Soil Chemistry and Management Study Guide

STUDY GUIDE PSS EXAM III

Clays and Colloids:
- Small size = large surface area.
- Possess charge due to:
  - Permanent Charge: From isomorphous substitution.
  - pH-Dependent Charge: Alters with soil solution pH (reversible protonation/de-protonation).
- Highly reactive, influencing water properties and ion exchange. Cation Exchange Capacity (CEC)
Definition:
- Sum of exchangeable cations a soil can hold.
- Measured in cmolc/kg or meq/100g.
Key Influences:
- Higher clay and organic matter content = higher CEC.
- pH affects the charge density of organic matter and variable charge clays.
Significance:
- Determines soil fertility and nutrient retention.
- Affects adsorption of pollutants and biological material.

Effect on Plants:
- High salinity lowers osmotic potential, stunting growth by reducing water uptake.
Measurement:
- Standard method: Saturated paste extract (EC at 25°C).
- Expressed as Total Dissolved Solids (TDS) or dS/m.
Management:
- Leaching requirement calculated to avoid exceeding plant tolerance thresholds.
- Example: If irrigation water EC = 1.5 dS/m, calculate leaching fraction to maintain root zone EC below crop 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:
- Flocculating agents (e.g., gypsum) displace Na⁺.
- Avoid low EC water to prevent sodicity exacerbation.

Acidity Causes:
- Natural: Leaching in high-rainfall areas, feldspar dissolution releasing Al³⁺.
- Anthropogenic: Nitrogen/sulfur oxidation, acid sulfate soils.
Effects on Plants:
- Al³⁺ toxicity at low pH stunts roots and reduces growth.
Nutrient Availability:
- pH strongly influences solubility of nutrients.
Management:
- Liming acidic soils to neutralize pH and improve fertility.

Key Processes:
- Photosynthesis (oxidation of water) and respiration (oxidation of organic matter).
- Microbes utilize alternate electron acceptors in the absence of O₂: NO₃⁻ > Mn⁴⁺ > Fe³⁺ > SO₄²⁻ > CO₂.
Redox Potential (Eh):
- Measured with Pt electrodes; indicates the oxidizing/reducing environment. Hydric Soils
Formation:
- Saturation and anaerobic conditions create distinct redoximorphic features (e.g., gleying, iron mottles).
Identification:
- Redox potential measurement, Fe²⁺ staining, or IRIS tubes.
Organic Matter:
- Slow decomposition in hydric conditions leads to organic soil formation (Histosols).

Composition:
- Primarily derived from plant residues.
- Lignin and other recalcitrant compounds decompose slowly.
Decomposition:
- Microbial activity drives organic matter mineralization, influenced by soil moisture and oxygen availability.
Management:
- Enhance soil fertility by maintaining organic matter and managing decomposition rates.

Key Concepts for Soil Management

Nutrient Retention:
- Optimize CEC with organic amendments and clay management.
- Maintain favorable pH for nutrient availability.
Water and Salinity:
- Prevent salinization with adequate drainage and leaching practices.
- Reclaim sodic soils using amendments and controlled irrigation.
Environmental Protection:
- Use soil adsorption capacity to trap contaminants and protect groundwater.
- Manage redox dynamics in wetlands for carbon storage and ecosystem health. Definition of CEC CEC refers to the capacity of soil to hold and exchange positively charged ions (cations) on its surface. It is expressed in centimoles of charge per kilogram of soil (cmolc/kg). This property is critical for nutrient availability, soil fertility, and pollutant retention.

Methods for Measuring CEC Method 1: Summation Method

Process:
- Saturate soil exchange sites with a known cation (e.g., NH₄⁺ or K⁺).
- Displace these cations and measure the concentrations of common ions in the leachate (e.g., Al³⁺, Ca²⁺, Mg²⁺, Na⁺).
- Sum the contributions of each ion to calculate total CEC.
Example:
- A 5 g soil sample is extracted at pH 5.7 using a neutral salt solution. Leachate volume = 80 mL. Ion concentrations (mg/L):
  - Al³⁺: 23 mg/L (atomic mass = 27)
  - Ca²⁺: 40 mg/L (atomic mass = 40)
  - Mg²⁺: 19 mg/L (atomic mass = 24)
  - K⁺: 22 mg/L (atomic mass = 39)
  - Na⁺: 14 mg/L (atomic mass = 23)
- Steps:
  - Convert mg/L to cmolc/kg soil for each ion.
  - Sum all values to find the total CEC.
- Result: Total CEC = 11.69 cmolc/kg soil.

