Soil Science
The living foundation beneath your feet — what soil is, how it works, how to improve it
Chapter 1: What Is Soil — Minerals, Organic Matter, Water, and Air
Why this matters: Soil is not dirt. Dirt is what you wash off your hands. Soil is a living, breathing ecosystem that took thousands of years to form and can be destroyed in a single season of poor management. Everything you grow depends on it. Understanding soil from the ground up — literally — is the foundation of all food production, native plant work, and land restoration.
1.1 The Myth of Dirt
COMMON MISCONCEPTION
From: 1.1 The Myth of Dirt
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig01.png
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1.2 The Four Components of Soil
Healthy soil is roughly:
IDEAL SOIL COMPOSITION by volume
From: 1.2 The Four Components of Soil
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig02.png
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1.3 The Mineral Component
Minerals make up the bulk of soil and come from the weathering of parent rock over thousands to millions of years.
ROCK → MINERAL PARTICLES over time
From: 1.3 The Mineral Component
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig03.png
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Where Texas Minerals Come From
TEXAS PARENT MATERIALS (what the soil came from)
From: 1.3 The Mineral Component
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1.4 The Organic Matter Component
Organic matter is the most important component of soil that you can actually build and improve. Despite being only 2-8% of soil by weight, it controls nearly everything important.
ORGANIC MATTER = everything that was once alive
From: 1.4 The Organic Matter Component
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig05.png
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Organic Matter in Texas Soils
ORGANIC MATTER LEVELS by region (approximate)
From: 1.4 The Organic Matter Component
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig06.png
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1.5 Water in Soil
Water in soil is not just sitting there — it exists in three forms, each with different implications for plant growth.
THREE FORMS OF SOIL WATER
From: 1.5 Water in Soil
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig07.png
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1.6 Air in Soil
Air in soil is essential for root respiration and microbial activity — a fact most gardeners underestimate.
SOIL AIR COMPOSITION vs ATMOSPHERE
From: 1.6 Air in Soil
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig08.png
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1.7 Soil as an Ecosystem
The living component of soil is as important as the physical and chemical components.
THE SOIL FOOD WEB
From: 1.7 Soil as an Ecosystem
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig09.png
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1.8 Soil Formation — How Long It Takes
SOIL FORMATION RATES
From: 1.8 Soil Formation — How Long It Takes
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig10.png
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Chapter Summary
SOIL = minerals + organic matter + water + air + LIFE
From: Chapter Summary
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig11.png
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1. Soil is primarily composed of mineral particles
2. Organic matter typically makes up 50% of most healthy soils
3. Soil air and soil water together fill the pore spaces between particles
4. A teaspoon of healthy soil contains more organisms than there are people on Earth
5. Topsoil forms at a rate of roughly 1 inch per 100-500 years
6. Subsoil is typically richer in organic matter than topsoil
7. The O horizon consists of decomposed and decomposing organic material
8. The A horizon is the topsoil layer with the most biological activity
9. The B horizon (subsoil) accumulates minerals leached from above
10. The C horizon is the parent material — the rock being slowly broken down
11. All soils worldwide have the same number of horizons
12. Soil texture refers to the proportions of sand, silt, and clay
13. Soil structure refers to how particles clump together
14. Erosion can remove topsoil faster than it is formed
15. Plants get most of their mass from the soil
16. Earthworms improve soil structure and fertility
17. Soil color indicates organic matter content and drainage conditions
18. Sandy soils have lower water-holding capacity than clay soils
19. Compaction reduces soil pore space and biological activity
20. All soils in Texas are similar in composition and fertility
1. What are the five main components of soil?
2. Why is soil considered a living system?
3. Describe the major soil horizons from surface to bedrock.
4. How long does it take for 1 inch of topsoil to form?
5. Why do plants derive most of their mass from air rather than soil?
6. What is the difference between soil texture and soil structure?
7. How does organic matter improve soil quality?
8. What causes soil compaction and what are its effects?
9. Why is topsoil loss a serious problem for agriculture?
10. What does soil color tell you about soil conditions?
1. The five components of soil are minerals, water, air, , and living organisms.
2. The surface layer of soil with the most organic matter and biological activity is the horizon.
3. The subsoil layer where minerals accumulate from above is the horizon.
4. The unweathered parent rock at the base of the soil profile is the horizon.
5. The thin layer of decomposing organic material on the soil surface is the horizon.
6. Most of a plant's dry weight comes from carbon fixed from atmospheric .
7. Soil refers to the proportions of sand, silt, and clay particles.
8. Dark soil color generally indicates higher content.
9. Compressed soil with reduced pore space and poor drainage is described as .
10. The rate of topsoil formation is approximately 1 inch per years.
Complete in the field. Check each off as you go.
1. Soil Profile Excavation: Dig a hole 2-3 feet deep on your property. Photograph and sketch the different layers. Note color changes, texture changes, and root depth. Identify the horizons.
2. Soil Organism Count: Collect a cup of topsoil from a healthy spot. Spread on white paper. Count earthworms, beetles, and other visible organisms in 5 minutes. Repeat from a compacted bare area. Compare.
3. Organic Matter Comparison: Compare topsoil from: (1) under an established native plant, (2) cultivated garden, (3) bare compacted area. Note color, smell, and feel of each. Which is darkest?
4. Compaction Test: Use a wire flag or metal rod. Push it into the soil with your thumb and body weight. Record the depth reached before significant resistance. Compare different areas of your property.
5. Water Infiltration Test: Pour 1 cup of water into a ring pressed into the soil. Time how long it takes to absorb. Repeat in 5 different spots with different histories. Which absorbs fastest?
Practice Exercises
- You squeeze a moist soil sample and it forms a ribbon over 2 inches long before breaking. What does this tell you about the soil texture?
- After a heavy rain, your garden beds stay wet for 4-5 days. Root rot is becoming a problem. What is the likely physical cause and what does the soil need?
- Why does applying chemical fertilizer year after year eventually require MORE fertilizer to get the same plant growth?
- A Texas Hill Country ranch has thin rocky soil over limestone. What parent material created this and approximately how deep would you expect the topsoil to be?
- You add 2 tons of compost to an acre of garden. After two years, how many additional pounds of stable humus have you approximately added?
- What is caliche, where does it come from, and what practical problem does it create for growing trees?
Next Chapter → Soil Texture — Sand, Silt, and Clay
Connections to Other Topics
→ C03 Ch02: Soil Texture — sand/silt/clay in detail
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c03-soil-science/ch01/c03-soil-science_ch01_what_is_soil_fig12.png
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