore Size: small

• Reactivity: strong

Clay loam, sandy clay, silty clay, clay

Which of the Three separates have the > 2.0 mm?

sand

Which of the Three separates have the 0.05 mm - 2.0 mm?

silt

Which of the Three separates have the < 0.05 mm?

clay

What are the Three soil separates?

1. Sand

2. Silt

3. Clay

Soil texture serves as an indicator for which four physical characteristics?

1. pore size

2. surface area

3. water movement

4. reactivity

How many distinct soil texture classes are recognized based on the relative abundance of separates?

12 classes

Which manual assessment method uses grittiness and plasticity to estimate texture?

texture-by-feel

What mathematical law is used to determine the settling rates of particles from water?

Stokes’ Law

In Stokes’ Law, how does particle size relate to settling velocity?

large particles settle quickly, while small particles settle slowly

What is the formula for Stokes’ Law?

In the Stokes’ Law formula, what value is given for the constant k?

What is the settling velocity (V) for a 1 mm sand particle?

112 cm/sec

What is the settling velocity (V) for a 0.05 mm silt particle?

0.281 cm/sec

What is the settling velocity (V) for a 0.002 mm clay particle?

0.0004 cm/sec

The arrangement or grouping of individual soil particles into larger secondary units or aggregates.

Soil Structure

What are the individual secondary units of soil structure called?

Aggregates

What are the 7 soil structures?

1. granular

2. platy

3. wedge

4. prismatic

5. columnar

6. subangular blocky

7. angular blocky

What chemical force allows clay and organic matter to bind particles together?

Electrostatic attraction

Which biological agents aid in the compression and binding of particles via hyphae and exudates?

Roots and fungal hyphae

How do macro-organisms contribute to the formation of soil structure?

Through burrowing, tunneling, and waste production

The ability of soil aggregates to avoid disintegration when disturbed by tillage or erosion.

Aggregate Stability

The dry weight of soil divided by its volume, used as an indicator of soil compaction.

Bulk Density

Variations in soil bulk density are primarily caused by differences in total _____

pore space

Which type of pores are located within individual soil aggregates?

Intra-aggregate pores (small pores)

Which type of pores are located between distinct soil aggregates?

Inter-aggregate pores (large pores)

How does water behave in inter-aggregate pores?

It moves easily and is poorly retained.

How does water behave in intra-aggregate pores?

It moves slowly and is strongly retained.

List the two primary causes of soil color.

1. humus content

2. iron compounds

What can soil color indicate about the soil's environment?

Physical and chemical characteristics or specific soil types.

What are the factors that affect Aggregate Stability?

1. amount of clay

2. chemical elements

3. organic matter

4. biological activity

What are the factors that affect Soil Structure?

1. organic matter

2. soil organisms

3. tillage

4. freezing and thawing

5. water movement

___is the arrangement or grouping of individual soil particles into larger secondary units

Structure

___possess natural electrical charge which can electrostatically bind particles together

Clays and organic matter

____can aid in binding of individual particles into larger aggregates

Macro-organisms and microorganisms

____allows for both movement and retention of water via macropores and micropores, repectively

Aggregation

____structure can inhibit infiltration of water, water movement, growth of roots

Poor soil

Which of the 7 Soil Structure is this?

Granular

Which of the 7 Soil Structure is this?

Platy

Which of the 7 Soil Structure is this?

Wedge

Which of the 7 Soil Structure is this?

Subangular Blocky

Which of the 7 Soil Structure is this?

Angular Blocky

Which of the 7 Soil Structure is this?

Prismatic

Which of the 7 Soil Structure is this?

Columnar

Why is it impossible to determine bulk density for subsurface horizons using an auger profile?

determining bulk density requires an exposed horizon to push a collection can into the soil

Why measure bulk density?

it gives information on the ease with which roots can grow and water will filter through the different soil horizons of a profile

A good soil for growing plants should generally contain about (1)___% pore space and (2) ___% solids. The pore space should consist of (3)___air and (4)___water, and the solids should be made up mostly of (5)___(sand, silt, and clay particles) mixed with some (6)____

1. 50%

2. 50%

3. half (25%)

4. half (25%)

5. minerals (45%)

6. organic matter (5%)

In the soil, (1) ___and (2)___are found (3)___the pore spaces (4)___ the particles. If this ideal soil were dried out, and all the pore space were (5)___so that there was no air and no more space, all that would remain would be the (6) ___portion of the soil

1. air

2. water

3. within

4. between

5. compressed

6. solid

To calculate bulk density, we would measure the (1)___of the total (2)___ of a (3)___sample

1. mass

2. volume

3. soil

To calculate particle density, we would measure the (1)___of the (2)___of only the (3)___ in the (4)___sample

1. mass

2. volume

3. solids

4. soil

Accepted ___of soil mineral particles: 2.65 g/cm³

Density

Density of soil including the particles and the pore spaces

___ranges between 1.1 and 1.6g/cm³

(for mineral soil with 1-5% organic matter)

Bulk Density

Factors Affecting Bulk Density

1. Porosity

2. organic matter

3. aggregation

4. arrangement of particles

5. compaction

6. depth in profile

Factors Affecting Bulk Density:

(1)___are weightless

Pores (Porosity)

Factors Affecting Bulk Density:

(2)

Typical Mineral Soil: 1-5% ___ (bulk density = 1.1 - 1.6 g/cm³)

Organic Soils: > 20% ____ (bulk density = 0.1 - 0.6 g/cm³)

Organic Matter

Factors Affecting Bulk Density:

(3)____generally increases overall porosity, decreases density

Aggregation

Factors Affecting Bulk Density:

(4)

(5) compaction

packing arrangement

Factors Affecting Bulk Density:

(5) compaction

(6)

(1) ___of typical mineral soils range between 1.0 and 1.6 g/cm³.

(2) ___increases porosity, (3) ___ BD

(4) ___can have BD as low as 0.1 g/cm³.

(5)___decreases porosity, (6)___ BD

(7) ___increases porosity, (8) ___ BD

(9) ___in profile decreases porosity, (10) ___ BD

1. Bulk densities

2. Organic matter

3. decreases

4. Organic soils

5. Compaction

6. increases

7. Aggregation

8. decreases

9. Depth

10. increases