Comprehensive Notes on Soil Physical Properties and Classification

Introduction to Soil Physical Properties

• Soil properties are categorized based on physical attributes such as texture, structure, density, porosity, consistency, strength, temperature, color, water, and moisture content.
• Chemical properties, which build upon these physical understandings, are discussed in subsequent topics.
• For optimal plant growth and health, as well as for construction project viability, good soil in excellent physical condition is paramount.
• Essential for plants is a healthy balance of soil air and water relationships.
• For construction, the "buildability" of a soil refers to how it interacts with the specific project requirements.

Key Physical Properties of Soil

• Concerned Properties:
  • Porosity (pore space within the soil)
  • Mineral content (considered physical when referring to general soil type, but delving into its chemical makeup is a chemical property).
  • Rockiness
  • Air holding capacity
  • Water holding capacity
  • Color
  • Parent material (the geological material from which soil is formed)

Ideal Pore Space and Water Movement

• Ideal Composition: Ideally, soil pore space should consist of approximately $50\%$ water and $50\%$ air.
  • This $50/50$ relationship is considered perfect for plant growth, ease of digging, workability, pliability, and plant establishment.
• Water Movement: Soil must be permeable enough to allow water movement through it, which involves two key processes:
  • Infiltration: The process by which water enters the soil surface and moves down through the upper layers.
    • Sandy soils have a higher infiltration rate because of their larger particle size and pore spaces, allowing water to pass through quickly.
    • Clay soils have a lower infiltration rate; they can become clumpy and repel water, similar to a tarp, leading to runoff.
  • Percolation: The downward movement of water through the soil profile, once it has infiltrated the surface.
    • Sandy soils have a higher percolation rate, meaning water moves swiftly through the layers.
    • Clay soils are very effective at holding water, thus having a lower percolation rate.
• Perk Test: This is a crucial test conducted on properties without municipal sewer systems.
  • It measures the rate at which water infiltrates and percolates through the native soil.
  • The results of a perk test directly determine the type of septic system (e.g., sand mound) required for the property.
  • Performing perk tests, which often involves digging test holes, is a common practice in the industry.
• Soil Texture and Water Retention:
  • Tighter soils (e.g., clays) are more retentive of water due to smaller, more numerous pore spaces.
  • Looser soils (e.g., sands) allow water to percolate and move through more freely.
  • Soil type is the primary factor influencing these characteristics.

Organic Content in Natural Soil Classification

• When classifying natural soils based on texture, organic content is generally excluded.
• Reasons for exclusion:
  • Organic matter is typically concentrated in the topsoil layer and decomposes relatively quickly, making it transient.
  • A very small percentage of soils in the United States (less than $3\%$ or $4\%$) are classified as organic soils; the vast majority are mineral soils.
  • For example, a project sample showed only $0.85\%$ organic content over eight acres, indicating its minor contribution to the overall soil mass in most native soils.
  • Organic soils do not have distinct "separates" (sand, silt, clay) in the same way mineral soils do, making textural triangle classification inapplicable.

Mineral Soil Components (Soil Separates)

• The three primary mineral components of soil, known as soil separates, are sand, silt, and clay.
  • It's essential to understand these components and their relative sizes.
• Particle Size Order (Largest to Smallest):
  1. Sand (largest particles)
  2. Silt (medium particles)
  3. Clay (smallest particles)
• Hierarchy of Weathered Materials: Above sand in particle size would be gravel, then small unweathered parent material, larger unweathered parent material, rocky parent material, and finally bedrock.
• Clay particles are essentially highly weathered and broken-down forms of sand and silt over geological time.
  • Note: Specific numerical size ranges for sand, silt, and clay particles are not required knowledge, but their relative order is critical.

The Soil Textural Triangle

• Purpose: This chart is a universal tool used to classify soil types based on the percentage composition of sand, silt, and clay.
• Usage: To classify a soil, you plot the percentages of sand, silt, and clay (which must sum to $100\%$) on the triangle.
  • Example: If a soil has $70\%$ sand, $20\%$ clay, and $10\%$ silt, you would find the $70\%$ sand line, the $20\%$ clay line, and the $10\%$ silt line. The point where these lines intersect identifies the soil type (e.g., Sandy Clay Loam).
• Clay Dominance: If clay content significantly exceeds approximately $50\%$ to $60\%$, the soil is often simply classified as "Clay," regardless of other components, as its properties will be dominated by clay.
• True Classifications: The terms obtained from the soil triangle (e.g., silty clay, sandy clay loam, clay loam) are the true scientific classifications of soil and are used in official charts like GIS soil maps.
• "Best" Soil - Context Dependent: The ideal soil type depends heavily on its intended use:
  • Pond Construction: High clay content is desirable (e.g., in the upper left region of the triangle) to minimize water seepage.
  • Farming or Retaining Walls: Different textural compositions would be preferred.
• Loam - The "Sweet Spot": While the term "loam" is often used colloquially, scientifically, it represents a balanced soil texture generally considered ideal (`