When is Compost Finished?

The last stage of composting is called 'curing'. This is when the pile is set aside, not added to or turned, the temperature will lower to finish the process.

Curing can take anywhere from 1 month to 1 year. Many composters have their own preferences as to how long to cure a pile. Some will intentionally wait over a year to ensure higher quality compost, and others will use a shorter process because they need to make space for new piles.

There are a few ways to tell if your compost is ready to use:
  • 1 It looks like dark, crumbly topsoil
  • 2 It has a pleasant, earthy odor. It should not smell like ammonia
  • 3 The original organic materials (with a few exceptions) should no longer be recognizable
  • 4 The compost pile should have shrunk by half the size
  • 5 The pile should have returned to air temperature, about 50 degrees F
  • While the majority of organic materials should not be recognizable in finished compost, it’s okay if there are a few stubborn materials, such as corncobs or wood chips that do not decompose. These materials should not be used in the finished compost, though – they should be filtered out by a process called ‘screening’.

  • Screening passes the finished compost through a filter. Objects that are larger than the filter can be added to a new compost pile. They can be beneficial to a new compost pile because they contain microorganisms that will help jump-start the composting process.

  • One simple way to test finished compost is to take a handful and put it in a sealed plastic bag. After 3 days, open the bag and smell. Does it smell sour? If so, the compost is not finished curing and still has microorganisms at work. If it smells pleasant and earthy, it’s ready to use.

  • There are also a number of composting tests using plant germination that you can use to see if your compost is safe to use with plants.

  • Beyond your own testing, compost can be approved by third party organizations. If you are selling your compost these tests may be required by the retailer.

  • A widely used and respected program for compost quality testing is The US Composting Council’s Seal of Testing Assurance program.

Below are factors that affect the quality of compost:
  • Time to Mature

    Compost that has been allowed to cure, or mature, is healthier for soil than immature compost. Immature compost may contain plant inhibitors, such as bacteria that will compete with the plants for nitrogen in the soil. You will want to test the compost for maturity to make sure it is safe to use.

  • Source Materials

    The materials added to the compost pile will affect the nutrients, soluble salts, and contaminants found in the soil. Compost made from food scraps is typically higher in nutrients, but also higher in soluble salts (which are harmful to plant growth).

  • Hot vs. Cold Composting

    Cold compost is likely to have more nutrients than hot compost; however, hot compost is less likely to have pathogens and weed seeds.

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