Introduction: Understanding Composting and Material Biodegradability
Composting is the natural process of decomposing organic matter into nutrient-rich soil amendment. When materials decompose in oxygen-rich conditions, they transform into humus—a stable organic material that improves soil structure, retains moisture, and provides essential nutrients to plants. Understanding which materials are compostable is crucial for effective waste management, reducing landfill contributions, cutting greenhouse gas emissions, and creating valuable resources for gardens and agriculture.
Core Concepts and Principles of Composting
The Science Behind Composting
| Concept | Description | Practical Importance |
|---|---|---|
| Carbon-Nitrogen Ratio (C:N) | Balance between carbon-rich “browns” and nitrogen-rich “greens” | Optimal ratio 25-30:1 ensures efficient decomposition |
| Microbial Activity | Bacteria, fungi, and other decomposers break down organic matter | Activity levels determine decomposition speed |
| Aeration | Oxygen availability throughout compost pile | Prevents anaerobic conditions and bad odors |
| Moisture Content | Water level in composting materials | Should feel like a wrung-out sponge (40-60% moisture) |
| Particle Size | Physical dimensions of compostable materials | Smaller particles decompose faster but need more aeration |
| Temperature | Heat generated by microbial activity | Hot composting (120-150°F) kills pathogens and weed seeds |
Types of Composting Systems
- Backyard Composting: Home-scale bins, tumblers, or piles
- Vermicomposting: Using worms (typically red wigglers) to process materials
- Bokashi: Anaerobic fermentation using specialized microbes
- Industrial Composting: Large-scale facilities with controlled conditions
- In-Vessel Composting: Enclosed systems with mechanical turning
Comprehensive Compostable Materials List
Definitely Compostable: Green Materials (Nitrogen-Rich)
| Material | Decomposition Speed | Special Considerations | Best Practices |
|---|---|---|---|
| Fruit scraps | Fast | May attract pests; can contain seeds | Chop into smaller pieces; bury in pile |
| Vegetable scraps | Fast | Cruciferous vegetables may cause odors | Chop into smaller pieces; bury in pile |
| Coffee grounds | Medium | Excellent source of nitrogen | Include filters; distribute throughout pile |
| Tea bags/leaves | Medium | Remove staples and synthetic bags | Tear open bags if not paper-based |
| Fresh grass clippings | Very Fast | Can mat together and become slimy | Mix well with brown materials; add in thin layers |
| Fresh plant trimmings | Fast | Sappy or thick stems decompose slower | Chop or shred larger pieces |
| Eggshells | Slow | Calcium-rich but decompose slowly | Crush before adding to speed decomposition |
| Manure (herbivores only) | Medium-Fast | May contain weed seeds; potential pathogens | Avoid pet waste; best from farm animals like horses, cows |
| Seaweed/kelp | Fast | Contains valuable trace minerals | Rinse to remove salt if from ocean |
| Aquarium water | N/A | Rich in nitrogen and nutrients | Use to moisten dry compost materials |
Definitely Compostable: Brown Materials (Carbon-Rich)
| Material | Decomposition Speed | Special Considerations | Best Practices |
|---|---|---|---|
| Fallen leaves | Slow-Medium | May mat together when wet | Shred if possible; mix thoroughly with greens |
| Straw | Slow | Excellent for adding structure | Break into smaller pieces; moisten before adding |
| Cardboard | Medium | Remove tape and excessive printing | Tear into smaller pieces; wet before adding |
| Paper products | Medium | Avoid glossy paper and colored inks | Shred or tear; wet before adding |
| Newspaper | Medium | Mostly black and white ink is best | Shred or crumple; wet before adding |
| Egg cartons (paper) | Medium | Tear into pieces | Wet before adding to pile |
| Toilet paper/paper towel rolls | Medium | Tear into smaller pieces | Wet before adding to pile |
| Dryer lint | Medium | Only from natural fibers | Check source material; avoid if synthetic |
| Nutshells | Very Slow | Harder shells may never fully decompose | Crush before adding; best in small amounts |
