Soil Chemistry Basics Dec 18, 2025

3 Learning Outcomes

By the end of this week, students will be able to:

• Explain soil pH and its effect on nutrient availability.
• Define and describe CEC and why it matters for fertility.
• Identify key macro- and micronutrients and their availability at different pH levels.
• Conduct basic pH and nutrient tests and interpret results.

Part 1 – Soil pH

• Definition: Measure of acidity or alkalinity (scale 0–14).
• Acidic (<7), Neutral (=7), Alkaline (>7).
• Why it matters:
• Controls nutrient availability (e.g., iron locked up in high pH soils).
• Influences microbial communities (fungi thrive in acidic soils; bacteria in neutral–slightly alkaline).
• Examples:
• Blueberries require acidic soils (pH 4.5–5.5).
• Alfalfa grows best at near-neutral pH (6.5–7.0).
• Visual Aid: Nutrient availability chart across pH scale.
• Engagement Question: “Why might adding lime help some crops but harm others?”

Part 2 – Cation Exchange Capacity (CEC)

• Definition: Soil’s ability to hold positively charged ions (cations) such as Ca²⁺, Mg²⁺, K⁺, NH₄⁺.
• How it works:
• Clay particles & organic matter are negatively charged.
• They attract and hold cations, preventing leaching.
• CEC Levels:
• High CEC (clay, organic soils): hold nutrients well but may lock them.
• Low CEC (sandy soils): nutrients leach quickly, require frequent replenishment.
• Practical Example: Compost raises CEC, making sandy soils more fertile.
• Analogy: Think of soil as a “nutrient bank account.” High CEC = many slots to hold currency (nutrients).

Part 3 – Nutrient Availability Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K).

• Secondary Nutrients: Calcium (Ca), Magnesium (Mg), Sulfur (S).
• Micronutrients: Iron, Zinc, Boron, Manganese, Copper, Molybdenum.
• How Chemistry Affects Availability:
• Acid soils = lock up Ca & Mg.
• Alkaline soils = lock up Fe, Mn, Zn.
• pH 6–7 = “sweet spot” for most nutrients.
• Case Example: Iron chlorosis (yellow leaves) in alkaline soils.

Part 4 – Synthesis

• Soil pH controls nutrient solubility.
• CEC controls nutrient storage and exchange.
• Both together determine how much nutrition plants can access.

Lab Activity

Title: Basic Soil Chemistry Analysis

Objective: Test pH, measure approximate nutrient levels, and interpret results.

Materials Needed:

• Soil samples (from Week 1/2)
• pH testing kits (field strips or digital meters)
• Simple soil test kits for N-P-K (colorimetric capsules or solutions)
• Optional: CEC estimation using ammonium acetate method (demo only if equipment available)
• Lab notebooks

Procedure:

1. pH Test:

• Mix soil with distilled water, test with pH strips/meter.
• Record values for each sample.

1. Nutrient Tests:

• Use soil test kit for Nitrogen, Phosphorus, Potassium.
• Compare colors to chart, record results.

1. Observation:

• Discuss differences between soils collected from different sites.
• Relate findings to Week 2 texture results.

Lab Report Prompt:

• Record pH and NPK levels for each soil.
• Classify soils as acidic, neutral, or alkaline.
• Predict how these results would impact plant growth.
• Suggest one amendment (lime, compost, sulfur, etc.) for improvement.

Week 3: Soil Chemistry Basics

Learning Outcomes

By the end of this week, students will be able to:

• Explain soil pH and its effect on nutrient availability.
• Define and describe CEC (Cation Exchange Capacity) and its importance for fertility.
• Identify key macro- and micronutrients and how their availability changes with soil pH.
• Conduct basic pH and nutrient tests and interpret results.

