Is My Compost Finished: How Long Does Compost Take To Mature

By: Susan Albert

Composting is one way many gardeners recycle garden waste. Shrub and plant trimmings, grass clippings, kitchen waste, etc., can all be returned to the soil in the form of compost. While seasoned composters know from experience when their compost is ready for use, newcomers to composting may need some direction. Read on for help in learning “when is compost done.”

Is My Compost Finished?

There are many variables that contribute to the timing of finished compost. It depends upon the particle size of materials in the pile, how often it is turned to supply oxygen, the moisture level and temperature of the pile, and the carbon to nitrogen ratio.

How Long Does Compost Take to Mature?

It can take from one month to a year to achieve a mature product, factoring in the above variables, plus the intended usage. For example, to use compost as a top dressing takes the least amount of time. Finished compost, or humus, is needed to use it as a growing medium for plants. Unfinished compost can be detrimental to plants if it is incorporated into the soil before it reaches the humus stage.

Finished compost looks dark and crumbly and has an earthy smell. The volume of the pile is reduced by about half, and the organic items added to the compost pile are no longer visible. If the hot composting method is used, the pile should not be producing much heat any longer.

Compost Maturity Test

There are scientific methods of testing the compost for maturity, but they can take some time. The quickest method is to place some compost into two containers and sprinkle them with radish seeds. If 75 percent of the seeds germinate and grow into radishes, your compost is ready to use. (Radishes are recommended because they germinate and develop quickly.)

More complicated methods of calculating germination rates include a “control” group and can be found on university extension websites. The phytotoxins in unfinished compost can prevent seeds from germinating or kill the sprouts soon thereafter. So, if an acceptable germination rate is attained, the compost is considered safe to use in any application.

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