| Pine needles | Very Slow | Can increase acidity slightly | Use in moderation; mix well with other materials |
| Sawdust | Medium | Use only untreated wood | Add in thin layers; ensure it’s not from treated wood |
| Wood chips | Very Slow | Provide structure but slow to break down | Use for aeration; will persist through one cycle |
| Paper coffee filters | Medium | Add with coffee grounds | Tear into pieces for faster decomposition |
| Cotton and wool rags | Slow | Must be 100% natural fibers | Cut into small pieces; avoid synthetic blends |
| Hair and fur | Slow | Rich in nitrogen | Distribute throughout pile; avoid clumps |
Conditionally Compostable Materials (Special Handling Required)
| Material | Home Compost | Industrial Compost | Special Considerations |
|---|---|---|---|
| Citrus peels | Yes, limited | Yes | Can be acidic and antimicrobial; use in moderation |
| Onions and garlic | Yes, limited | Yes | Strong odors; may repel worms in vermicomposting |
| Bread and grains | With caution | Yes | May attract rodents; bury deep in pile |
| Cooked rice and pasta | With caution | Yes | Can become slimy; may attract pests |
| Corn cobs | Yes, chopped | Yes | Very slow to break down; need to be chopped |
| Avocado pits | Yes, crushed | Yes | Extremely slow unless broken down first |
| Certified compostable plastics | No | Yes | Requires high temperatures of industrial composting |
| Compostable food containers | No | Yes | Requires industrial conditions; check certification |
| Bioplastics (PLA) | No | Yes, if certified | Need sustained high temperatures to break down |
| Parchment paper | Yes, torn up | Yes | Avoid waxed or silicone-coated varieties |
| Cooked meat scraps | Not recommended | Yes | Attracts pests; potential pathogens |
| Dairy products | Not recommended | Yes | Attracts pests; potential odors |
| Eggs (whole) | Not recommended | Yes | Potential pathogens; shells alone are fine |
| Wood ash | Yes, sparingly | Yes | Very alkaline; use in small amounts only |
| Black and white newspaper | Yes | Yes | Colored inks may contain heavy metals |
| Wine corks (natural) | Yes, chopped | Yes | Must be natural, not synthetic |
| Dryer sheets | No | No | Contain synthetic chemicals |
Non-Compostable Materials (Avoid These)
| Material | Why It’s Problematic | Alternative Disposal |
|---|---|---|
| Meat bones | Attract pests; do not break down | Municipal compost or trash |
| Fats, oils, grease | Disrupt decomposition; attract pests | Used oil collection or trash |
| Fish and seafood scraps | Strong odors; attract pests | Municipal compost or trash |
| Pet waste (cats, dogs) | Contains pathogens | Special pet waste composters or trash |
| Walnut shells | Contains juglone, toxic to many plants | Municipal compost or trash |
| Pressure-treated wood | Contains toxic chemicals | Hazardous waste disposal |
| Coal or charcoal ash | May contain harmful substances | Trash |
| Glossy or coated paper | Contains plastics and chemicals | Recycling for some types; otherwise trash |
| Stickers on fruits/vegetables | Made of plastic; do not decompose | Trash |
| Synthetic fabrics | Do not decompose; may contain microplastics | Textile recycling or trash |
| Plastic of any kind | Does not decompose | Recycling when possible; otherwise trash |
| Biodegradable plastics (non-certified) | Require industrial conditions; may leave microplastics | Check local facilities; often trash |
| Diseased plants | May spread disease to garden | Municipal compost (hot composting) or trash |
| Large branches | Too slow to decompose | Chipping for mulch or municipal yard waste |
| Invasive weeds | May spread when compost is used | Municipal composting or trash |
Comparing Home vs. Industrial Composting Capabilities
| Feature | Home Composting | Industrial Composting |
|---|---|---|
| Temperature Range | 90-140°F (32-60°C) | Up to 160°F (71°C) |
| Duration | 3 months to 2 years | 3-6 months |
| Certified Compostable Packaging | Generally not suitable | Specifically designed for these systems |
| Pathogen Elimination | Limited | Complete at proper temperatures |
| Weed Seed Destruction | Limited | Complete at proper temperatures |