Lecture Content

Part 1 – Soil pH

• Definition: pH measures soil acidity/alkalinity (scale 0–14).
• Acidic (<7), Neutral (=7), Alkaline (>7).
• Why it Matters:
• Controls nutrient availability (iron becomes unavailable in high-pH soils).
• Shapes microbial life: fungi thrive in acidic soils, bacteria in neutral/slightly alkaline soils.
• Crop Examples:
• Blueberries: acidic soils (pH 4.5–5.5).
• Alfalfa: near-neutral soils (pH 6.5–7.0).
• Visual Aid: Nutrient availability vs. pH chart.
• Engagement Question:
• “Why might adding lime help some crops but harm others?”
• Answer: Lime raises pH. Crops like alfalfa benefit from neutral soils, but acid-loving crops like blueberries may lose access to iron and other nutrients if pH is raised too high.

Part 2 – Cation Exchange Capacity (CEC)

• Definition: Soil’s ability to hold positively charged ions (cations: Ca²⁺, Mg²⁺, K⁺, NH₄⁺).
• How it Works:
• Clay and organic matter are negatively charged.
• They attract and hold cations, preventing nutrient leaching.
• CEC Levels:
• High CEC (clay, organic soils): more nutrients, but can hold too tightly.
• Low CEC (sandy soils): nutrients leach quickly, frequent fertilization needed.
• Analogy: Soil as a “nutrient bank account.”
• High CEC = many storage slots for nutrients.
• Low CEC = few slots, money (nutrients) spent quickly.

Part 3 – Nutrient Availability

• Macronutrients: N (Nitrogen), P (Phosphorus), K (Potassium).
• Secondary Nutrients: Ca (Calcium), Mg (Magnesium), S (Sulfur).
• Micronutrients: Fe (Iron), Zn (Zinc), B (Boron), Mn (Manganese), Cu (Copper), Mo (Molybdenum).
• Chemistry Influence:
• Acid soils = Ca & Mg deficiencies.
• Alkaline soils = Fe, Mn, Zn deficiencies.
• pH 6–7 = “sweet spot” for most nutrients.
• Case Example: Iron chlorosis (yellowing leaves due to Fe deficiency) in alkaline soils.

Part 4 – Synthesis

• Soil pH = controls solubility (how much nutrient is available).
• CEC = controls storage and exchange (how long nutrients remain accessible).
• Together, they determine plant nutrition success.

Lab Activity: Basic Soil Chemistry Analysis

Objective: Test pH, measure approximate nutrient levels, and interpret results.

Materials:

• Soil samples (from Week 1/2 collection).
• pH testing kits (strips or digital meters).
• Soil test kits (colorimetric capsules for N-P-K).
• Optional: CEC demo using ammonium acetate method.
• Lab notebooks.

Procedure:

1. pH Test – Mix soil with distilled water, test with strips/meter. Record values.
2. Nutrient Tests – Test Nitrogen, Phosphorus, Potassium with kit. Record results.
3. Observation & Comparison – Discuss differences across samples and link back to Week 2 (texture).

Lab Report Prompt:

• Record pH and NPK for each soil.
• Classify soil (acidic, neutral, alkaline).
• Predict impact on plant growth.
• Suggest one amendment (lime, sulfur, compost, etc.).

Research Recommendations

• Compare soil nutrient availability charts from multiple agricultural universities (e.g., Texas A&M, Cornell, NC State).
• Research specific crop requirements (pH ranges and nutrient needs).
• Study field methods for testing pH, NPK, and CEC.
• Explore soil amendment guides (lime vs. sulfur, compost vs. synthetic fertilizers).

Homework / Extended Assignment

• Write a one-page reflection comparing two soil types you’ve tested (e.g., sandy roadside soil vs. garden loam).
• Include pH, NPK, texture (from Week 2), and CEC considerations.
• Suggest how each soil could be amended to better support plant growth.
• To receive a list of engagement question related to this topic, please email your 1-page reflection to gulftexlawns@gmail.com. Once we receive you 1-page reflection we will email the questions.

Engagement Questions for Week 3: Soil Chemistry Basics

1. What pH range is optimal for most nutrients to be available?
2. Why would sandy soils need fertilization more often than clay soils?
3. What deficiency is common in alkaline soils?
4. If your soil test shows pH 5.0 and low calcium, what amendment would you recommend?
5. Why might organic matter improve sandy soils?