Odor is the most prevalent problem at yard waste composting sites. Avoiding prolonged anaerobic conditions helps to minimize odor problems. Under anaerobic conditions, volatile organic acids, alcohols, and ammonia and sulfur compounds can be produced. The names of some of these compounds, like cadaverine and putrescine, suggest the offensiveness of the odors they generate. In contrast, when conditions are aerobic, the compost pile exudes only a mild, earthy smell.
Odors can develop when the windrow is too large.
When the windrow is first formed there is a high demand for oxygen because of the abundant degradable matter. If the windrows are too large, sufficient oxygen cannot penetrate through the windrow. An anaerobic core develops as decomposition slows and acid fermentation, or pickling, occurs, producing odors. If the piles become saturated, they will produce leachate. These liquids are similar to those that occur during decomposition in a forest, but they are more concentrated.