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When is Compost Ready?

Posted by Catherine Wainwright, on March 14, 2024.

Compost can take from a few months to over a year to reach maturity. Factors that can influence speed include the size of the compost pile, the type of compost bin you have, the balance of carbon-rich (brown) and nitrogen-rich (green) materials, how often the it is turned, and environmental conditions such as temperature and moisture. Smaller quantities may take longer to mature, as they do not generate enough heat or retain moisture as effectively, while warm weather and regular turning can accelerate the composting process.

What Does Mature Compost Look Like?

Mature compost is dark brown with a crumbly texture. It should have a deep, earthy smell that is devoid of unpleasant odours. The final product should no longer resemble any of the organic material added to the pile, having broken down into fine particles. It should feel spongey and moist but not overly wet and it should be cool to the touch, indicating the pile is no longer decomposing. Consider that your compost pile may not mature evenly — some bits of larger, undecomposed material can remain in a mature compost, which can simply be added to your next batch.

How to Test for Maturity

The easiest way to tell whether compost is mature is by sight, smell, and texture. To test for texture, take a handful from the pile’s centre and squeeze in a gloved hand. A mature, healthy compost should have the texture of a damp sponge, holding its shape while remaining lightly crumbly and not releasing much water. To take a more scientific approach, use the radish seed test. Fill a few small pots with your compost and sprinkle several radish seeds on the surface. If around 75% of the seeds germinate, within 7 to 10 days, your compost is likely to be mature and ready to use.

The Four Stages of Composting

Composting progresses through four stages before reaching maturity. The initial stage, known as the mesophilic phase, lasts a few days as the compost begins to heat up. Next, the thermophilic phase sees sustained temperatures for a few weeks to several months. As most of the materials decompose, the pile cools down, entering the cooling phase. Finally, during the curing phase, the compost stabilises and matures. This mature compost is now ready for use.

Can I use Immature Compost in the Garden?

Do not us immature compost in the garden. It can be detrimental to plants by introducing harmful acids, depleting nutrients, reducing oxygen availability, and lowering soil pH. It often has a higher concentration of organic acids, which can burn plant roots and inhibit growth. Additionally, as microbes continue to decompose organic material, they use up nitrogen and oxygen in the soil — this can stunt growth or even kill plants. You can use partially mature compost as a top dressing or mulch, but if you intend to use it as a growing medium or soil enhancer, ensure that your compost is fully mature.