Composting (FS-2023-0687)

Composting is a natural, biological process that transforms organic materials into a nutrient-rich soil conditioner called “compost.” Composting involves the controlled aerobic decomposition of organic waste, including kitchen waste (such as fruit, vegetables, and coffee grounds) and yard waste (such as leaves and grass clippings). The composting process produces heat, which decreases pathogens, viable seeds, and the volume of the organic materials. Composting reduces greenhouse gas emissions and allows nutrients from waste to be returned to the agricultural production systems, enhancing the circular bioeconomy.

Compost is often created from food waste or lawn clippings. Additional substrates suitable for composting include agricultural residues, livestock manure, food processing waste, brewery and distillery byproducts, coffee grounds, yard debris, paper products, aquatic plants, biodegradable plastics, and organic textiles. When composting woody materials and other complex substrates, it is necessary to reduce the particle size, using equipment such as a chipper, prior to adding to the compost pile. These complex substrates can be used as bulking materials to keep oxygen flowing through the compost pile.

Homeowner (or “backyard”) composting is best suited for plant-based materials, excluding those that are diseased, that contain seeds that could sprout, or that could attract pests. It is essential for homeowners to manage their compost to prevent odors and accelerate decomposition, which includes regular turning and maintaining a proper balance between food waste and yard waste.

Commercial composting facilities typically achieve higher temperatures than backyard efforts, enabling them to compost a wider range of materials, including those that pose challenges in terms of pathogens or seeds that require longer decomposition times. These facilities are regulated and must adhere to specific standards to ensure that the compost produced is safe and of high quality.

The following table provides a general guide to materials that are suitable for composting in homeowner backyard operations and commercial operations (Christensen, 2009; Ruggero et al., 2019; Schwarz and Bonhotal, 2011; Sullivan, 2010; SUEPA, 2017). Please note that facility capabilities may vary depending on the composting technology utilized and management practices in place.

Composting involves microbial activity that breaks down organic matter through the following stages:

1. Collection: Organic materials are gathered to provide a proper carbon-to-nitrogen ratio (30:1) so that the microbes involved in the composting process can work efficiently.
    
2. Mixing: For effective composting, mix the waste materials manually or with equipment, such as a front loader, grinder, or rotating drum. Mixing allows the material to remain relatively homogeneous, maintain contact with the microbes, and brings materials from the outer parts into the middle of the pile, which has a higher temperature.
    
3. Aeration: Oxygen is crucial for the aerobic microorganisms responsible for decomposition. Using mechanical turning or forced aeration into the pile ensures that oxygen reaches all parts of the compost.
    
4. Decomposition: Microorganisms, such as bacteria, fungi, and actinomycetes, break down organic materials and convert complex organic compounds into simpler substances, which generates heat in the process to raise the compost pile temperature and help kill pathogens and weed seeds. The carbon, nitrogen, and water content should be managed to ensure decomposition generates heat within the compost pile at and above 133°F.
    
5. Curing: After the active decomposition phase, the compost must mature or “cure.” Curing typically takes several months to a year, during which time the compost stabilizes and develops its final characteristics for field application.

• To achieve successful composting, it is essential to use a mixture of “brown” to “green” materials to achieve a 30:1 carbon-to-nitrogen ratio. Livestock manure is often near this optimal 30:1 ratio, while leaves and grass clipping (carbon-rich “browns”) can be mixed with food waste (nitrogen-rich “greens”) to reach the ideal C:N ratio.
   
• Keep the compost pile moist but not waterlogged. It should feel like a wrung-out sponge.
   
• Turn the pile regularly to provide oxygen to the microbes and promote decomposition evenly within the compost pile.
   
• Chop or shred materials into smaller pieces to speed up decomposition and create a more homogeneous mixture.
   
• Homeowners conducting compost in non-commercial, backyard operations should not add meat, dairy, fat, grease, small animal waste, and other materials to their composting pile to avoid the potential of hosting pathogens. These operations may not reach the proper temperature nor have the time necessary to sufficiently remove possible pathogens within these materials during the composting process.

