Soil Science
The living foundation beneath your feet — what soil is, how it works, how to improve it
Chapter 3: Soil Structure — Aggregates, Pores, and Tilth
Why this matters: Two soils with identical texture can perform completely differently depending on their structure. Structure is what you build over years of good management — it determines drainage, aeration, root penetration, and workability. "Good tilth" is what every farmer and gardener is ultimately working toward.
3.1 What Is Soil Structure?
SOIL TEXTURE vs SOIL STRUCTURE
From: 3.1 What Is Soil Structure?
Image file: ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig01.png
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3.2 Aggregate Formation and Stability
HOW AGGREGATES FORM
From: 3.2 Aggregate Formation and Stability
Image file: ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig02.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig02.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig02.png" alt="HOW AGGREGATES FORM"></figure>
3.3 Tilth — What It Feels Like
GOOD TILTH
From: 3.3 Tilth — What It Feels Like
Image file: ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig03.png
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3.4 The Pore System in Detail
PORE SIZE CLASSES
From: 3.4 The Pore System in Detail
Image file: ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig04.png
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3.5 Compaction — The Enemy of Structure
COMPACTION: reduction of pore space
From: 3.5 Compaction — The Enemy of Structure
Image file: ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig05.png
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Chapter Summary
STRUCTURE = how particles are arranged into aggregates
From: Chapter Summary
Image file: ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig06.png
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1. Soil aggregates are clusters of soil particles bound together by organic matter and biology
2. Granular soil structure is ideal for most crops
3. Tillage improves soil structure over time
4. Earthworms contribute to soil aggregate formation
5. Fungi hyphae bind soil particles into aggregates
6. A soil aggregate test (slake test) reveals structural stability
7. Platy soil structure has flat horizontal layers and indicates compaction
8. Blocky soil structure is common in subsoil B horizons
9. Prismatic soil structure has vertical columns and is common in subsoil
10. Good soil structure has large interconnected pore spaces
11. Cover crops improve soil structure
12. Heavy machinery compaction destroys soil aggregates
13. No-till farming preserves soil structure better than conventional tillage
14. Sandy soils have better natural structure than clay soils
15. Glomalin is a protein produced by mycorrhizal fungi that helps bind soil aggregates
16. Soil with poor structure drains slowly and roots penetrate it with difficulty
17. Adding gypsum can improve structure in sodic clay soils
18. Once destroyed, soil structure cannot be rebuilt
19. A well-structured soil crumbles easily when moist
20. A structureless soil falls apart when dry or sticks in a solid mass when wet
1. What is a soil aggregate and what holds aggregates together?
2. Describe five types of soil structure and where each is typically found.
3. What is the slake test and what does it measure?
4. How does tillage affect soil structure over time?
5. What is glomalin and why is it important for soil structure?
6. How do earthworms contribute to soil structure?
7. What farming practices best preserve or rebuild soil structure?
8. Why is soil structure important for roots?
9. How does compaction affect soil structure?
10. Describe the relationship between soil structure and water movement.
1. Clusters of soil particles bound by organic glues and fungal hyphae are called soil .
2. The soil structure type with flat horizontal layers indicating compaction is called structure.
3. Vertical column-shaped aggregates common in subsoil are called structure.
4. The protein produced by mycorrhizal fungi that glues soil aggregates is called .
5. The test where a dry aggregate is dropped into water to measure its stability is the test.
6. Soil with crumbly, friable consistency and good structure is described as having good .
7. Repeated tillage breaks down aggregates and creates a hard layer below plow depth.
8. Cover crops improve structure by adding and feeding soil biology.
9. No-till farming preserves structure by leaving the network intact.
Complete in the field. Check each off as you go.
1. Slake Test: Collect dry soil aggregates from 3 locations (garden bed, pasture, bare compacted area). Drop into water. Observe which disintegrate quickly vs hold together. Rate structural stability 1-5.
2. Tillage Comparison: Compare soil from a regularly tilled garden to an untilled no-disturbed area. Feel the difference. Dig to 12 inches and observe: which has more earthworms? Better aggregation?
3. Cover Crop Experiment: In fall, plant winter rye or crimson clover in a section of bare soil. Terminate in spring. Compare soil structure to untreated area after one season.
4. Earthworm Count: Count earthworms in a 12"×12"×12" block of soil from 5 different areas of your property. Which area has the most? Why? What does earthworm count indicate?
5. Root Penetration Observation: Dig up a plant root system from a well-structured soil and from a compacted area. Note the difference in root branching, depth, and overall health.
Practice Exercises
- Two soils have identical texture (loam) but one has good aggregate structure and one does not. How would they differ in behavior after a heavy rain?
- What is glomalin, who produces it, and why is it so important for soil structure?
- You notice a strong earthy smell after the first rain on your garden. What does this indicate about your soil biology?
- Why does a single pass of a rototiller initially feel like it improves structure but actually makes structure worse over time?
Next Chapter → Soil pH — What It Is and Why It Matters
Connections to Other Topics
→ C03 Ch07: Soil Biology — organisms that build structure
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c03-soil-science/ch03/c03-soil-science_ch03_soil_structure_fig07.png
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