Why does my compost smell bad – specifically, it smells like ammonia or rotten eggs? I’m trying to create a healthy compost for my garden, but the odor is quite strong and unpleasant. What are the most common causes of a smelly compost pile, and how can I troubleshoot to identify the exact problem in my setup? What adjustments to the ingredients or composting process should I make to eliminate the bad smell and create a balanced, odorless compost instead? Are there specific ratios of “greens” to “browns” that I should be aiming for, or perhaps I’m adding something I shouldn’t be? Also, could my composting bin itself be contributing to the problem (e.g., inadequate ventilation)?

Answer

A compost pile smells bad primarily due to anaerobic decomposition, which occurs when there isn’t enough oxygen present. This leads to the production of unpleasant byproducts like ammonia, hydrogen sulfide, and other volatile organic compounds. Several factors can contribute to this lack of oxygen and subsequent bad smell:

1. Lack of Oxygen (Anaerobic Conditions):

• Insufficient Airflow: Compost needs good airflow to support aerobic bacteria, which break down materials efficiently and without foul odors. Compaction, excessive moisture, or a lack of bulky materials can reduce airflow.
• Overly Wet Compost: Excessive moisture fills the air spaces in the compost pile, preventing oxygen from circulating. This can be caused by heavy rains, overwatering, or adding too many wet materials without balancing them with dry materials.
• Compaction: Heavy materials or excessive weight on the pile can compact it, squeezing out air and creating anaerobic zones.
• Too Many Fines: Using too many small particles like clay soil or finely shredded materials can restrict airflow.

2. Imbalance of Green and Brown Materials:

• Excess of "Green" (Nitrogen-Rich) Materials: Green materials like grass clippings, vegetable scraps, and coffee grounds are high in nitrogen. Too much of these without enough "brown" (carbon-rich) materials can lead to excess nitrogen, producing ammonia (which smells like urine).
• Insufficient "Brown" (Carbon-Rich) Materials: Brown materials like dried leaves, shredded paper, and wood chips provide carbon and bulk, helping to create air pockets and balance the nitrogen. A shortage of brown materials means there isn’t enough carbon to support the aerobic bacteria.

3. Problematic Materials:

• Meat, Dairy, and Oily Foods: These materials attract pests and decompose slowly, often creating foul odors as they rot anaerobically. They should generally be avoided in backyard compost piles.
• Pet Waste: While technically compostable under specific conditions, pet waste (especially from cats and dogs) can contain pathogens and often smells strongly, making it generally unsuitable for home composting, especially if the compost is intended for use in vegetable gardens.
• Diseased Plants: Composting diseased plants can spread the disease throughout the compost and then to your garden if the compost isn’t properly heated to kill the pathogens. They can also cause bad odors depending on the specific disease.
• Excessive Citrus Peels or Onions/Garlic: While technically compostable, large amounts of these can create a strong, unpleasant smell.

4. Inadequate Decomposition:

• Cold Compost: If the compost pile isn’t large enough or doesn’t have the right balance of ingredients, it may not heat up sufficiently. A hot compost pile (130-160°F) is essential for killing pathogens and breaking down materials quickly and efficiently. A cold pile decomposes slowly and is more likely to develop odors.
• Insufficient Turning: Turning the compost pile regularly introduces oxygen and helps to distribute moisture and materials, promoting even decomposition. A lack of turning can lead to anaerobic pockets.

5. Specific Odors and Their Causes:

• Ammonia (Urine-like Smell): Too much nitrogen, not enough carbon.
• Rotten Egg Smell (Hydrogen Sulfide): Anaerobic conditions due to compaction, excessive moisture, or lack of aeration.
• Sour or Rancid Smell: Decomposing fats or oils, or excessively wet conditions.
• General "Rotting" Smell: Slow, incomplete decomposition due to lack of oxygen, inappropriate materials, or insufficient heat.