Composting is the controlled aerobic decomposition of organic material by microorganisms. The end product — mature compost — is a stable, humus-rich amendment that improves soil structure, supplies nutrients slowly, and supports microbial biodiversity. On a garden scale, it converts kitchen and garden waste into a useful soil input at near-zero cost. On a farm scale, it transforms livestock manure and crop residues into a legally compliant organic fertiliser that satisfies EU Regulation 2018/848 requirements for organic certification.
The process is not complicated, but it has specific requirements. Meeting them consistently is what separates compost that reaches maturity in 8–12 weeks from material that sits inert for two years and remains raw.
The Four Variables That Control Decomposition Rate
Carbon-to-nitrogen ratio (C:N)
Soil bacteria require carbon as an energy source and nitrogen for protein synthesis. When the C:N ratio of the feedstock is too high (above 35:1), bacterial populations are nitrogen-limited and decomposition slows. When it is too low (below 15:1), excess nitrogen is lost as ammonia — producing the smell associated with poorly managed compost heaps — and bacterial activity is still inhibited.
The target range at the start of a composting pile is 25:1 to 30:1. Achieving this in practice means combining carbon-rich "brown" materials with nitrogen-rich "green" materials in roughly the right proportions:
- High-carbon materials (browns): straw (C:N ≈ 80:1), sawdust (C:N ≈ 500:1), cardboard (C:N ≈ 350:1), dry leaves (C:N ≈ 60:1), wood chips (C:N ≈ 400:1)
- High-nitrogen materials (greens): fresh grass clippings (C:N ≈ 15:1), food scraps (C:N ≈ 15:1), fresh cattle manure (C:N ≈ 20:1), chicken litter (C:N ≈ 10:1), legume crop residues (C:N ≈ 15:1)
A pile built from equal volumes of straw and fresh cattle manure reaches approximately 25:1 — workable without calculation. If the heap smells of ammonia, add browns. If it shows no heat generation after five days, add greens.
Moisture content
The optimal moisture content for aerobic composting is 50–60% by weight. At lower moisture levels, microbial activity drops sharply; above 65%, air is excluded from pore spaces and anaerobic conditions develop, producing methane and hydrogen sulphide rather than CO₂. The practical test: a handful of compost material squeezed firmly should release a few drops of water but not form a stream.
In Polish summer conditions, uncovered windrows lose moisture rapidly in July and August. Covering with breathable fabric (not impermeable plastic sheeting) prevents surface desiccation while allowing gas exchange. In winter, outdoor composting slows substantially below 5°C — the lower temperature limit for mesophilic bacterial activity — but does not stop entirely in the interior of a well-insulated heap.
Temperature
Active composting generates heat through microbial metabolism. A correctly built heap of sufficient volume (minimum 1 cubic metre) should reach 50–65°C in the core within three to five days. This temperature range selects for thermophilic bacteria that decompose material faster than mesophiles, and — critically — kills weed seeds and most plant pathogens including Fusarium and Pythium species that survive in raw manure.
EU organic certification requires that animal manure used as fertiliser either be composted to a minimum of 55°C for three days (with turning) or stored for a defined period. Monitoring with a long-stem compost thermometer — inserted to 30–40 cm depth — is the only reliable way to confirm the temperature has been reached across the heap, not just at the surface.
Turning the heap when temperature exceeds 65°C (above which even thermophiles are inhibited) reintroduces oxygen, redistributes cooler outer material to the hot centre, and restarts the heating cycle. Three to five turnings over 6–8 weeks typically produces mature compost.
Aeration
Oxygen supply is the rate-limiting factor in most garden and small farm composting systems. Passive aeration — natural convection through a heap with sufficient structural porosity — is adequate for slow cold composting. Active turning or forced aeration (perforated pipes within the heap connected to a low-power fan) accelerates the process substantially.
Wood chips or coarse straw added at 10–15% by volume create structural channels that maintain porosity as softer materials compact during decomposition. On farm-scale windrows, a turning machine or front-end loader turning every 7–10 days maintains aerobic conditions without additional infrastructure.
Garden Composting: What Works in Polish Conditions
Most Polish kitchen gardens generate a combination of vegetable peelings, coffee grounds, eggshells, grass clippings, and autumn leaves — a reasonably balanced feedstock by default, since cool-season Polish gardens produce relatively little high-carbon material in summer (when decomposition is fastest) and large quantities of fallen leaves in autumn (when decomposition slows).
Practical adaptations for domestic scale:
- Store dry autumn leaves in a separate bin or bags through summer. Add them gradually to active summer heaps to balance wet grass clippings, which on their own pack down and exclude air.
- Avoid composting diseased tomato or potato material if late blight (Phytophthora infestans) is active in the garden. Heap temperatures rarely reach 55°C consistently at garden scale, and blight spores survive cold composting.
- Eggshells do not decompose within a standard composting cycle — they remain as calcium carbonate fragments. They can be incorporated anyway; they improve drainage in heavy soils, but should not be expected to contribute plant-available calcium within one season.
- Cooked food, meat, fish, and dairy should not be composted in open bins — they attract rodents and produce anaerobic conditions before they can be covered. Bokashi fermentation (anaerobic acidification in sealed containers) is an alternative system for this waste stream.
Farm-Scale Composting: Manure and Crop Residues
Farm composting is regulated under Polish environmental law implementing the EU Nitrates Directive. Storage of uncomposted manure requires covered, sealed infrastructure to prevent run-off and ammonia loss. Composting — which reduces total nitrogen content as ammonia is released during the hot phase — paradoxically produces a more stable, lower-emission product for field application.
A three-bay concrete composting pad — each bay holding one turning cycle's volume — allows continuous throughput: fill bay 1, turn to bay 2 at week 3, turn to bay 3 at week 6, apply to fields from week 9. This system, common on 50–200 ha mixed farms in Warmia-Mazury and Podlaskie, processes cattle manure through three temperature cycles reliably and produces compost that meets the EU organic regulation definition of "processed manure".
Windrow composting on an outdoor pad is lower-cost but requires covering during heavy rainfall to prevent nutrient leaching. Polish data from IUNG trials show that uncovered windrows lose up to 30% of total nitrogen during autumn and winter months compared to covered pads.
Maturity Testing
Immature compost applied to growing crops can cause nitrogen immobilisation — soil bacteria consuming available nitrogen to continue breaking down the incompletely decomposed material — which temporarily reduces plant-available nitrogen and can depress yields. Mature compost does not cause this effect.
Three practical tests for compost maturity:
- Smell: Mature compost smells like woodland soil — earthy, slightly sweet, not sour or ammonia-like. Sour smell indicates anaerobic conditions; ammonia smell indicates incomplete decomposition with nitrogen loss.
- Temperature: A mature heap no longer heats above ambient temperature after watering and turning. Continued heating indicates residual active decomposition.
- Germination test: Fill two small pots with mature compost and two with potting soil (control). Sow 10 cress seeds in each. After 5 days, if germination in the compost is within 20% of the control, the compost is not phytotoxic. Lower germination rates suggest residual decomposition by-products (organic acids, ammonia) that require further maturation.
