Manure Cycling
Part of Crop Rotation
Animal manure is the most abundant fertility resource available to pre-industrial and subsistence farming systems. Managed correctly, it replaces nitrogen, phosphorus, and potassium removed by harvested crops, improves soil structure, and feeds soil biology. Managed poorly, it runs off and pollutes water, loses most of its nitrogen to the atmosphere, and can introduce pathogens. Systematic collection, composting, and applied rotation of manure closes the fertility loop between animals and crops.
Manure Nutrients by Animal Type
All animal manures contain nitrogen (N), phosphorus (P), and potassium (K), but in very different ratios and concentrations. Understanding these differences determines which crops and soils benefit most from each type.
Fresh manure approximate nutrient content (kg per tonne):
| Manure Type | N (kg/t) | P (kg/t) | K (kg/t) | Water % | Notes |
|---|---|---|---|---|---|
| Dairy cattle | 5–6 | 1–2 | 5–6 | 85–90 | High K; good for grass and brassicas |
| Beef cattle | 6–7 | 2–3 | 5–6 | 80–85 | Drier than dairy; handles better |
| Horse | 5–7 | 1.5–2.5 | 5–7 | 70–80 | High wood shaving content; slow to break down |
| Pig | 5–7 | 3–5 | 3–5 | 85–90 | High P; good for root crops |
| Sheep/goat | 8–12 | 3–4 | 7–10 | 60–75 | Most concentrated; easiest to handle |
| Poultry (broiler) | 20–30 | 10–15 | 12–16 | 50–60 | Very high N; must be composted before use |
| Rabbit | 15–25 | 8–12 | 5–8 | 50–60 | Can be applied fresh (cool manure) without burning |
Fresh poultry manure applied directly to growing crops causes nitrogen burn — too concentrated. Always compost poultry manure for at least 6–8 weeks, or apply in autumn for spring crops.
Collection Systems
From Housed Animals
The most efficient system captures all dung and urine together:
- Deep litter system: Add dry bedding (straw, wood chips) on top of accumulated manure. Animals live on an ever-deepening bed. Turn or add bedding monthly. The bottom layers compost in place. Clean out every 6–12 months. The resulting material is partially composted, easier to handle, and has lower nitrogen volatilization loss than raw manure.
- Regular collection: Scrape yards and stalls daily, collecting into a manure pile or bin. Covered collection reduces rain dilution and nitrogen loss.
From Grazing Animals
Animals that graze open fields deposit manure spread across large areas — difficult to collect but self-applying. Supplement with:
- Overnight housing in a yard or shed where dung can be collected
- Night paddocks where animals are concentrated each evening — collect from these concentrated areas regularly
Approximate annual manure production per animal:
| Animal | Fresh Manure per Year |
|---|---|
| Dairy cow | 20–25 tonnes |
| Beef animal | 10–15 tonnes |
| Horse | 10–12 tonnes |
| Pig (sow + litter) | 2–4 tonnes |
| Sheep | 0.5–1 tonne |
| 100 laying hens | 6–8 tonnes |
| 100 meat rabbits | 2–3 tonnes |
Composting Manure
Raw manure, particularly from pigs and poultry, contains weed seeds, pathogens (Salmonella, E. coli, Cryptosporidium), and excessively high ammonia concentrations that can burn plant roots. Composting eliminates these problems.
Hot Composting Method
Building a thermophilic (hot) compost pile from manure and bedding:
- Pile size: Minimum 1 m × 1 m × 1 m. Smaller piles do not retain enough heat.
- Ratio: Mix 1 part manure (nitrogen, wet) with 2–3 parts dry bedding or straw (carbon, dry). Target C:N ratio of 25:1–30:1.
- Moisture: Squeeze a handful — it should feel like a wrung-out sponge. If it drips freely, add dry material. If it crumbles, add water.
- Aeration: Turn the pile every 3–5 days for the first 2 weeks to maintain aerobic conditions.
- Temperature: A well-built pile will reach 55–70°C within 3–5 days. Maintain above 55°C for a minimum of 3 days to kill weed seeds and pathogens. Use a metal probe thermometer or stick your hand into the pile center — if you cannot hold it there for more than 2–3 seconds, the pile is hot enough.
