Environmental Chemistry Lecture 10: Soil Chemistry

Environmental Chemistry - Lecture 10: Soil Chemistry

• Course: CHEMISTRY 321, Fall 2024
• Instructor: Maryam Izadifard
• University: University of Calgary
• Lecture 10 focuses on Soil Chemistry

Part I: Introduction

• Suggested Readings:
  • Manahan chapters 9.1-9.3 and 10
  • CB/MC 4th or 5th edition: Chapter 16
  • GW Van Loon and SJ Duffy "Environmental Chemistry" 3rd ed. Chapters 14, 17-18
• YouTube video link provided

Objectives

• Describe the composition of soil
• Understand that soil is a 3-phase mixture
• Recognize kaolinite, montmorillonite, and illite (differences)
• Classify soil by physical properties (particle size, texture, permeability)
• Use the soil texture triangle to determine soil texture
• Understand the pathways of rock transformations to soil (physical, chemical, and biological weathering)
• Recognize components of soil organic matter
• Define chemical properties (organic matter, pH, available elements, cation exchange capacity)
• Understand that colloids have a large specific surface area and cation exchange capacity
• Define total and available elements, cation exchange capacity, and base saturation

Why Soil is Cheap

• Soil remains abundant in many areas, making collection/sale inexpensive
• The market undervalues soil's ecological importance and long-term effects of degradation
• Soil is often treated as surplus or waste from industries like construction

Terrestrial Environment

• Terrestrial environments cover 29% of Earth's surface area
• The land is primarily composed of rocks and soil—the geosphere—which comprises the outer 40 km of Earth's crust
• Environmental chemistry of terrestrial ecosystems focuses on the study of soils

Why Study Soil?

• Soil is the link between air, water, rocks, and organisms
• Soil is responsible for many crucial functions (carbon and nutrient cycling, agriculture, forestry, water cycling, natural waste decomposition)
• Soil provides habitats for microorganisms, insects, etc.

What is Soil?

• Soil is a product of weathering—physical, chemical, and biological processes—of rocks.
• It's a medium conducive to plant growth.
• Soil has four fundamental components: finely divided mineral matter, various levels of organic matter, water, and air (gas phase).
• Soil is a three-phase mixture: solid, liquid, and gas

Soil Profile: Vertical Layering of Soil Horizons

• Soil has distinct horizontal layers, or horizons, with varying characteristics
• Each layer, or horizon, has a specific name.

Weathering by Rainwater

• Rainwater percolating through soil transports soluble and colloidal materials downwards
• This process creates banded layers known as horizons

Silicate Minerals in Soil

• Table showing the most common elements in soil (ppm)

Four Major Components of Soil

• Finely divided mineral matter (primary and secondary)
• Various levels of organic matter (1-6%)
• Water (liquid phase)
• Air/Gas phase

Why Clays are Important?

• Clay particles are smaller than sand and silt, creating a vast surface area.
• This large surface area is crucial for various soil processes

The Soil Particle Size

• Classification system for soil particles (clay, silt, sand, gravel)

Clays

• Aluminosilicates
• Important minerals
• Natural water barriers
• Constituent of soils
• Sheet-like structures (illite, kaolinite, montmorillonite)

Clay Minerals

• Detailed chemical structures of kaolinite, illite, and montmorillonite

Model Diagram of Montmorillonite Clay Structure

• Illustrates the structure of montmorillonite clay

Additional Clay Mineral Information

• Descriptions of kaolinite, illite, and montmorillonite crystal structures

Weathering by Hydration, Acid-Base Reaction, and Hydrolysis

• Chemical processes leading to soil formation from rocks by adding water, reactions with carbonic acid, and reactions with water

Weathering by Complexation and Redox Chemistry

• Processes of chemical weathering involving complexation with organic molecules and oxidation/reduction reactions

Biological Weathering

• Role of living organisms in soil formation (trees, microbes, animals/insects)

Soil Properties: Physical Properties

• Size: Particle size classification (clay, silt, fine, coarse, gravel)
• Texture: Combining percentages of clay, silt, and sand
• Permeability: Ability to conduct water

Soil Properties: Chemical Properties

• Total elements: Abundance in soil (table provided)
• Organic matter: Sources and role in soil function
• Available elements: Essential elements in useable forms for plants
• CEC (Cation exchange capacity): Capacity to retain nutrients, and their measurement

Example 2: Finding CEC in Soil

• A practical example illustrating how to calculate CEC

Soil Organic Matter

• Describing Humus, humic acids, and fulvic acids, their functions in soil

Soil Chemical Properties: Available Elements

• How plant availability of elements is measured in soil.

Clay and Humic Materials

• Colloidal materials, surface properties, electrostatic attraction, importance for nutrients and pollutants

Large Surface Area

• Explanation of surface area calculation

Negative Charge

• Discussion of how clays and humic materials develop negative charges

Soil Chemical Properties: Soil pH

• Dependence on soil composition, carbonate minerals, humic materials, and redox

Part II: Environmental Issues Associated with Soils

• Issues addressed: Soil erosion, too little/much water, nutrient leaching, acidification, salinity, metal contamination
• Further readings in related chapters

Objectives

• Awareness of environmental issues associated with soil
• Addressing questions related to the issues
• Definition and plotting stages of soil buffering