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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Agriculture / Horticulture / Landscaping › Soils: Properties, Health, and Management for Horticulture and Landscaping › Lesson 8
AGR-HORT49-02 · Lesson 8 · 12 clock hours · lab

Soil Biology, Organic Matter, and Amendments

Objective. Students will describe the roles of bacteria, fungi, nematodes, and earthworms in nutrient cycling and will evaluate the quality of a compost sample using physical and chemical indicators.

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This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Soil Biology, Organic Matter, and Amendments

Healthy soil is not simply a mixture of minerals and water — it is a living system. Understanding the organisms that drive nutrient cycling allows you to make better amendment decisions, improve plant establishment, and maintain long-term soil health in both landscaping and food-production settings.

The Soil Food Web

The soil food web describes the community of organisms that live in and on soil and the feeding relationships among them. The key groups you must know are:

Each feeding relationship transfers energy and releases nutrients. When decomposers consume carbon-rich residues, they release nitrogen, phosphorus, and sulfur in plant-available mineral forms — a process called mineralization.

The Nitrogen Cycle: Key Steps

1. Decomposition / Mineralization — bacteria and fungi break down organic nitrogen (proteins, urea) into ammonium (NH₄⁺). Warmer temperatures and adequate moisture speed this step.

2. Nitrification — specialized bacteria convert ammonium first to nitrite (NO₂⁻) and then to nitrate (NO₃⁻). Nitrate is highly mobile in soil water. This step slows in cold, waterlogged, or very acidic conditions.

3. Plant uptake — roots absorb ammonium or nitrate directly.

4. Immobilization — when microbes consume high-carbon residues (high C:N ratio), they pull mineral nitrogen back into their bodies, temporarily reducing plant availability.

5. Denitrification — anaerobic bacteria convert nitrate to nitrogen gas (N₂) under waterlogged conditions, causing nitrogen loss from the soil.

Understanding these steps helps you time organic amendments and avoid nutrient lock-up.

Organic Matter and Its Importance

Soil organic matter (SOM) improves nearly every soil property: it increases water-holding capacity, supplies nutrients slowly over time, feeds soil organisms, and binds soil particles into stable aggregates. Even a small increase in SOM percentage can dramatically improve compacted or sandy soils.

Common organic amendments include:

Evaluating Compost Quality

Before applying any amendment, assess it against recognized quality indicators:

| Indicator | Finished Compost (Pass) |

|---|---|

| Color | Dark brown to black |

| Odor | Earthy, no ammonia or sulfur smell |

| Moisture | Moist but not dripping (feels like a wrung sponge) |

| Particle size | Fine and uniform; no large uncomposted chunks |

| pH | Generally 6.0–8.0 |

Ammonia odor indicates incomplete decomposition. A sulfur (rotten-egg) smell indicates anaerobic conditions. Both are signs that the material is not ready for direct plant contact.

Amending Compacted, Low-OM Clay Soils

Clay soils have small particles with high surface area, good nutrient-holding capacity, and poor drainage when compacted. The amendment strategy for a new landscape or garden bed typically involves:

1. Choosing the product — finished compost is the standard first choice.

2. Application rate — follow current university extension guidelines for your region. Rates are commonly expressed in inches of amendment tilled to a given depth. Always consult published, peer-reviewed extension recommendations rather than guessing.

3. Timing — incorporate amendments before planting; fall application allows weathering and partial breakdown before spring.

4. Incorporation method — till or spade to the depth of the root zone (typically 6–12 inches for landscape beds); avoid over-tilling, which destroys soil structure.

Safety in the Soil Lab

Key terms9

MineralizationThe microbial process that converts organic nitrogen compounds into inorganic ammonium (NH₄⁺) that plants can absorb.
NitrificationThe two-step bacterial conversion of ammonium to nitrite, then to nitrate — the most mobile form of soil nitrogen.
ImmobilizationThe temporary uptake of mineral nitrogen by soil microbes consuming high-carbon residues, reducing plant-available nitrogen.
DenitrificationThe anaerobic bacterial conversion of nitrate to nitrogen gas, causing nitrogen loss from soil; promoted by waterlogged conditions.
MycorrhizaeSymbiotic fungi that colonize plant roots and extend their reach for water and phosphorus in exchange for plant-produced sugars.
Soil Organic Matter (SOM)The fraction of soil composed of plant and animal residues in various stages of decomposition; improves water retention, nutrient supply, and soil structure.
C:N RatioThe ratio of carbon to nitrogen in an organic material; high C:N (e.g., straw) slows decomposition and can cause temporary nitrogen immobilization; low C:N (e.g., manure) speeds release.
BioturbationThe physical mixing of soil layers by soil-dwelling organisms such as earthworms, improving aeration and organic matter distribution.
Finished CompostFully decomposed organic material that is stable, dark, earthy-smelling, and safe for direct contact with plant roots.