Composting offers a range of environmental and practical benefits:

• Waste Reduction: Composting diverts organic waste from landfills, reducing greenhouse gas emissions, and extends the landfill lifespan.
   
• Water Retention: Compost improves soil structure and enhances water retention capacity. Compost mixed with poor or sandy soils can improve water retention and fertility.
   
• Soil Enrichment: Compost improves aeration and nutrient content in the soil, which supports beneficial soil microorganisms, resilient plant growth, and root development. Cured compost can be mixed with soil in garden applications, applied to lawns in landscaping, used directly on crops, or placed in potting mixes.
   
• Erosion Control: Adding compost to soil helps prevent erosion by binding soil particles together, reducing runoff during heavy rain.
   
• Reduced Need for Chemical Fertilizers: Compost can provide essential nutrients to plants, reducing the need for synthetic fertilizers.
   
• Lower Greenhouse Gas Emissions: Composting plays a significant role in mitigating greenhouse gas emissions and reducing the environmental impact of waste. When organic waste is sent to landfills without composting, it undergoes anaerobic decomposition, producing significant amounts of uncaptured methane, a potent greenhouse gas. Composting occurs in aerobic (oxygen-rich) conditions, preventing the formation of methane during anaerobic decomposition. Additionally, greenhouse gas emissions are offset by using compost and reducing greenhouse gas emissions associated synthetic fertilizer production, thereby creating a more circular bioeconomy.

Users may encounter these issues when composting:

• Smell: If your compost pile smells unpleasant, it may be too wet or have too much “green” material. Overly soggy piles can benefit from more “brown” materials to absorb excess moisture. Turn the pile to aerate it and add more “brown” or carbon-rich absorbent materials and bulking materials, such as wood chips, to balance the moisture content and improve air circulation to eliminate anaerobic conditions. Improving drainage can also help, but make sure any leachate coming off the pile is properly treated.
   
• Lack of Decomposition: If the compost pile is not breaking down as expected, check the carbon-to-nitrogen ratio to ensure it is balanced (near 30:1). If you smell ammonia, add more “brown” carbon-rich that will readily break down. If a pile is not decomposing without being too wet or smelling of ammonia, it may be too carbon-rich. Add nitrogen as "green” material to balance the carbon to nitrogen ratio.
   
• Pests: If pests (flies or rodents) are attracted to your compost pile, the pile can be enclosed in a bin if adequate aeration levels are maintained. Adding meat or dairy products without high pile temperatures maintained especially by backyard, non-commercial operations may attract pests.

For more information on the Animal Waste Technology factsheet series and the Maryland Animal Waste Technology assessment submitted to the Maryland Department of Agriculture go to https://go.umd.edu/AWTF

This material is based upon work supported by the Maryland Department of Agriculture under Grant # MDA-2072-FY22.

• Christensen, E. (2009). Best management practices (BMPs) for incorporating food residuals into existing yard waste composting operations. The United States Composting Council. https://www.compostfoundation.org/Portals/1/ Documents/BMP-for-FW-to-YW.pdf.
   
• Ruggero, F., Gori, R., Lubello, C., (2019). Methodologies to assess biodegradation of bioplastics during aerobic composting and anaerobic digestion: A review. Waste Management & Research, 37(10), 959-975.
   
• Schwarz, M., J. Bonhotal, (2011). Composting at Home - The Green and Brown Alternative. Cornell Waste Management Institute. https://ecommons.cornell.edu /items/27eaaa88-7597-4a8f-bf2e-9301ce8eeb3e.
   
• Sullivan, D., (2010). Converting waste to resources benefits college students, campus farms and gardens, facilities management and the greater community. Part III. In the Recycling Food Waste: 101 series. BioCycle, Vol. 51, No. 12. https://www.biocycle.net/recycling-food-waste-101/
   
• United States Environmental Protection Agency, (2017). Composting at Home. https://19january2017snapshot.epa.gov/recycle/composting-home_.html