2 Soil Texture Pre-lab Discussion
Soil Texture Pre-lab Discussion Notes
What is Soil Texture?
- Soil texture refers to the relative proportion of sand, silt, and clay in a soil sample.
- Fine Earth Fraction includes:
- Sand, Silt, Clay (particles < 2 mm in diameter)
- Coarse Fractions / Fragments include gravel and cobbles (up to 250 mm in diameter).
Composition of Soil Separates
Size and Characteristics
- Soil Separates diameter (USDA classification):
- Sand: 2 mm - 0.05 mm (Coarse, gritty)
- Silt: 0.05 mm - 0.002 mm (Smooth, powdery)
- Clay: < 0.002 mm
Properties of Sand vs. Clay
| Property | Sand | Clay |
|---|---|---|
| Total Porosity | Low (more macropores) | High (more micropores) |
| Water Holding Capacity | Low (droughty) | High |
| Aeration | Very good aeration | Poor aeration |
| Tillage Ease | Easy to till | Difficult to till |
| Stickiness | Non-sticky | Very sticky |
| Nutrient Holding Capacity | Low (less fertile) | High (more fertile) |
Importance of Soil Texture
- Soil texture affects:
- Porosity
- Water movement (hydraulic conductivity)
- Water-holding capacity
- Internal drainage
- Tillage properties
- Nutrient retention
- Soil texture is a fundamental property and remains unchanged; a sandy soil stays sandy, and a clay soil stays clay.
Soil Textural Triangle
- Used to classify soil texture based on the proportions of sand, silt, and clay.
- Examples of textural classes:
- Sand (85% sand or more)
- Loamy sand (75-85% sand)
- Loam (less than 52% sand, 7-27% clay, 28-50% silt)
- Clay (≥40% clay, <45% sand, <40% silt)
Methods of Determining Soil Texture
Roll Method
- Knead a moistened soil sample into a wire and mold it into a ring.
Feel Method
- Rub moist soil between fingers to estimate proportions based on texture (grittiness, smoothness, stickiness).
Sedimentation Method
- Based on particle size; larger particles settle faster in water compared to smaller particles.
Hydrometer Method
- Uses Stokes' Law to separate particles based on settling velocities of different sizes.
- Measures specific gravity of liquids using a hydrometer (ratio of liquid density to water density).
- Formula: V = KD²
- Where:
- V = velocity of settling
- D = diameter of the soil particle
- K = constant for temperature effects.
Key Points for Note-Taking
- Understanding soil texture is crucial for effective soil management.
- The various methods for determining soil texture should be practiced for accurate assessments.