Study Guide: PSS Exam III

1. Adsorption and Cation Exchange

General Concepts

• Clays and Colloids:
  • Small size leads to large surface area.
  • Charge sources:
    • Permanent Charge: From isomorphous substitution.
    • pH-Dependent Charge: Changes with soil solution pH.
  • Highly reactive, influencing water properties and ion exchange.

Cation Exchange Capacity (CEC)

• Definition:
  • Sum of exchangeable cations soil can hold.
  • Measured in cmolc/kg or meq/100g.
• Key Influences:
  • Higher clay and organic matter content leads to higher CEC.
  • pH affects charge density of organic matter and charge clays.
• Significance:
  • Determines soil fertility and nutrient retention.
  • Affects adsorption of pollutants and biological materials.

2. Salinity and Sodicity

Salinity

• Effect on Plants:
  • High salinity lowers osmotic potential, reducing water uptake and stunting growth.
• Measurement:
  • Saturated paste extract method (EC at 25°C).
  • Expressed as Total Dissolved Solids (TDS) or dS/m.
• Management:
  • Leaching requirement calculated to avoid exceeding plant tolerance.

Sodicity

• Definition:
  • High sodium levels disrupt soil structure and water infiltration.
• Indicators:
  • Sodium Adsorption Ratio (SAR) in water.
  • Exchangeable Sodium Percentage (ESP) in soil.
• Management:
  • Use flocculating agents (e.g., gypsum) to displace Na⁺.
  • Avoid low EC water to prevent worsening sodicity.

3. Acid Soils

• Acidity Causes:
  • Natural: Leaching, feldspar dissolution releasing Al³⁺.
  • Anthropogenic: Nitrogen/sulfur oxidation, acid sulfate soils.
• Effects on Plants:
  • Al³⁺ toxicity stunts roots and reduces growth.
• Nutrient Availability:
  • pH influences nutrient solubility.
• Management:
  • Lime application to neutralize pH and improve fertility.

4. Redox Reactions and Hydric Soils

Redox Reactions

• Key Processes:
  • Photosynthesis and respiration involve oxidation processes.
  • Microbes use alternate electron acceptors in absence of O₂.

Hydric Soils

• Formation:
  • Saturation and anaerobic conditions create redoximorphic features.
• Identification:
  • Redox potential measurement, Fe²⁺ staining, or IRIS tubes.

5. Soil Organic Matter and Decomposition

• Composition:
  • Derived from plant residues; lignin decomposes slowly.
• Decomposition:
  • Microbial activity drives mineralization, influenced by moisture and oxygen.
• Management:
  • Maintain organic matter and manage decomposition for fertility.

Key Concepts for Soil Management

• Nutrient Retention:
  • Optimize CEC, manage pH.
• Water and Salinity:
  • Prevent salinization, reclaim sodic soils.
• Environmental Protection:
  • Trap contaminants, manage redox in wetlands.

Methods for Measuring CEC

Method 1: Summation Method

1. Saturate soil with cation, displace, and measure concentrations.
2. Sum ion contributions to calculate CEC.

Method 2: Displacement Method

1. Saturate with index cation, displace, measure displaced cation.
2. Calculate CEC using concentration, volume, and soil mass.

Applications of CEC Calculations

1. Base Saturation Percentage (BSP)
2. Exchangeable Sodium Percentage (ESP)
3. Leaching Requirement for managing salinity.