Lecture Notes: Soil Composition and Properties

Introduction

• Topic: Soil composition and properties (Topic 4.3)
• Objective: Describe similarities and differences between properties of different soil types.
• Key Knowledge:
  • All soils consist of sand, silt, and clay.
  • Particle size affects water and nutrient holding capabilities.
  • Classification of soil by texture (percentages of sand, silt, clay).
  • Factors affecting water holding capacity.
  • Basic tests to measure soil fertility.

Soil Composition

• Components: Sand, silt, clay, humus, organic matter, water, air.
• Geological Focus: Rock-based components (sand, silt, clay).

Soil Texture

• Particle Size:
  • Sand: Large particles, large pores.
  • Silt: Intermediate size.
  • Clay: Small particles, small pores.
• Porosity: Space between particles; affects air and water entry.
• Texture Classification:
  • Use a soil texture chart.
  • Calculate texture by settling soil in water and measuring layers.

Soil Texture Classification Example

• Example soil in mason jar: 45% sand, 35% silt, 20% clay → classified as loam.
• Using a Soil Texture Chart:
  • Start with sand percentage.
  • Move to where sand and silt meet, calculate clay.
  • Ensure total percentage adds to 100.

Porosity and Permeability

• Porosity: Space between soil particles; affects water entry.
• Permeability: Ability of water to drain through soil.
• Relationship:
  • Inverse relation with water holding capacity.
  • Sandy soil: High porosity, low water holding.
  • Clay soil: Low porosity, high water holding.

Ideal Soil for Plants

• Ideal Balance: Loam (40-40-20 mix of sand, silt, clay).
• Benefits:
  • Moderate porosity/permeability.
  • Supports water and oxygen access.

Factors Affecting Soil Fertility

• Nutrient Factors:
  • Key nutrients: Nitrogen, phosphorus, magnesium, calcium, potassium.
  • Increased by organic matter, clay, and bases like calcium carbonate.
  • Decreased by soil acidity, excessive farming, erosion.
• Water Factors:
  • Improved by good aeration, organic matter, and deep-rooted plants.
  • Reduced by soil compaction, erosion.

Soil Quality Tests

• Tests:
  • Texture: Measure sand, silt, clay percentages.
  • Permeability: Time for water to drain through a soil column.
  • pH: Soil acidity using pH test strips.
  • Color: Indicates humus content.
  • Nutrient Levels: Chemical tests for nitrogen and phosphorus.

Practical Application

• FRQ Practice: Describe a soil test method and its implications for advising farmers.

Conclusion

• Encourage further learning and engagement with soil studies.

Remember: Think like a mountain, write like a scholar.