Composting timeline:
| Week | Temperature | Activity |
|---|---|---|
| 1–2 | 55–70°C | Active microbial decomposition; frequent turning required |
| 3–4 | 45–55°C | Slowing; less frequent turning |
| 5–8 | 30–45°C | Stabilizing; worms begin colonizing if not too hot |
| 8–12 | Near ambient | Mature compost; earthy smell; no recognizable manure |
Passive Composting
If hot composting labor is unavailable, pile manure and bedding in a sheltered heap and leave for 6–12 months. Turns will not happen; the pile composted passively and more slowly. Pathogens are killed by time rather than heat; weed seeds may survive in the cool outer layers. Apply this compost to established perennial crops or incorporate in autumn for spring planting rather than using around food crops directly.
Application Rates
Over-application wastes manure, pollutes water, and causes nitrogen burn. Under-application leaves soil deficient. Target rates depend on crop need and soil status.
General application guide (composted or well-rotted farmyard manure):
| Crop Type | Application Rate (tonnes/ha) | Application Timing |
|---|---|---|
| Heavy feeders (brassicas, maize, squash) | 20–40 | Autumn or 4 weeks before planting |
| Moderate feeders (potatoes, leeks, celery) | 15–25 | Autumn or early spring |
| Light feeders (roots, beans, peas) | 0–10 | Autumn only (fresh growth not desired for roots) |
| Permanent pasture | 15–30 | Any time except frozen ground |
| Orchards (established) | 10–20 | Autumn, around drip line |
Never apply fresh manure to root vegetables (carrots, parsnips, beetroot) — it promotes excessive forking and can make roots inedible. Always apply in autumn for spring-planted roots, allowing 6+ months between manure application and harvest.
Simplified Calculation for Small Holdings
For a kitchen garden of 100 m² (0.01 ha):
- Heavy feeder beds: 40 t/ha × 0.01 ha = 400 kg = roughly 4 wheelbarrow loads of compost
- Light feeder beds: 10 t/ha × 0.01 ha = 100 kg = roughly 1 wheelbarrow load
Apply compost to the top 15–20 cm of soil and incorporate by digging or heavy rainfall will do the work over winter.
Storage Between Collection and Application
Manure loses nitrogen rapidly if left exposed:
- Covered storage: A simple lean-to roof over the manure heap prevents rain from leaching nutrients and reduces ammonia volatilization from rain splash.
- Compact piles: Loose, airy piles lose nitrogen fastest. Compact the pile after each addition to reduce air space and slow volatilization.
- Timing: Apply manure when plants are actively growing to capture released nutrients. Manure applied to bare soil in winter loses 20–40% of its nitrogen through leaching before spring crops can use it.
Nitrogen retention by storage method:
| Storage Method | N Retained After 3 Months |
|---|---|
| Hot compost (turned) | 50–60% |
| Passive heap (covered) | 40–50% |
| Passive heap (uncovered, rain-exposed) | 25–35% |
| Slurry (liquid manure) in covered tank | 70–80% |
| Spreading immediately after collection | 90%+ (if incorporated within 24 h) |
The fastest way to conserve nitrogen is to incorporate fresh manure immediately by shallow digging or discing. Within 24 hours of application, 10–30% of ammonia nitrogen can volatilize from unincorporated manure on a warm day. Incorporation traps it in the soil.
Rotation Integration
In a crop rotation, apply manure strategically:
- Heavy-feeder position (brassicas, maize, squash): Apply before this crop — highest nutrient demand.
- After legume year: Little or no manure needed — the legume has already fixed nitrogen.
- Before root crop year: Apply the previous autumn if at all — roots grown in freshly manured ground fork and become inedible.
| Rotation Position | Manure Application | Notes |
|---|---|---|
| Before brassicas | Heavy (25–40 t/ha) | Best use of concentrated manure |
| Before potatoes | Moderate (15–25 t/ha) | Improves yield significantly |
| Before cereals | Light to moderate (10–20 t/ha) | Autumn-applied |
| Before legumes | None | Legumes fix their own nitrogen; manure promotes excessive leaf at cost of yield |
| Before roots (carrots) | None (autumn previous year) | Fresh manure causes forking |
Manure Cycling Summary
Collect all animal manure systematically — deep litter housing is the most efficient capture system. Compost using hot methods (55–70°C for 3+ days) to kill pathogens and weed seeds before applying to food crops. Apply at 15–40 tonnes per hectare depending on crop type: heaviest before brassicas and maize, lightest or not at all before legumes and carrots. Store under cover to retain nitrogen. Apply immediately before incorporation when possible. In a crop rotation, always apply manure to the heavy-feeder position, never before root crops or legumes.