Unit 3: Soil Physical Properties

1. Soil Texture

Soil texture is considered the fundamental physical property of soil because it is not easily altered by management practices. It determines the basic physical framework of the soil.

Definition

Soil texture refers to the relative proportion of mineral particles of different sizes (sand, silt, and clay) in the soil mass. It specifically refers to the inorganic fraction of the soil less than 2mm in diameter (fine earth).

Soil Separates (USDA Classification)

The United States Department of Agriculture (USDA) classifies soil particles based on diameter:

Properties of Soil Separates

1. Sand:
   • Low specific surface area.
   • Large pore spaces (macropores) facilitate rapid drainage and good aeration.
   • Chemically inert (mostly quartz).
   • Low water holding capacity and low nutrient retention.
2. Silt:
   • Intermediate surface area.
   • Microsand properties (mostly quartz and feldspars).
   • Highly susceptible to wind and water erosion due to lack of cohesion (unlike clay) and small size (unlike sand).
3. Clay:
   • High specific surface area (colloidal behavior).
   • High porosity (total pore space) but very small pores (micropores), leading to slow drainage.
   • Carry negative charges; high Cation Exchange Capacity (CEC).
   • Exhibits plasticity and stickiness when wet.

Soil Textural Classes

Soils are assigned to textural classes based on the percentage of sand, silt, and clay, visualized using the Textural Triangle. There are 12 main classes, grouped broadly into:

• Sandy Soils (Coarse): Sand, Loamy sand.
• Loamy Soils (Medium): Sandy loam, Loam, Silt loam, Silt.
• Clayey Soils (Fine): Sandy clay, Silty clay, Clay.

Note: "Loam" is not an equal mix of separates, but a mix that exhibits equal properties of the three. A typical loam might be 40% sand, 40% silt, and 20% clay.

2. Soil Structure

While texture defines the size of particles, structure defines how they are arranged.

Definition

Soil structure is the arrangement of primary soil particles (sand, silt, clay) into secondary units called aggregates or peds.

Mechanism of Aggregate Formation

1. Flocculation: The electro-chemical grouping of clay particles (often induced by divalent cations like ).
2. Cementation: The binding of flocculated particles by organic matter, iron/aluminum oxides, or carbonates.

Classification of Soil Structure

Structure is described by Type (Shape), Class (Size), and Grade (Distinctness).

A. Types (Shape)

1. Platey: Horizontal plates. Found in compacted surface soils or E horizons. Impedes downward water movement.
2. Prismatic/Columnar: Vertical pillars.
   • Prismatic: Flat tops. Common in B horizons.
   • Columnar: Rounded "biscuit" tops. Common in sodic soils (high sodium).
3. Blocky: Cube-like blocks. Common in B horizons.
   • Angular Blocky: Sharp edges.
   • Sub-angular Blocky: Rounded edges.
4. Spheroidal (Granular/Crumb): Small, porous spheres. Characteristic of A horizons (topsoil) high in organic matter. Ideal for agriculture.

B. Grades (Strength/Stability)

• Structureless: Single grain (sand) or Massive (cemented clay).
• Weak: Peds barely distinguishable; crumble easily.
• Moderate: Peds clearly visible; moderately durable.
• Strong: Peds distinct; resist displacement; separate cleanly.

Importance

• Influences infiltration and permeability.
• Modifies the effect of texture (e.g., a clay soil with granular structure can drain well).
• Determines root penetration capability.

3. Bulk Density and Particle Density

Particle Density ()

• Definition: The mass per unit volume of the soil solids alone (excluding pore space).
• Typical Value: Most mineral soils average 2.65 g/cm³ (the density of quartz).
• Factors: It is relatively constant unless high organic matter is present ( decreases) or heavy minerals like magnetite are present ( increases).
• Formula:
  

Bulk Density ()

• Definition: The mass per unit volume of dry soil (including both solids and pore space).
• Typical Range: 1.1 to 1.6 g/cm³.
• Significance: It is an indicator of soil compaction and porosity.
• Formula:
  

Factors Affecting Bulk Density:

1. Texture: Fine-textured soils (clays) generally have lower bulk density (1.1–1.3 g/cm³) than sandy soils (1.4–1.6 g/cm³) because clays have high microporosity and form aggregates that create volume.
2. Organic Matter: Increases pore space and is light in weight; lowers .
3. Compaction: Tillage or heavy machinery increases by crushing macropores.
   • Root growth is restricted if .

