Soil and Its Physical Properties
- 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.
- 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:
Colour | Cause |
Black/Dark Grey | High organic matter |
Red | Unhydrated ferric oxides (Fe₂O₃) |
Brown/Yellow | Hydrated iron oxides (Fe₂O₃·3H₂O) |
White/Grey | Silica, lime, and salts |
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
- 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):
ODR Level | Effect |
> 40 × 10⁻⁸ g/cm²/min | Normal plant growth |
< 40 × 10⁻⁸ g/cm²/min | Growth reduced |
< 20 × 10⁻⁸ g/cm²/min | Root growth ceases |
Air Content by Soil Type:
Soil Type | Air (%) |
Sandy | ≥ 25% |
Loamy | 15–20% |
Clayey | < 10% |
- Soil Temperature
Soil temperature influences:
- Seed germination
- Root growth
- Microbial activity
- Nutrient availability
Optimal Temperature Ranges:
Activity | Temperature (°C) |
Root Development | 10–27°C |
Crop Growth | 15–45°C |
Soil Heat Absorption:
Soil Type | Solar Radiation Absorbed |
Black Cotton Soil | ~86% |
Grass-covered Soil | ~60% |
Alluvial Soil | ~40% |
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
- 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
- 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
- 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
Type | Tension Range | Remarks |
Superfluous Water | ≤ 0.3 atm | Excess water, harmful for plants |
Available Water | 0.3 – 15 atm | Main source for crop use |
Unavailable Water | > 15 atm | Held too tightly to be used |
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
Moisture Type | Tension (atm) | pF | Availability |
Gravitational Water | < 0.1 – 0.3 | ~0–2.5 | Not available |
Capillary (Available) | 0.1 – 15 | 2.54–4.2 | Available |
Hygroscopic Water | > 31 | ~6.0 | Unavailable |
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
