Part of Soil Science
Composting is the managed aerobic decomposition of organic materials into a stable, nutrient-rich soil amendment. It is humanity’s most ancient and most universally applicable soil improvement technology. With nothing more than organic waste — kitchen scraps, plant material, manure, fallen leaves — and the right management, you can convert waste streams into a material that transforms poor soils into productive ones. In any rebuilding scenario, composting is one of the first systems to establish.
What Composting Accomplishes
Composting converts raw organic materials that would decompose slowly (and possibly cause problems in the field) into a stable, mature product that:
- Improves soil structure in both sandy and clay soils
- Provides slow-release nitrogen, phosphorus, sulfur, and micronutrients
- Increases soil cation exchange capacity (nutrient-holding ability)
- Introduces diverse beneficial soil biology
- Suppresses some soil-borne plant pathogens
- Generates heat during active decomposition that kills weed seeds and pathogens
A mature compost applied at 5–10 tonnes/hectare per year can supply enough nutrition for moderate yields of most vegetable crops without any other fertilization. For grain crops in fertile soils, compost can supply sufficient nutrition as the primary fertility input.
The Biology of Decomposition
Composting is a biological process, not a chemical one. Microorganisms — primarily bacteria and fungi — are the workers. They consume organic material, respiring it as CO2 and water (aerobic decomposition) while building their own biomass. When they die, their nutrients become plant-available.
Temperature Phases
Mesophilic phase (10–40°C): The initial phase. Easily decomposable materials (sugars, starches, proteins) are consumed by mesophilic bacteria (those that function best at moderate temperatures). Pile heats up as microbial respiration generates heat.
Thermophilic phase (40–70°C): If the pile is large enough and materials are properly mixed, heat production exceeds heat loss and the pile reaches thermophilic temperatures. Thermophilic bacteria (different species that prefer heat) take over. This is the critical phase:
- Weed seeds are killed above 50°C (most seeds destroyed after 30–60 minutes at 55°C)
- Human and animal pathogens are killed above 55°C (Salmonella, E. coli destroyed within hours at 55°C)
- Fly pupae and other insects are killed
- Many plant pathogens are destroyed
The thermophilic phase typically lasts 3–7 days in a well-constructed pile, then temperatures drop as the most easily decomposable materials are consumed.
Cooling and maturing (mesophilic): After thermophilic heating, the pile cools. A new community of mesophilic bacteria, fungi, actinomycetes, protozoa, and eventually earthworms colonize the maturing compost. Fungi — white, thread-like hyphae visible throughout the pile — break down cellulose and lignin that bacteria cannot. Actinomycetes (grey-white powdery patches) produce earthy smell compounds and break down difficult materials. This maturation phase takes 2–6 weeks in active management, longer in passive systems.
Requirements for Aerobic Composting
Four factors control composting rate and quality:
1. Oxygen (aeration): Aerobic decomposition requires continuous oxygen supply. Without oxygen, anaerobic bacteria take over — producing methane, hydrogen sulfide (rotten egg odor), and toxic organic acids, while decomposing much more slowly. Maintain oxygen by:
- Turning the pile to introduce fresh air
- Building the pile with structural materials (woody stalks, wood chips) that create air pockets
- Not overly compacting the pile
2. Moisture: Microbes require moisture. Target: 40–60% water content by weight, or the moisture content of a wrung-out sponge — moist throughout but not dripping. Test: squeeze a handful of compost. Properly moist compost releases 1–2 drops of water; if it streams freely, too wet; if nothing comes out, too dry.
Too wet: aerobic conditions collapse, pile goes anaerobic, smells bad, slows dramatically. Too dry: microbial activity stalls, decomposition stops, pile stays cold.
3. Carbon-to-Nitrogen ratio (C:N): The ratio of carbon-rich (brown) to nitrogen-rich (green) materials determines decomposition speed and finished compost quality. Target C:N ratio: 25–30:1. See the dedicated green-brown ratio article for details.
4. Particle size: Smaller particles have more surface area for microbial attack and decompose faster. Shredding or chopping inputs to 2–5 cm pieces speeds composting. However, too fine (sawdust, flour) particles can compact and block airflow. Large woody pieces (branches greater than 2 cm) decompose too slowly and should be chipped first.
