Soil Science
The living foundation beneath your feet — what soil is, how it works, how to improve it
Chapter 7: Soil Biology — Bacteria, Fungi, and the Soil Food Web
Why this matters: The living organisms in soil are not just passengers — they ARE the fertility system. They cycle nutrients, build structure, suppress disease, and protect plants. Understanding them changes everything about how you manage soil. You stop trying to feed the plant and start trying to feed the community that feeds the plant.
7.1 The Scale of Soil Life
IN ONE TEASPOON OF HEALTHY SOIL:
BACTERIA: 100 million – 1 billion cells
FUNGI: several yards of hyphae
PROTOZOA: several thousand
NEMATODES: dozens to hundreds
MICROARTHROPODS: 5-10
Total biological weight in one acre of topsoil:
1,000 – 10,000 lbs of living organisms
(equivalent to one to ten full-grown cows)
This is the community you are managing
when you manage soil
COMPARED TO DEGRADED SOIL:
Intensively tilled, chemical-treated soil:
Bacteria: 1-10 million (10-100x fewer)
Fungi: minimal or absent
Protozoa: few
Nematodes: mostly plant-parasitic species
Earthworms: rare or absent
The result is a soil that must be fed externally
because its own cycling machinery is gone7.2 Bacteria — The Most Numerous
BACTERIA in soil
From: 7.2 Bacteria — The Most Numerous
Image file: ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig01.png
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7.3 Fungi — The Structural Engineers
FUNGI in soil
From: 7.3 Fungi — The Structural Engineers
Image file: ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig02.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig02.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig02.png" alt="FUNGI in soil"></figure>
7.4 The Larger Organisms
PROTOZOA (single-celled, larger than bacteria)
From: 7.4 The Larger Organisms
Image file: ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig03.png
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7.5 Building Soil Biology
THE BIOLOGY BUILDING PROGRAM
From: 7.5 Building Soil Biology
Image file: ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig04.png
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1. Bacteria are the most numerous organisms in healthy soil.
2. Fungi can access nutrients in forms unavailable to bacteria.
3. Protozoa eat bacteria, releasing nutrients into soil solution.
4. Nematodes are always harmful pests in the soil food web.
5. Earthworms improve soil structure and nutrient availability.
6. A teaspoon of healthy topsoil contains up to 1 billion bacteria.
7. The rhizosphere is the zone of soil directly around roots.
8. Decomposers break down organic matter and release nutrients.
9. Mycorrhizal fungi live inside and outside root cells.
10. The soil food web refers to the feeding relationships among soil organisms.
11. Tillage disrupts soil biology significantly.
12. Biochar has been shown to support soil biology.
13. Bacteria fix nitrogen in legume root nodules.
14. Cover crops feed soil biology over winter.
15. Chemical pesticides always improve soil biology.
16. Earthworm casts are more nutrient-rich than surrounding soil.
17. Nematodes that feed on plant roots are always beneficial.
18. Fungal-dominated soils tend to support forests, while bacterial-dominated support grasslands.
19. Adding compost inoculates soil with beneficial microorganisms.
20. Soil biology can recover after disturbance if organic matter is restored.
1. What is the soil food web?
2. Name five types of soil organisms and their roles.
3. What is the rhizosphere and why is it important?
4. How do bacteria contribute to nutrient cycling?
5. How do predator-prey relationships among soil organisms release nutrients?
6. Why does tillage damage soil biology?
7. What is the difference between bacterial-dominated and fungal-dominated soils?
8. How do cover crops support soil biology?
9. What farming practices build soil biology?
10. How do earthworms benefit soil?
1. The most numerous organisms in healthy soil are .
2. The zone of soil immediately surrounding roots is called the .
3. Nitrogen-fixing bacteria that live freely in soil are in the genus .
4. Bacteria that convert ammonium to nitrate are called bacteria.
5. Earthworm excretions that are highly nutrient-rich and well-structured are called .
6. Fungi that colonize plant roots for mutual benefit are fungi.
7. The process where protozoa eat bacteria and release their nutrients is called .
8. Adding is the most direct way to inoculate soil with beneficial organisms.
9. Tillage disrupts soil biology by destroying networks.
Complete in the field. Check each off as you go.
1. Soil Biology Observation: Collect a handful of healthy garden soil and a handful of compacted bare soil. Compare: smell (earthy petrichor = geosmin from Streptomyces), color, feel, visible organisms. Which seems more biologically active?
2. Earthworm Count: Count earthworms in a 1-foot cube of soil in 5 different locations. Which areas have most? Map results and identify why.
3. Microscope Observation: Prepare a thin smear of soil diluted in water on a glass slide. Under 100x magnification, look for moving organisms: bacteria (tiny rods/spheres), protozoa (larger amoeba-like), fungal threads. Sketch what you see.
4. Compost Tea: Make a simple compost tea (steep 1 cup compost in 5 gallons water with aeration for 24 hours). Observe the tea color and smell. Apply to a garden bed and to a control bed. Monitor plant growth.
5. Building Biology: Plant a cover crop mix. After termination, test: earthworm count, aggregate stability, and organic matter before and 12 months after. Record changes.
Chapter Summary
SOIL BIOLOGY = the actual fertility engine
From: Chapter Summary
Image file: ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig05.png
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Next Chapter → Mycorrhizal Fungi — The Underground Network
Connections to Other Topics
→ C03 Ch08: Mycorrhizal Fungi — specialized fungal relationships
From: Connections to Other Topics
Image file: ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig06.png
Save image as ../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig06.png in this folder, then replace this block with:<figure><img src="../../../images/s01-foundation/c03-soil-science/ch07/c03-soil-science_ch07_soil_biology_fig06.png" alt="→ C03 Ch08: Mycorrhizal Fungi — specialized fungal relationships"></figure>