Unit 1: Physical Properties of Soil

1. Texture

• Definition: Soil texture refers to the fineness or smoothness of soil aggregates, which include particles like:
  • Gravel

  • Silt

  • Sand

  • Clay

• Soil Types:
  • Clay: Fine particles, smooth texture, retains water.

  • Sand: Coarse particles, gritty texture, drains water quickly.

  • Loam: A mixture of sand, silt, and clay; ideal for most crops.

• Methods to Determine Soil Texture:
  • Sedimentation Method: Based on the settling of soil particles in water.

  • Feel Method: Feeling the soil’s texture by hand.

  • Sieve Method: Using different sieves to separate particles based on size.

2. Soil Colour

• Definition: Soil colour is an observable feature, which helps in identifying soil types and fertility.

• Factors Affecting Soil Colour:
  • Organic matter content

  • Moisture content

  • Parent material

• Common Soil Colours:
  • Red/Brown: Well-drained, healthy soil.

  • Grey: Poor drainage, infertile soil.

  • Black/Dark Brown: High organic matter, fertile soil.

3. Soil Temperature

• Definition: Soil temperature is the degree of hotness or coldness of the soil particles, impacting various processes.

• Influences of Soil Temperature:
  • Germination of seeds

  • Microbial activity

  • Soil formation

  • Root growth and flowering

4. Soil Depth

• Definition: Soil depth refers to the thickness of the soil cover above the parent material (rock).

• Soil Depth Categories:
  • Shallow soil: Parent rock is near the surface, suitable for shallow-rooted crops.

  • Deep soil: Parent rock is deep, suitable for deep-rooted crops.

• Influences of Soil Depth:
  • Affects soil temperature, water retention, and drainage.

5. Soil Consistency

• Definition: Soil consistency is how easily soil particles can be crushed by fingers or cultivation tools. It depends on the moisture content.

• Methods to Determine Soil Consistency:
  • Moist Soil:
    • Loose: Does not hold together when moist.

    • Firm: Crushed under moderate pressure.

  • Wet Soil:
    • Stickiness: Non-sticky, slightly sticky, sticky, very sticky.

    • Plasticity: Ability to be reshaped, ranging from non-plastic to very plastic.

  • Dry Soil:
    • Described as either loose (breaks easily) or very hard (requires force to break).

6. Soil Structure

• Definition: The arrangement of soil particles into aggregates called peds. These peds have spaces for air and water.

• Types of Soil Structures:
  1. Blocky: Irregular, found in dry soils.

  2. Cuboidal: Equal horizontal and vertical dimensions.

  3. Columnar: Vertical structures with salt caps, poor water drainage.

  4. Prismatic: Found in the B horizon, vertical columns, poor water circulation.

  5. Platy: Thin plates, found in clay pan soils, poorly aerated.

  6. Granular: Found in topsoil, suitable for root growth, good air and water spaces.

  7. Crumb: Formed by humus and clay particles, friable, good for crops.

  8. Single Grain: Common in sandy soils, poor water retention.

7. Soil Porosity

• Definition: Soil porosity is the amount of pore spaces in the soil that hold air and water.

• Porosity Formula:
  

8. Effects of Physical Properties of Soil on Crop Production

• Soil Texture: Affects water retention, drainage, and nutrient availability.

• Soil Colour: Indicates soil fertility.

• Soil Temperature: Influences seed germination, microbial activity, and growth.

• Soil Structure: Affects root penetration and soil fertility.

• Soil Depth: Determines the suitability for different crops (deep vs. shallow-rooted).

• Soil Consistency: Influences tillage and ease of root growth.

Maintaining Soil Structure

• Crop Rotation: Helps replenish nutrients and maintain organic matter.

• Adding Organic Matter: Improves microbial activity and moisture retention.

• Liming: Reduces acidity, improving nutrient availability.

• Preventing Soil Erosion: Use reforestation, conservation tillage, and cover crops.

• Maintaining Ground Cover: Reduces erosion, increases fertility.

Destruction of Soil Structure

• Causes:
  • Soil Erosion: Caused by wind, water, and human activities (deforestation, overgrazing, poor farming).

  • Poor Soil and Water Management: Reduces soil fertility and structural integrity.

  • Compaction: Due to heavy machinery, reducing porosity and root penetration.

  • Reduced Organic Matter: Leads to poor soil stability.