Compost Inputs: What to Use
Green (Nitrogen-Rich) Inputs
| Material | C:N Ratio | Notes |
|---|---|---|
| Fresh grass clippings | 15–20:1 | Excellent; can go anaerobic in thick layers |
| Kitchen vegetable scraps | 12–20:1 | Good; attracts pests if not buried |
| Fresh animal manure (chicken) | 7–10:1 | Excellent nitrogen; high pathogen risk if not heated |
| Fresh animal manure (horse, cow) | 15–25:1 | Good; horse manure may contain weed seeds |
| Fresh legume plants | 15–20:1 | Very good |
| Food waste (fruit, vegetables) | 10–20:1 | Good; bury under browns |
| Coffee grounds | 20:1 | Good nitrogen source |
| Fresh seaweed | 10–20:1 | Excellent; wash salt if from ocean |
| Human urine | ~0.8:1 | Extremely nitrogen-rich; use diluted as activator |
Brown (Carbon-Rich) Inputs
| Material | C:N Ratio | Notes |
|---|---|---|
| Dry straw | 50–100:1 | Very good structural material |
| Dry autumn leaves | 40–80:1 | Excellent; shred to prevent matting |
| Cardboard (corrugated) | 150–500:1 | Good structural filler; remove tape/staples |
| Wood chips | 200–700:1 | Best for structure; slowest to decompose |
| Sawdust | 200–700:1 | Very high C; use sparingly or add extra N |
| Paper (shredded) | 100–200:1 | Good; glossy paper less suitable |
| Corn stalks | 60–80:1 | Good; chop for faster decomposition |
| Woody prunings | 100–400:1 | Shred finely |
What Not to Compost
Avoid:
- Meat, fish, dairy: attracts rodents; creates odors; can harbor pathogens even in hot compost
- Diseased plant material (unless you can guarantee thermophilic temperatures throughout)
- Weeds with persistent rhizomes (couch grass, bindweed) — unless pile heats above 60°C
- Human feces or cat/dog waste — requires dedicated high-temperature system (“humanure”) to safely process; not for beginners
- Treated or painted wood — may contain toxins
- Plants treated with persistent herbicides — some herbicides survive composting and damage crops
With caution:
- Citrus peels: fine in small quantities; break down slowly and can deter worms
- Onion skins: fine in moderation
- Cooked food: increases pest attraction; bury deeply or use enclosed bins
Active vs. Passive Composting
Active (hot) composting: The pile is built all at once with correct C:N ratio and moisture, and turned every 3–5 days. This maintains thermophilic temperatures throughout and produces finished compost in 30–90 days. Requires more labor and planning but produces higher-quality compost faster, and reliably kills weed seeds and pathogens.
Passive (cold) composting: Materials are added continuously over months. No regular turning; pile heats only at its active core. Takes 6–18 months. Weed seeds and pathogens may survive in outer zones. Produces good but slower compost. Lower labor input. Appropriate when materials accumulate gradually.
Both approaches are legitimate depending on resources and goals. In a survival or rebuilding scenario, passive composting can be started immediately with whatever materials are available, while an active system requires accumulating enough material to build a proper pile at once.
Active Composting Step by Step
Step 1: Choose Location
- Accessible but not in a high-traffic area
- Partial shade reduces drying in hot climates; full sun acceptable in cool, wet climates
- Level ground or slight slope (not a depression where water pools)
- Near water source for moistening
- Downwind of living areas during active phase (some smell is normal)
Step 2: Gather Materials
Before building, assemble enough material to construct a complete pile in one session. Minimum useful pile size: 1 cubic meter (1m × 1m × 1m). Smaller piles lose heat too rapidly to reach thermophilic temperatures.
Calculate roughly equal volumes of green and brown materials (by volume, not weight — greens are denser). Have a water source available.
Step 3: Build in Layers
Layer greens and browns alternately, 10–15 cm thick per layer. Sprinkle water on each layer as you build if materials are dry. A pile that is dry at the center is the most common cause of failure.
Many composters add a thin layer of finished compost or garden soil (1–2 cm) between layers to introduce microbial inoculants. This is helpful but not essential — microbes colonize from the environment within days.
Step 4: Monitor and Turn
Check pile temperature with a long-stem thermometer or by inserting your hand 30 cm into the pile. Active piles reach 50–65°C within 2–5 days. If no heat after 5 days:
- Too wet: add dry browns and turn
- Too dry: add water and turn
- Too much brown material: add green material
- Too small: add more material to reach minimum size
Turn pile when:
- Temperature drops below 45°C (indicates core is being depleted of oxygen and food)
- Typically every 3–5 days in active phase
Turn by moving the pile completely — outer cool material goes to the inside, hot core material goes to outside. After several turns (2–4 weeks), thermophilic phase is complete.
Step 5: Cure
After active turning phase, let the pile cure (mature) without turning for 2–4 weeks. This allows secondary decomposition, fungal colonization, and stabilization of nutrients. Earthworms will colonize the curing pile naturally if present in the area.
Step 6: Assess Maturity
Finished compost:
- Dark brown to black color, uniform texture
- Earthy smell (like forest floor after rain) — not ammonia, not sour, not rotten
- Cool to the touch even in center
- No recognizable plant material except possibly small wood pieces
- Crumbles easily, not slimy
- pH 6.5–7.5
Immature compost applied to soil suppresses germination, ties up nitrogen, and may contain pathogens. When in doubt, wait another 2–4 weeks.
Application Rates and Methods
| Crop Type | Application Rate | Method |
|---|---|---|
| Vegetables | 3–5 cm layer, incorporate | Broadcast and dig in before planting |
| Grain crops | 5–10 tonnes/ha | Broadcast and incorporate |
| Fruit trees | 5–10 cm ring mulch | Surface apply around drip line |
| New garden beds | 10–15 cm, incorporate | Deep mix into top 30 cm |
| Lawn establishment | 2–3 cm | Broadcast, lightly rake in |
Apply in spring before planting or in fall to allow nutrient integration. On sandy soils, incorporate immediately to prevent surface application from drying out.
Composting is among the most impactful single practices available to a rebuilding farmer. It converts waste into fertility, closes nutrient cycles, and produces the most versatile soil improvement tool available without industrial inputs.