Millennium E-learning Centre
PHYSICAL PROPERTIES OF SOIL
Agriculture Form 3 Complete Study Guide
Understanding Soil Properties
The physical properties of soil play a critical role in determining its suitability for agricultural purposes. Understanding these properties allows farmers to make informed decisions about crop production and land management.
🌱 Key Insight: Mastering soil physical properties is essential for optimizing crop production and ensuring sustainable agricultural practices.
1. Soil Texture
Definition: Soil texture refers to the relative proportions of different-sized soil particles and influences the soil’s ability to retain moisture and nutrients. The fineness or coarseness of the soil is determined by the presence of aggregates such as gravel, silt, sand, and clay.
Main Soil Categories
🏖️ SAND
Large particles, good drainage, poor water retention
🧱 CLAY
Very fine particles, poor drainage, excellent water retention
🌱 LOAM
Balanced mixture, ideal for most crops, good properties
Methods of Determining Soil Texture
🥤 Sedimentation Method
Soil particles are separated based on their weight by allowing them to settle in water.
✋ Feel Method
This involves rubbing the soil between fingers to gauge its smoothness or coarseness.
🕳️ Sieve Method
Soil particles are passed through sieves of various sizes to separate them by particle size.
2. Soil Colour
Soil colour is an easily observable property and can provide valuable insights into its composition and fertility. The appearance of the soil is primarily influenced by organic matter content, moisture content, and parent material.
Common Soil Colours and Their Implications
🟤 Red/Brown Soil
Indicates well-drained soil
⚫ Grey Soil
Generally infertile
⚫ Black/Dark Brown
High in organic matter
🟡 Yellow/White/Grey
Specific mineral presence
3. Soil Temperature
Definition: Soil temperature refers to the degree of hotness or coldness of the soil. This property significantly influences various biological, physical, and chemical processes in the soil.
Effects on Agricultural Processes
🌱 Seed Germination
Optimal temperatures required for seeds to sprout
🦠 Microbial Activity
Temperature affects beneficial soil organisms
🌸 Flowering & Root Growth
Controls plant development stages
⚡ Nutrient Uptake
Rate of nutrient absorption by plants
4. Soil Depth
Definition: Soil depth measures how thick the topsoil layer is, which is important for determining the types of crops that can be grown. Soil depth can vary from shallow to deep soils.
Implications of Soil Depth
🌳 Deep Soils
- Suitable for deep-rooted crops
- Better water retention
- More nutrient storage
🌿 Shallow Soils
- Better for shallow-rooted crops
- Quick drainage
- Temperature changes faster
5. Soil Consistency
Definition: Soil consistency describes the ease with which soil particles can be broken down by finger pressure or by the use of cultivation tools. It is closely related to the soil’s moisture content.
Methods for Determining Soil Consistency
💧 Moist Soil
Loose
Does not hold together when moist
Friable
Easily broken apart
Firm
Requires moderate pressure to break
🌊 Wet Soil
Stickiness Levels:
- • Non-sticky
- • Slightly sticky
- • Sticky
- • Very sticky
Plasticity Range:
- • Non-plastic
- • Slightly plastic
- • Plastic
- • Very plastic
☀️ Dry Soil
Soft (Loose)
Easily broken down
Hard
Resistant to pressure
6. Soil Structure
Definition: Soil structure refers to the arrangement of soil particles into aggregates, or “peds.” These peds allow for the movement of water and air within the soil, which is essential for root growth and nutrient uptake.
Types of Soil Structure
🧱 Blocky
Found in dry soils, irregular and closely packed with little air space.
Common in B-horizon🎲 Cuboidal
Aggregates equal in horizontal and vertical dimensions, includes granular, crumb, and blocky types.
🏛️ Columnar
Vertical structures with salt caps at the top, found in arid areas.
Poor drainage📏 Prismatic
Made up of vertical columns of soil, often found in B-horizon.
Poor water circulation📄 Platy
Thin horizontal plates that overlap, common in clay soils.
Poor aeration🌰 Granular
Found at the topsoil, porous and allows air/water movement.
Ideal for crops🍞 Crumb
Humus and clay particles stick together, creating good pore space.
⚪ Single Grain
Particles lie side by side with high air space.
Poor water retention7. Soil Porosity
Definition: Soil porosity refers to the number of pore spaces within the soil, which are occupied by air and water. Higher porosity allows for better drainage and root development.
📊 Porosity Formula
Porosity = (Amount of water added to soil / Total volume of soil sample) × 100%
Factors That Destroy Soil Structure
🌪️ Soil Erosion
Wind and water erosion, along with human activities such as deforestation and poor farming methods, lead to land degradation.
💧 Poor Management
Improper soil and water management practices reduce soil fertility and alter soil properties.
🚜 Compaction
Heavy machinery and repeated plowing compact the soil, reducing porosity.
🍂 Reduced Organic Matter
Lack of organic matter decreases soil stability and leads to poor structure.
Ways to Maintain Soil Structure
Crop Rotation
Increases soil nutrients and organic matter, improving soil structure.
Adding Organic Matter
Enhances microbial activity, reduces compaction, and improves moisture retention.
Liming
Reduces soil acidity and improves biological activity, benefiting soil structure.
Preventing Soil Erosion
Reforestation, conservation tillage, and cover crops protect soil structure.
Ground Cover
Cover crops improve organic matter, fertility, and reduce soil erosion.
Effects of Physical Soil Properties on Crop Production
🏖️ Soil Texture
Affects nutrient availability, water holding capacity, and aeration.
🎨 Soil Colour
Indicates soil fertility levels and composition.
🌡️ Soil Temperature
Influences microbial activity, seed germination, and plant growth.
🏗️ Soil Structure
Determines suitability for crops based on fertility, drainage, and aeration.
📏 Soil Depth
Helps in selecting crops suitable for deep or shallow rooting.
✋ Soil Consistency
Affects cultivation practices and root penetration.
Key Takeaway
Understanding the physical properties of soil is essential for optimizing crop production and ensuring sustainable agricultural practices. These properties work together to determine soil health, fertility, and suitability for different crops.
