Soil Science
The living foundation beneath your feet — what soil is, how it works, how to improve it
Chapter 4: Soil pH — What It Is and Why It Matters
Why this matters: pH controls what nutrients are available to your plants regardless of how much fertilizer you add. A plant with yellowing leaves in a nutrient-rich soil is often not nutrient-deficient — it just can't access the nutrients because pH is wrong. Understanding pH is the difference between treating symptoms and solving the actual problem.
4.1 The pH Scale
pH = measure of hydrogen ion (H⁺) concentration
From: 4.1 The pH Scale
Image file: ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig01.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig01.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig01.png" alt="pH = measure of hydrogen ion (H⁺) concentration"></figure>
4.2 Why pH Controls Nutrient Availability
NUTRIENT AVAILABILITY BY pH
From: 4.2 Why pH Controls Nutrient Availability
Image file: ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig02.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig02.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig02.png" alt="NUTRIENT AVAILABILITY BY pH"></figure>
4.3 Iron Chlorosis — Texas's Most Common Nutrient Problem
IRON CHLOROSIS in Texas
From: 4.3 Iron Chlorosis — Texas's Most Common Nutrient Problem
Image file: ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig03.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig03.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig03.png" alt="IRON CHLOROSIS in Texas"></figure>
4.4 Testing Soil pH
METHODS for testing pH
From: 4.4 Testing Soil pH
Image file: ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig04.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig04.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig04.png" alt="METHODS for testing pH"></figure>
4.5 Adjusting Soil pH
LOWERING pH (acidifying — for acid-loving plants)
From: 4.5 Adjusting Soil pH
Image file: ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig05.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig05.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig05.png" alt="LOWERING pH (acidifying — for acid-loving plants)"></figure>
Chapter Summary
pH SCALE: 0-14, 7 = neutral, log scale (each unit = 10x)
From: Chapter Summary
Image file: ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig06.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig06.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig06.png" alt="pH SCALE: 0-14, 7 = neutral, log scale (each unit = 10x)"></figure>
1. pH 7.0 is neutral
2. Values below 7 indicate acidic conditions
3. Values above 7 indicate alkaline (basic) conditions
4. Most plants grow best in slightly acidic soil (pH 6.0-6.8)
5. Texas soils are often alkaline (pH 7.5-8.5) due to limestone parent material
6. Lowering soil pH requires adding agricultural lime
7. Sulfur lowers soil pH
8. Iron becomes unavailable to plants in highly alkaline soils
9. Iron chlorosis (yellow leaves with green veins) often indicates alkaline soil
10. Blueberries require strongly acidic soil (pH 4.5-5.5)
11. Adding lime to soil raises pH
12. Phosphorus availability is highest at pH 6.5
13. Manganese toxicity can occur in very acidic soils
14. Azaleas and rhododendrons prefer alkaline soil
15. A pH test kit can be purchased for home use
16. Commercial soil labs can measure pH precisely
17. Adding sulfur to alkaline Texas soil immediately lowers pH permanently
18. Organic matter tends to buffer soil toward neutral pH
19. Calcium carbonate (caliche) in Texas soil causes high pH
20. Blueberries can be grown in Texas alkaline soil without amendment
1. What is soil pH and why does it matter for plant nutrition?
2. What is the ideal pH range for most vegetable crops?
3. Why are Texas soils often alkaline?
4. What is iron chlorosis and how does pH cause it?
5. How do you lower soil pH?
6. How do you raise soil pH?
7. What is caliche and what problems does it create?
8. Which nutrients become less available at high pH?
9. How do you test soil pH at home?
10. Name three acid-loving plants and describe how you would grow them in alkaline Texas soil.
1. The pH scale runs from to __________.
2. A soil pH of 7.0 is described as .
3. Most vegetables grow best at pH to __________.
4. Texas limestone soils are often (acidic/alkaline).
5. Adding agricultural lime (calcium carbonate) soil pH.
6. Adding elemental sulfur soil pH.
7. Yellow leaves with green veins on a plant in alkaline soil indicates deficiency.
8. The hard cemented layer of calcium carbonate common in Texas soils is called .
9. Phosphorus is most available to plants at pH approximately .
10. To grow acid-loving blueberries in Texas, you would the soil pH.
Complete in the field. Check each off as you go.
1. pH Testing: Purchase a pH test kit or strips. Test soil from 5 different spots on your property. Test: under live oak, in the vegetable garden, in a grass area, near caliche, and from a compost pile. Compare results.
2. Iron Chlorosis Identification: Walk your property and look for plants with yellow leaves and green veins (especially on young growth). This is iron chlorosis. Which plant types show it most?
3. Sulfur Application Test: Apply elemental sulfur at the recommended rate to a 4×4 ft plot. Test pH monthly for 6 months. How quickly does it change?
4. Caliche Location: Find a spot with shallow caliche. Dig a hole and note the depth to the white/gray cemented layer. Pour water — how long does it take to drain? What plants grow (or don't) above it?
5. Acid-Loving Plants in Texas: Research the requirements for growing blueberries in Texas. What pH is needed? What amendments are required? What varieties are recommended for Texas?
Practice Exercises
- A gardener adds compost, blood meal, and balanced fertilizer to a bed but plants still show yellow leaves with green veins. Soil test shows pH 8.2. What is the diagnosis and what should be done?
- Why is it impractical to grow blueberries in most Texas garden soils without special management?
- You want to lower your garden's pH from 7.5 to 6.5 in a clay loam soil. Approximately how much elemental sulfur per 100 square feet would you need, and how long would it take?
- A native Texas plant like Texas live oak is planted in pH 8.0 soil. Will it show iron chlorosis? Why or why not?
- What is the danger of using aluminum sulfate repeatedly to lower soil pH?
Next Chapter → Macronutrients — Nitrogen, Phosphorus, Potassium
Connections to Other Topics
→ C03 Ch05: Macronutrients — pH affects N, P, K availability
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig07.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig07.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch04/c03-soil-science_ch04_soil_ph_fig07.png" alt="→ C03 Ch05: Macronutrients — pH affects N, P, K availability"></figure>