Composting is an aerobic process, which is a fancy way of saying it needs air. What’s more, air is probably even more important than food the average compost pile runs out of air long before it runs out of food.
If there isn’t enough air, decomposition becomes anaerobic, which is bad news for two reasons. First, it’s much slower than aerobic composting, and second, some of the products, such as ammonia and hydrogen sulfide, don’t smell very nice.
Traditional advice is to turn compost piles, specifically to introduce more air, but the perceived need to turn piles regularly is enough to put many gardeners off the whole idea of composting. Which brings us to a central dilemma: nitrogen-rich materials such as annual weeds and kitchen waste decompose quickly, but lack structural strength. As soon as they begin decomposing, they lose what little structure they have and collapse into an airless, smelly mess.
It’s easy to add materials with more fiber, such as tree and hedge prunings, and these will stop the pile from collapsing. But they decompose much more slowly than soft material, so you end up with coarse, twiggy compost. Shredding woody waste first helps it to break down more quickly, but reduces its usefulness in imparting structure to the pile.
Why does woody waste decompose so slowly? Because it has a high C:N ratio and the composting bacteria are not very good at breaking it down. Fungi are much better, but work only slowly and usually don’t like quite the same conditions as bacteria, so a pile that’s just right for bacteria will be less good for fungi.
One excellent solution is a waste material that every household produces in abundance and is recycled far less often than it should be: paper and cardboard.