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Hot vs. Cold Composting

Each method has its advantages and drawbacks.

Some Like It Hot
The advantage of hot composting relate mainly to its fast turnover. Even in cooler climates you can process six or more batches in a season. If you have a big garden and limited room for composting, this is the way to go. It’s also the most effective way to build fertility when you’re just starting out a new location. The other major advantage to this method is its temperature. Few weed seeds and pathogens can survive thermophilic temperatures, especially if they are maintained for several weeks. This gives you more leeway to compost materials that should otherwise be avoided. However, it’s best to avoid composting materials that may carry diseases or weed seeds until you are sure of your hot-composting skills.

The major disadvantage of quick composting, with the exception of static piles that use forced aeration, is the labor involved. Not everyone is enthusiastic enough — or able — to be out there turning the compost every few days, especially if the pile is much larger than a 3-foot cube. This is also a less forgiving process than others; if the moisture level or carbon:nitrogen ratio is wrong, you have to make adjustments. Another drawback is that the whole pile must be built at once.

Hot composting conserves less nitrogen than cooler methods, since extra nitrogen is required to stimulate fast bacterial growth and some inevitably drifts off in the form of ammonia. However, a cool pile that sits in the rain for more than a year also loses much of its nitrogen content.

Studies at the Ohio Agricultural Research and Development Center have shown that compost produced at higher temperatures has less ability to suppress soil-borne diseases than does cool compost. This is because the beneficial bacteria and fungi that attack pathogens cannot survive the higher temperatures.

A Cool Alternative
Some compost professionals tend to turn up their noses at slow, cool methods, deriding them as “let it happen” compost. However, if you have the space but not the time or stamina to work with your compost, this is the easiest approach to take. Compost made in this manner will still heat up at first, but not to the levels of hot compost — 120ºF (49ºC) is a maximum. The mesophilic organisms will carry most of the burden of humus making, which will occur in six months to two years, depending on climate, materials used and aeration conditions.

The advantages and disadvantages of cool composting mirror those of hot composting: It involves less work but longer lag time until the compost is finished. It fails to kill pathogens or weeds but spares disease-suppressing microbes. It conserves nitrogen but must be protected from the elements longer. It has the advantage of allowing you to add materials a little at a time until you have a critical mass. The drawback to this is that you must be careful to balance carbon and nitrogen as well as wet and dry materials as you go. Otherwise, you can create anaerobic conditions or unpleasant smells.

There is a fierce debate regarding which method produces humus of a higher quality. Although cool compost generally results in more undecomposed bits of high-carbon materials, these can easily be screened out and added into the next pile.