Method 2: Displacement Method

Process:
- Saturate soil exchange sites with an index cation (e.g., NH₄⁺).
- Displace the index cation with another (e.g., K⁺) and measure the displaced index cation in the leachate.
Example:
- Soil Sample: 4 g, saturated with NH₄⁺, displaced with K⁺.
- Leachate: 10 mL, NH₄⁺ concentration = 500 ppm (equivalent to mg/L).
- Steps:
  - Convert NH₄⁺ concentration from ppm to cmolc/kg soil using the formula: CEC=(Concentration (mg/L)FW of NH₄⁺ (18 g/mol))×(Volume of leachate (L)Mass of soil (kg))\text{CEC} = \left(\frac{\text{Concentration (mg/L)}}{\text{FW of NH₄⁺ (18 g/mol)}}\right) \times \left(\frac{\text{Volume of leachate (L)}}{\text{Mass of soil (kg)}}\right)CEC=(FW of NH₄⁺ (18 g/mol)Concentration (mg/L))×(Mass of soil (kg)Volume of leachate (L))
  - Result: CEC = 6.99 cmolc/kg soil.

Important Conversions

ppm to cmolc/kg:
- Convert mass of ions to molar equivalents using atomic mass.
- Multiply by ion charge to get charge equivalents.
- Adjust for soil mass and leachate volume.

Estimating CEC Using Soil Properties

Formula: Estimated CEC=(%Sand×CEC of Sand)+(%Silt×CEC of Silt)+(%Clay×CEC of Clay)+(%SOM×CEC of SOM)\text{Estimated CEC} = (% \text{Sand} \times \text{CEC of Sand}) + (% \text{Silt} \times \text{CEC of Silt}) + (% \text{Clay} \times \text{CEC of Clay}) + (% \text{SOM} \times \text{CEC of SOM})Estimated CEC=(%Sand×CEC of Sand)+(%Silt×CEC of Silt)+(%Clay×CEC of Clay)+(%SOM×CEC of SOM)
- Convert percentages to proportions by dividing by 100.
Example:
- Soil Composition:
  - Sand = 30%, Silt = 40%, Clay = 30%, SOM = 3%.
  - Standard CEC values:
    - Sand = 0 cmolc/kg, Silt = 0 cmolc/kg, Clay = 20 cmolc/kg, SOM = 200 cmolc/kg.
- Calculation: Estimated CEC=(0.3×0)+(0.4×0)+(0.3×20)+(0.03×200)=12 cmolc/kg\text{Estimated CEC} = (0.3 \times 0) + (0.4 \times 0) + (0.3 \times 20) + (0.03 \times 200) = 12 \text{ cmolc/kg}Estimated CEC=(0.3×0)+(0.4×0)+(0.3×20)+(0.03×200)=12 cmolc/kg

Applications of CEC Calculations

Base Saturation Percentage (BSP):
- Formula: BSP=(Sum of base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺)Total CEC)×100\text{BSP} = \left(\frac{\text{Sum of base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺)}}{\text{Total CEC}}\right) \times 100BSP=(Total CECSum of base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺))×100
- High BSP indicates fertile soil.
Exchangeable Sodium Percentage (ESP):
- Formula: ESP=(CEC of Na⁺Total CEC)×100\text{ESP} = \left(\frac{\text{CEC of Na⁺}}{\text{Total CEC}}\right) \times 100ESP=(Total CECCEC of Na⁺)×100
- High ESP (>15%) indicates sodicity.
Leaching Requirement:
- Calculate irrigation needs to manage salinity using: Leaching Requirement=Leaching Fraction (LF)×Root Zone Depth\text{Leaching Requirement} = \text{Leaching Fraction (LF)} \times \text{Root Zone Depth}Leaching Requirement=Leaching Fraction (LF)×Root Zone Depth

Transcript for:Soil Chemistry and Management Study Guide