| Material Variety | More limited | Wider range of materials |
| Monitoring & Control | Manual, inconsistent | Automated, consistent |
| Scale | Small batches | Tons of material |
Common Challenges and Solutions
Troubleshooting Composting Problems
| Problem | Causes | Solutions |
|---|---|---|
| Foul odor | Too wet; too many greens; poor aeration | Add browns; turn pile; adjust moisture |
| Slow decomposition | Too dry; too many browns; cold weather; large particles | Add greens; add water; chop materials smaller |
| Attracting pests | Food scraps exposed; meat/dairy present | Bury food scraps; use enclosed bin; remove problematic materials |
| Too wet/slimy | Excess moisture; too many greens | Add browns; improve drainage; cover during heavy rain |
| Too dry | Insufficient water; too many browns | Add water; add greens; cover to retain moisture |
| Weed growth in compost | Immature compost; weed seeds | Ensure hot composting; remove weeds before they seed |
| Not heating up | Too small; improper C:N ratio; insufficient moisture | Build larger pile (3’x3′ minimum); adjust brown/green ratio; add water |
Seasonal Composting Adjustments
| Season | Challenges | Adjustments |
|---|---|---|
| Summer | Rapid drying; potential odors | Add more water; cover pile; add more browns |
| Winter | Slow decomposition; freezing | Insulate with straw; use larger pile; continue adding materials |
| Fall | Excess leaves | Store extra leaves for year-round use as browns |
| Spring | Wet conditions; limited browns | Turn pile frequently; add stored fall leaves |
Best Practices and Practical Tips
For Efficient Home Composting
- Maintain proper C:N ratio: Approximately 3 parts browns to 1 part greens by volume
- Chop or shred materials: Smaller pieces decompose faster
- Layer materials: Alternate browns and greens to prevent matting
- Turn regularly: Every 1-2 weeks for faster composting
- Monitor moisture: Should feel like a wrung-out sponge
- Keep a container for kitchen scraps: Makes collection easier
- Store extra browns: Save fall leaves to balance year-round greens
- Consider location: Partial shade prevents drying out
- Use a thermometer: Monitor temperature to ensure proper decomposition
- Keep a “prohibited items” list: Post near kitchen collection bin
For Vermicomposting
- Choose appropriate worms: Red wigglers (Eisenia fetida) work best
- Avoid acidic materials: Limit citrus, onions, and garlic
- Maintain proper bedding: Shredded newspaper, cardboard, or coir
- Feed in moderation: Only what worms can process in 2-3 days
- Bury food scraps: Place under bedding to reduce fruit flies
- Maintain moisture: 70-80% (wring out bedding and only 1-2 drops should come out)
- Control temperature: Keep between 55-77°F (13-25°C)
- Harvest regularly: Remove finished compost every 3-6 months
For Industrial/Municipal Composting
- Check local acceptance guidelines: Materials vary by facility
- Look for proper certification: BPI, ASTM D6400, EN 13432 for compostable items
- Remove produce stickers: Even from commercially composted food
- Break down larger items: Tear or cut cardboard boxes
- Keep non-compostables separate: Follow facility guidelines strictly
- Use compostable bags: If permitted by your municipal program
Resources for Further Learning
Books and Publications
- The Rodale Book of Composting by Grace Gershuny and Deborah L. Martin
- Worms Eat My Garbage by Mary Appelhof (vermicomposting)
- Let It Rot! by Stu Campbell
- Compost Everything: The Good Guide to Extreme Composting by David the Good
Online Resources
- Composting Council: www.compostingcouncil.org
- Cornell Waste Management Institute: cwmi.css.cornell.edu
- EPA Composting At Home: www.epa.gov/recycle/composting-home
- BioCycle Magazine: www.biocycle.net
Certification Programs
- Biodegradable Products Institute (BPI): Certifies compostable products
- ASTM D6400: Standard specification for compostable plastics
- EN 13432: European standard for compostable packaging
Mobile Apps
- Compost Guide: Reference for compostable materials
- ShareWaste: Connects people with food scraps to local composters
- Compost Tracker: Logs additions to monitor brown/green ratio