How to Compost Coffee Grounds

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Compost -- black gold for the garden -- is free for anyone with a garden corner and a bin or box in which to make it. Building a "heap" takes a bit of planning, but a compost bin can save wear and tear on a garbage disposal in addition to providing organic enrichment for garden soil. Coffee grounds contribute nitrogen to compost, but they have other benefits when added to the heap.

The Compost Scheme

  • You don't have to be a certified organic farmer to assemble a successful compost pile. If you have cardboard, paper, corn husks in season, vegetable waste and coffee grounds, you're good to go. A successful compost pile balances carbon-rich materials, including dry leaves, paper and wood chips, with nitrogen-dense materials, including grass clippings. Coffee grounds add to the nitrogen side of the balance.

    An analysis performed by the Soil and Plant Laboratory in Bellevue, Washington for Sunset Magazine found used coffee grounds provided by Starbucks Coffee to contain 2.28 percent nitrogen by weight. To derive 2.28 pounds of nitrogen, you'd need 100 pounds of coffee grounds. The carbon-nitrogen balance in a healthy heap ranges between 25 parts carbon to 1 part nitrogen and 40 parts to 1, so a 5-pound can of spent grounds could probably gobble up as much cardboard as you could generate in a weekend.

Coffee Grounds Chemistry

  • Most of the acid in the coffee bean ends up in your cup, leaving grounds with a pH between 6.5 and 6.8, according to Oregon State University's Lane County Extension Service Compost Specialist program. In addition to approximately 2 percent nitrogen, grounds contain 0.06 percent phosphorus and 0.6 percent potassium.

    Grounds also contain plenty of fiber, which contributes to the carbon part of the equation. Coffee grounds have a 20-to-1 carbon-nitrogen ratio themselves, so mixing equal weights of coffee grounds and dead leaves would approach the ideal ratio of 30-to-1.

    Coffee grounds also help elevate temperatures in the compost heap. OSU reported sustained temperatures of 140 to 160 degrees Fahrenheit when 25 percent of the volume was coffee grounds.

Compost Recipes

  • Add coffee grounds to existing piles or start new piles with them. OSU suggests layering one-third each by weight coffee grounds, grass clippings and dry leaves to start a pile or on top of an existing pile. Add equal weights of grounds and dry leaves or paper and incorporate into a pile by turning well. *Filters from your coffeemaker8 also provide carbon. Master Gardener Linda Chalker-Scott of the Puyallup Research and Extension Center at Washington State University recommends a compost heap containing no more than 20 percent coffee grounds.

Other Ideas

  • Coffee grounds can also be composted directly in the garden. Chalker-Scott suggests using the grounds as a soil conditioner or a 1/2-inch layer covered by 4 inches of coarse organic matter, such as wood chips, as a mulch that will decompose slowly.

    Vericomposting uses worms to generate compost in a small bin that can stay in the kitchen. Fill a 1-foot deep bin with newspaper and cardboard bedding and sprinkle it with water and coffee grounds. The coffee grounds provide grit for the worms' digestive systems. As vegetable scraps are gradually added, the worms eat a mix of plants and paper and generate compost for spring planting.

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