Home Composting for Beginners: Hot vs Cold, C:N Ratios, and Troubleshooting

All composting is microbial decomposition of organic matter, but the temperature at which this decomposition occurs has profound effects on the speed, quality, and safety of the finished product. Cold composting is the default mode of most garden heaps: material is added gradually as it becomes available, the heap is rarely turned, and decomposition proceeds slowly at ambient temperatures — typically 10–20°C in a British garden. A cold heap can take 12–18 months to produce usable compost and will not reach temperatures sufficient to kill weed seeds (most weed seeds survive below 55°C) or pathogens. The advantages are minimal effort and the ability to add material at any time. Hot composting is an actively managed process in which a sufficiently large volume of correctly balanced material is assembled, moistened, and turned regularly to maintain the aerobic conditions in which thermophilic (heat-loving) bacteria thrive. When conditions are correct, the centre of a hot heap reaches 55–70°C within 48–72 hours of turning, held at these temperatures for long enough to kill weed seeds and most pathogens. A well-managed hot heap can produce finished compost in as little as 6–8 weeks. The minimum viable volume for hot composting is approximately 1 cubic metre (1 m × 1 m × 1 m) — below this, heat dissipates too rapidly from the surface for the centre to reach and maintain temperatures. This volume requirement means hot composting is more practical with a dedicated bay system or a large tumbler composter than in a standard plastic dalek-style bin.

The carbon-to-nitrogen ratio (C:N ratio) is the fundamental parameter governing composting speed and quality. Soil microorganisms — primarily bacteria and fungi — require carbon as an energy source and nitrogen for building proteins and cell structures. The optimal C:N ratio for active decomposition is approximately 25–30:1 (25–30 parts carbon to 1 part nitrogen by weight). A ratio higher than this (too much carbon relative to nitrogen) produces a slow, cold heap in which nitrogen-limited microorganisms cannot sustain active decomposition. The material dries out, becomes pale and woody, and may sit for months with negligible progress. A ratio lower than this (too much nitrogen relative to carbon) produces excess ammonia as microorganisms process more nitrogen than they can use — this is the cause of the characteristic sulphurous smell of nitrogen-rich heaps and represents a genuine loss of valuable nutrients into the atmosphere. In practice, composters rarely measure the C:N ratio directly — instead, they work with the conceptual shorthand of 'greens' (nitrogen-rich materials) and 'browns' (carbon-rich materials). A roughly equal mix by volume of greens and browns approximates the target C:N ratio for most materials, though the specific ratios of materials vary considerably: wood chip has a C:N ratio of approximately 400:1 and needs a very high proportion of nitrogen-rich material to balance; food scraps range from 15:1 (meat, fish) to 25:1 (vegetable peelings); freshly cut grass is typically 15–20:1.

Greens are nitrogen-rich materials that provide the protein and nutrient content that drives active microbial decomposition. They are typically moist and break down relatively quickly. Examples include: fresh grass clippings, vegetable and fruit peelings, fresh plant trimmings, coffee grounds (despite their colour, these are nitrogen-rich), tea leaves, fresh seaweed, chicken manure, and most food scraps. Browns are carbon-rich, typically dry or woody materials that provide the structural matrix of the heap, absorb excess moisture, and supply the carbon energy source for the microbial population. Examples include: cardboard and paper (torn or shredded), autumn leaves, straw, wood chip, dry plant stems and stalks, sawdust from untreated wood, egg boxes, and paper bags. The practical rule is to add roughly equal volumes of greens and browns, adding a layer of browns each time a significant quantity of nitrogen-rich material goes in. This rule of thumb prevents two of the most common failure modes: a slimy, smelly heap (excess greens, insufficient browns) and a dry, inactive heap (excess browns, insufficient greens).

Understanding what not to compost is as important as knowing what to add. Meat and fish should not be composted in a standard open or cold heap — they decompose anaerobically, producing extremely unpleasant odours, and attract rats and other vermin. They can be composted in a sealed, rat-proof bin using bokashi pre-treatment (see below) or in a purpose-built digestor system. Dairy products have the same problems as meat and fish: slow anaerobic decomposition, odour, and pest attraction. Diseased plant material should not be composted in a cold heap because the pathogen will survive at low composting temperatures and be reintroduced to the garden when the finished compost is applied. Blight-affected tomato or potato haulm is the most important example in UK gardens — this must be binned or burned, not composted, unless using a hot composting system that reaches and sustains 55°C+ for several days. Perennial weed roots — bindweed, couch grass, creeping thistle — should not be composted in a cold heap because they often survive and regenerate from fragments. Cooking them in a bucket of water for a few weeks until they are fully dead before adding to the heap, or leaving them in the sun in a sealed black bag for a full summer, renders them safe. Dog and cat faeces should not be composted in standard domestic systems due to the risk of pathogen (particularly Toxocara) transmission.

Turning the compost heap introduces oxygen, which is the essential requirement for aerobic (hot) decomposition. Aerobic bacteria decompose organic matter approximately 40–50 times faster than anaerobic bacteria and produce heat, carbon dioxide, and water rather than the methane, hydrogen sulphide, and organic acids that characterise anaerobic decomposition. For hot composting, turning every 3–7 days during the active phase maintains oxygen levels above the threshold for thermophilic activity and ensures that all material in the heap passes through the hot centre, not just the material already positioned there. For cold composting, turning every 4–6 weeks is sufficient and simply re-introduces oxygen and redistributes moisture. Moisture management is equally critical. The optimal moisture content for composting is approximately 50–60% by weight — the material should feel like a wrung-out sponge, with moisture visible when compressed but not dripping. Too dry (below 40% moisture) and bacterial activity slows dramatically; too wet (above 65%) and the material becomes anaerobic regardless of turning. In UK conditions, rain can waterlog an open heap in winter — covering with a breathable lid (hessian, old carpet, or a commercial heap cover) prevents waterlogging while maintaining the gas exchange necessary for aerobic activity.

The most common composting problems and their solutions fall into predictable patterns. A smelly, wet heap indicates excess nitrogen and insufficient aeration — add browns generously, turn to introduce oxygen, and consider adding a layer of wood chip or shredded cardboard to absorb moisture. A dry, inactive heap with no visible breakdown indicates excess carbon, insufficient moisture, or both — water the heap thoroughly, add nitrogen-rich greens, and turn to mix. White fungal mycelium in the heap is entirely normal and beneficial — these are the fungi responsible for breaking down woody material, and their presence indicates a healthy, aerobic environment. Rats in or around the heap are caused by composting cooked food, meat, or dairy — exclude these materials or switch to a rat-proof container. Slow breakdown in winter is normal and expected — cold temperatures suppress microbial activity dramatically and there is little that can practically be done other than insulating the heap with straw bales and waiting for spring. Bokashi composting is a radically different system developed in Japan that uses a specific community of effective microorganisms (EM — principally lactic acid bacteria) to ferment rather than aerobically decompose food waste. All food can be processed in a bokashi system, including meat and dairy. The food is mixed with bokashi bran (inoculated with EM), placed in an airtight bucket, and fermented over 2–4 weeks at room temperature. The result is not finished compost but a pre-digested, pickled material that must then be either buried in the garden (where it is rapidly incorporated into the soil by earthworms and microorganisms), added to a conventional compost heap as a nitrogen-rich activator, or mixed into a hot composting system. Bokashi does not smell like rotting food — the fermentation process produces a mild, pickled odour. It is ideal for small gardens or flats where outdoor composting space is limited.