Worked examples4

1. A compost sample has a pH of 7.2, a dark brown color, an earthy odor, and moist but not dripping texture. Particles are mostly fine with a few small chunks. Does it pass a basic quality check?
  1. Check pH: 7.2 falls within the acceptable range of 6.0–8.0. ✓
  2. Check color: dark brown indicates advanced decomposition. ✓
  3. Check odor: earthy smell, not ammonia or sulfur. ✓
  4. Check moisture: moist but not dripping — like a wrung sponge. ✓
  5. Check particle size: mostly fine; a few small chunks are borderline but acceptable for landscape use if screened before application.
Answer: The sample passes all five indicators. It is suitable for landscape bed application. For vegetable gardens, screening out remaining chunks would be recommended.
2. You are amending a 200 sq ft raised landscape bed with finished compost to a depth of 2 inches. How many cubic feet of compost do you need?
  1. Convert the amendment depth to feet: 2 inches ÷ 12 = 0.167 ft
  2. Multiply area by depth: 200 sq ft × 0.167 ft = 33.4 cubic feet
  3. Round up to the nearest practical unit: approximately 33–34 cubic feet of compost needed.
Answer: You need approximately 33–34 cubic feet (roughly 1.25 cubic yards) of finished compost for this bed.
3. A compost pile has a C:N ratio of 35:1. Will decomposition be fast or slow, and why? What amendment could you add to speed it up?
  1. An ideal C:N ratio for active composting is roughly 25:1 to 30:1. A ratio of 35:1 means the pile is carbon-heavy.
  2. Microbes need nitrogen to build cell mass; excess carbon means nitrogen is limiting, so microbes work slowly and may immobilize available nitrogen.
  3. To lower the C:N ratio, add a nitrogen-rich material such as fresh grass clippings, vegetable scraps, or blood meal and mix thoroughly.
  4. After adding the nitrogen source, monitor temperature and moisture to confirm decomposition has accelerated.
Answer: Decomposition will be slow because carbon is excessive relative to nitrogen. Adding a low-C:N nitrogen source (grass clippings, manure, etc.) will bring the ratio closer to the target range and accelerate microbial activity.
4. A pH meter reads 5.3 when placed in a compost sample slurry. The target range for finished compost is 6.0–8.0. By how many pH units is the sample outside the acceptable range, and what does a low pH suggest about the compost?
  1. Calculate the difference from the lower limit: 6.0 − 5.3 = 0.7 pH units below the acceptable minimum.
  2. A pH of 5.3 is acidic. In compost, acidic conditions often indicate active fermentation or incomplete decomposition — organic acids produced by anaerobic microbes have not yet been neutralized.
  3. This compost likely needs more time, turning to introduce oxygen, and possibly moisture adjustment before it is ready for use.
Answer: The sample is 0.7 pH units below the acceptable minimum. The low pH suggests incomplete or anaerobic decomposition; the compost is not finished and should not be applied directly to plants.
In class: Soil food web mapping: using a provided food web diagram, students trace the path of a nitrogen atom from organic matter through decomposer bacteria to plant uptake; annotate each step with the process name (mineralization, nitrification, etc.) and one factor that speeds or slows it. · Compost quality assessment: evaluate three compost/amendment samples (finished compost, raw manure, and a peat-based mix) for color, odor, moisture, particle size, and pH; record a pass/fail against industry quality criteria and discuss which is most appropriate for a new landscape bed vs. a vegetable garden. · Amendment trial design: given a scenario of a compacted, low-organic-matter clay soil, student groups propose an amendment plan specifying product type, application rate (from published extension guidelines), timing, and incorporation method; groups present findings to class for peer critique.
Materials: Soil food web diagram (USDA NRCS Soil Biology Primer or equivalent), compost and amendment samples, pH meter, moisture meter, odor and visual assessment rubric, extension guidelines for organic amendment rates, lab notebooks
Check: Nitrogen pathway diagram complete and accurate (at least 4 of 5 steps correct); compost assessment form complete with justification; amendment plan contains product, rate, timing, and method — rubric score 75% or higher to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Soil science and plant nutrition
1. Which soil organism is primarily responsible for converting ammonium (NH₄⁺) to nitrate (NO₃⁻) in the nitrogen cycle?
Soil management and amendments
2. A fresh manure sample smells strongly of ammonia during a lab assessment. What does this most likely indicate?
Plant science and soil biology
3. Mycorrhizal fungi benefit host plants primarily by:
Soil science and nutrient management
4. Denitrification causes nitrogen loss from the soil. Under which condition is it most likely to occur?
Soil management and amendments
5. A student evaluates three compost samples and finds one that is dark brown, has an earthy odor, a pH of 6.8, and moist texture with fine particles. Which conclusion is best supported?
Landscaping and soil management
6. When incorporating organic amendments into a compacted clay landscape bed, what is the generally recommended minimum incorporation depth for a typical landscape planting bed?
Soil science and nutrient management
7. A compost pile has a very high carbon-to-nitrogen ratio. Which of the following effects is most likely?
Safety and health in agricultural settings
8. Which personal protective equipment is most critical when handling raw manure samples in the soil lab?
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