Porosity ()

The percentage of soil volume occupied by pore space. It is mathematically related to densities:

4. Soil Consistency

Definition

Soil consistency describes the resistance of soil material to deformation or rupture and the degree of cohesion and adhesion at various moisture contents.

States of Consistency

1. Wet: Stickiness (adhesion to objects) and Plasticity (ability to be molded).
2. Moist: Friability (ability to crumble under gentle pressure). This is the ideal state for tillage.
3. Dry: Hardness (resistance to rupture).

Atterberg Limits

Used to define the boundaries between different consistency states in fine-grained soils:

1. Liquid Limit (LL): Moisture content where soil begins to flow like a liquid.
2. Plastic Limit (PL): Moisture content where soil can be rolled into a thread without breaking.
3. Plasticity Index (PI):
   . Represents the range of water content where soil exhibits plastic properties.

5. Soil Temperature

Soil temperature governs the rate of chemical and biological reactions.

Sources of Soil Heat

• Solar Radiation: The primary source (about 95%).
• Conduction: From the Earth's interior (negligible for surface soils).
• Biological/Chemical: Decomposition of organic matter releases heat.

Factors Affecting Soil Temperature

1. Albedo: The fraction of solar radiation reflected. Dark soils absorb more heat (low albedo); light soils reflect more (high albedo).
2. Aspect/Slope: In the Northern Hemisphere, south-facing slopes receive more direct sunlight and are warmer.
3. Soil Moisture: Water has a high Specific Heat Capacity (requires more energy to warm up). Wet soils warm up much slower than dry soils in spring ("Cold Soils").
4. Vegetative Cover: Acts as an insulator, reducing temperature fluctuations (cooler in summer, warmer in winter).

Importance

• Germination: Seeds require specific minimum temperatures (e.g., Maize > 10°C).
• Microbial Activity: Nitrification ceases below 5°C.
• Nutrient Availability: Phosphorus uptake is inhibited in cold soils.

6. Soil Air

Soil air occupies the pore spaces not filled with water.

Composition (Soil Air vs. Atmospheric Air)

Note: Soil air is higher in and lower in due to root and microbial respiration.

Mechanisms of Gas Exchange

1. Mass Flow: Movement of air due to pressure differences (wind, temperature changes, water entering displacing air). Minor contributor.
2. Diffusion: Movement of gases from partial pressure to lower partial pressure. This is the primary mechanism of soil aeration. diffuses into the soil; diffuses out.

Oxygen Diffusion Rate (ODR)

If ODR drops below a critical level, plant roots cannot respire, leading to:

• Inhibition of nutrient and water uptake.
• Accumulation of toxic byproducts (e.g., methane, hydrogen sulfide).
• Reduction of iron () creating gleyed (gray/blue) colors.

7. Soil Water

Water acts as a solvent and transport medium for nutrients.

Structure and Properties

• Polarity: The water molecule is dipolar (positive H end, negative O end).
• Hydrogen Bonding: Water molecules stick to each other.
• Adhesion: Water sticks to soil solids (matrix).
• Cohesion: Water sticks to other water molecules.

Soil Water Potentials

Water moves from high energy to low energy. Total soil water potential () is the sum of:

1. Matric Potential (): Negative pressure due to adhesion/capillarity. The dominant force in unsaturated soils.
2. Osmotic Potential (): Due to dissolved salts. Reduces water availability.
3. Gravitational Potential (): Due to gravity. Positive force pulling water down.

Biological Classification of Soil Water

1. Superfluous (Gravitational) Water: Water held at potentials > -1/3 bar (or -33 kPa). Drains away freely due to gravity. Unavailable to plants.
2. Available Water: Held between Field Capacity (-1/3 bar) and Permanent Wilting Point (-15 bar). Capillary water used by plants.
3. Unavailable Water: Held at potentials < -15 bar. Includes hygroscopic water held tightly to soil particles.

Soil Moisture Constants

1. Saturation: All pores filled with water (Potential 0).
2. Field Capacity (FC): Water remaining after gravitational water has drained (usually 1-3 days after rain).
   • Potential: -0.33 bar (-33 kPa).
   • The upper limit of plant-available water.
3. Permanent Wilting Point (PWP): Soil water content where plants wilt and cannot recover (roots cannot overcome the matric suction).
   • Potential: -15 bar (-1500 kPa).
   • The lower limit of plant-available water.
4. Hygroscopic Coefficient: Water held as a thin film (vapor) on particles.
   • Potential: -31 bar.

Calculation of Plant Available Water (PAW)