Soil and Its Physical Properties

1. Soil Consistence

Soil consistence refers to the resistance of soil to deformation or rupture. It varies with:

• Particle size, shape, and arrangement
• Nature of water films around particles
• Type and amount of organic and inorganic colloids

As soil dries, cohesive forces and mechanical strength increase due to the closer bonding of particles. Soil consistence plays a crucial role in tillage, root penetration, and water movement.

2. Soil Colour

Soil colour is influenced by organic matter, mineral content, and moisture level. It provides clues about:

• Soil fertility
• Drainage conditions
• Aeration and temperature

Main Colouring Agents:

Yellow colour indicates higher moisture than red.

Impact on Soil Temperature:

• Darker soils absorb more heat → higher temperature
• Light-coloured soils reflect more → cooler soil surface

3. Soil Air

Soil air occupies the macropores of soil and is essential for root respiration and microbial activity.

Ideal Proportions in Well-Aerated Soil:

• ~1/3 pore space → Air
• ~2/3 pore space → Water

Oxygen Diffusion Rate (ODR):

Air Content by Soil Type:

4. Soil Temperature

Soil temperature influences:

• Seed germination
• Root growth
• Microbial activity
• Nutrient availability

Optimal Temperature Ranges:

Soil Heat Absorption:

Specific Heat of Soil:
Ranges from 0.20 to 0.23 (compared to water: 1.0)

💧 5. Soil Water

Soil water exists in various forms and determines plant water availability, nutrient movement, and microbial activity.

(A) Physical Classification of Soil Water

1. Hygroscopic Water

• Held tightly by soil colloids at tensions of 31 – 10,000 atm
• Exists as thin films (15–20 molecular layers)
• Non-liquid, biologically inactive
• Moves in vapour form

2. Capillary Water

• Held between field capacity (0.3 atm) and wilting point (15 atm)
• Present in micropores
• Available water for plants
• Moves from thick to thin films

3. Gravitational Water

• Held at < 0.3 atm tension
• Free water present in macropores
• Drains out quickly
• Can leach nutrients
• Considered unavailable to plants

(B) Biological Classification of Soil Water

 Important Concepts and Definitions

Field Capacity (FC)

• Soil moisture content after macropores have drained
• Only micropores remain filled
• Matric tension: 0.1–0.3 atm
• pF: 2.54 – 4.2
  ✅ Favorable for plant growth

Permanent Wilting Point (PWP)

• Plants wilt permanently and do not recover
• Concept by Briggs & Shantz (1912)
• Tension: 15 atm
• pF: ~4.2
  📌 Remaining water is in the smallest micropores

Hygroscopic Coefficient

• Water held after complete drying of micropores
• Tension: ~31 atm
• pF: ~6.0

Moisture Equivalent

• Introduced by Briggs & McLane (1907)
• Defined as: Weight % of water retained by 1 cm thick saturated soil sample after 1000× gravity for 30 minutes
• Approximation:

Wilting Point = Moisture Equivalent ÷ 1.84

• pF ≈ 2.54

Maximum Capillary Capacity (MCC)

MCC = Water Holding Capacity – Hygroscopic Coefficient

Summary Table: Soil Moisture Tension & Availability

Quick Revision Pointers for Competitive Exams

• ODR below 40 × 10⁻⁸ g/cm²/min: Plant growth suffers
• Field Capacity tension: 0.1–0.3 atm
• PWP tension: 15 atm
• Hygroscopic coefficient tension: ~31 atm
• Wilting Point (W.P.) ≈ Moisture Equivalent ÷ 1.84
• MCC = WHC – Hygroscopic Coefficient