What characteristics indicate that compost is mature and safe to use in the garden, and what is a typical ideal C:N ratio for finished compost?

• A. Odor-free, crumbly, dark, no recognizable food scraps; stable, no pathogens; C:N around 12:1 to 20:1.
• B. Smelly, chunky, bright; unstable; C:N around 5:1.
• C. Wet and slimey; high pathogens; C:N around 40:1.
• D. Odor-free but wet; stable; C:N around 60:1.

Answer: (A)

Mature compost is stable, safe to use, and ready to feed plants. The best indicators are an earthy, odorless smell; a crumbly, dark, soil-like texture; and no recognizable food scraps, showing that the material has broken down fully. Stability means microbial activity is low, so it won’t heat up or change much once applied, and it’s typically free of pathogens when produced under proper conditions. The usual target for finished compost is a carbon-to-nitrogen ratio around 12:1 to 20:1, which supports a balanced release of nutrients to plants and helps maintain the material’s stability in the garden.

Why the other descriptions don’t fit: compost that smells or feels unstable indicates unfinished material with ongoing decomposition or anaerobic pockets. A very low C:N ratio around 5:1 signals excessive nitrogen, often with odors and potential plant burn. A high C:N around 40:1 suggests slow decomposition and nutrient release. Being odor-free but wet with a C:N near 60:1 points to overly carbon-rich material that remains sluggish and poorly integrated into soil.